Immunity and colds
Everything in the library studied for it, medical and holistic together. Within each kind, what was shown to help comes first, strongest evidence first. What was tested and not shown to help stays on the list.
Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on
Medication
Acute sinus infection in adults
Shown to cure slightly more adults with acute sinusitis than placebo, 5 to 11 more per 100, while 13 more per 100 had side effects.
Compared with placebo or no treatment [1]; placebo [2]
The numbers and sources
- Effect
- Without antibiotics, 46% were cured at 1 week and 64% at 14 days. With a clinical diagnosis, antibiotics raised cure (OR 1.25, 95% CI 1.02 to 1.54), a number needed to treat of 19, and 10 when the diagnosis was confirmed on imaging. Side effects were more frequent (OR 2.21), one extra person harmed for every 8 treated [1]. In individual patient data from 9 trials, 15 adults needed antibiotics for one more to be cured, and common signs and symptoms could not pick out who would benefit [2].
- Population
- 3,057 adults in 15 trials in ambulatory care [1]; 2,547 adults in 9 trials [2].
- Sources
- [1], [2]
- Caveats
- Children, people with weakened immune systems and severe sinusitis were not studied [1]. The trials used a range of antibiotics.
Doxycycline after sex to prevent sexually transmitted infections
Shown to cut chlamydia and syphilis by about three quarters with doxycycline after sex, mostly in men who have sex with men, with a smaller or no effect on gonorrhea.
Compared with standard care without doxycycline [15][16]; no prophylaxis [17]
The numbers and sources
- Effect
- In people on HIV prevention pills, a sexually transmitted infection was found at 10.7% of quarterly visits on doxycycline 200 mg within 72 hours of sex and 31.9% on standard care (RR 0.34), with similar results in people living with HIV (RR 0.38). Tetracycline-resistant gonorrhea occurred in 5 of 13 cases on doxycycline and 2 of 16 on standard care [15]. Pooled trials found lower chlamydia (RR 0.22) and syphilis (RR 0.24) and a smaller drop in gonorrhea (RR 0.79) [16]. Another meta-analysis found chlamydia (RR 0.35) and syphilis (RR 0.23) lower and no significant effect on gonorrhea (RR 0.90) [17].
- Population
- 501 men who have sex with men and transgender women [15]; 1,278 participants in 3 trials [16]; 1,727 participants in 4 trials, 73% men who have sex with men [17].
- Sources
- [15], [16], [17]
- Caveats
- Open-label trials. Long-term effects on resistance remain uncertain [16][17].
Sore throat
Shown to reduce sore throat at day 3 against placebo, with fewer than 6 treated for one extra person without it, though 82% on placebo were symptom-free by one week.
Compared with placebo or no treatment [3]
The numbers and sources
- Effect
- Sore throat at day 3 was less frequent on antibiotics (RR 0.70, 95% CI 0.60 to 0.80), and at 1 week (RR 0.50), when the number needed to treat was 18. Fever at day 3 did not differ (RR 0.75, 95% CI 0.53 to 1.07). Antibiotics lowered ear infections within 14 days (Peto OR 0.21), quinsy (Peto OR 0.16) and acute rheumatic fever (Peto OR 0.36), but not sinusitis [3].
- Population
- 15,337 cases of sore throat in 29 trials, mostly adults [3].
- Sources
- [3]
- Caveats
- Most trials were run in the 1950s, when complications such as rheumatic fever were far more frequent. Few trials separated bacterial from viral sore throat, and harms were poorly reported [3].
Ear infections in children
Shown to slightly reduce children's ear pain at 2 to 3 days, 20 treated for one fewer in pain, but not at 24 hours, with extra vomiting, diarrhea or rash.
Compared with placebo [7]; waiting to see if antibiotics were needed [7]
The numbers and sources
- Effect
- Pain at 24 hours did not differ (RR 0.89, 95% CI 0.78 to 1.01), but was a third less frequent at 2 to 3 days (RR 0.71, number needed to treat 20). Abnormal tympanometry at 2 to 4 weeks (RR 0.83), eardrum perforation (RR 0.43) and infection in the other ear (RR 0.49) fell, and late recurrences did not change. Vomiting, diarrhea or rash rose (RR 1.38, one extra per 14 treated). Against waiting, immediate antibiotics did not lower pain at 3 to 7 days (RR 0.75) and raised these side effects (RR 1.87) [7].
- Population
- 3,401 children in 13 placebo trials and 1,556 in 6 trials of immediate antibiotics against waiting, all in high-income countries [7].
- Sources
- [7]
- Caveats
- Serious complications were rare in both groups. Results may not apply where complications are more common [7].
Bladder infection in women
Shown to cure uncomplicated bladder infections in women far more often than placebo, about 5 times the odds, while amoxicillin-clavulanate cured fewer than ciprofloxacin, 58% against 77%.
Compared with placebo [8]; ciprofloxacin [9]
The numbers and sources
- Effect
- Clinical success (OR 4.81, 95% CI 2.51 to 9.21), cure (OR 4.67) and clearing bacteria at the end of treatment (OR 10.67) were more likely on antibiotics than placebo, and reinfection or relapse less likely (OR 0.27). Adverse events were more frequent (OR 1.64), and kidney infection did not differ [8]. A 3-day course of amoxicillin-clavulanate cured 58% against 77% on ciprofloxacin, and 60% against 77% even with susceptible bacteria, with vaginal E. coli remaining in 45% against 10% [9].
- Population
- 1,062 nonpregnant adult women with confirmed cystitis in 4 trials [8]; 370 women aged 18 to 45 [9].
- Sources
- [8], [9]
- Caveats
- The placebo trials did not single out the drugs on this page [8].
Immune response to vaccines and infections in older adults
Small trials of rapamycin analogs, not sirolimus, found a better flu vaccine response and fewer reported infections in older adults.
The numbers and sources
- Effect
- The rapamycin analog everolimus (RAD001) raised the response to influenza vaccine by about 20% in older volunteers [4]. A low-dose combination of everolimus and BEZ235 taken for 6 weeks was associated with fewer reported infections over the following year (P = 0.001) and a better influenza vaccine response [5].
- Population
- Older volunteers [4]; 264 older adults in a phase 2a trial [5].
- Sources
- [4], [5]
- Caveats
- These trials tested rapamycin analogs, not sirolimus itself. Infections were self-reported [5]. [5] lists Novartis funding.
Common cold
Mixed results: antihistamines slightly eased overall cold symptoms on days one and two in adults, 45% against 38% on placebo, with no difference after that.
Compared with placebo [7]
The numbers and sources
- How common
- Very common: more than 1 in 10 people.
- Effect
- On day one or two, 45% had a beneficial effect on overall severity with an antihistamine against 38% on placebo (OR 0.74, 95% CI 0.60 to 0.92). There was no difference at 3 to 4 days or 6 to 10 days. Sedating antihistamines slightly eased runny nose and sneezing on day three, an effect the authors called clinically non-significant [7].
- Population
- 4,342 participants, 212 of them children, in 18 randomized trials of natural and induced colds [7].
- Sources
- [7]
- Caveats
- The review pooled sedating and non-sedating antihistamines [7].
Common cold symptoms
Limited evidence: four trials in adults with a cold found mixed results, with nasal obstruction improved in two and sore throat or malaise not improved in two.
The numbers and sources
- Effect
- A Cochrane review found 4 trials and did not pool them because designs and outcomes differed. Nasal obstruction improved on acetaminophen in 2 of the 4. One found less runny nose and no difference in sneezing or coughing. Sore throat and malaise did not improve in 2 of the 4. Headache and achiness improved more than on placebo in 2 studies and did not differ in 1. No study reported how long cold symptoms lasted [15].
- Population
- 758 adults with a common cold in 4 trials, 3 of them lasting 4 to 6 hours [15].
- Sources
- [15]
- Caveats
- Short trials with inconsistent results. One used acetaminophen combined with pseudoephedrine [15].
COVID-19
Limited evidence: famotidine was not linked to fewer deaths in hospitalized COVID-19 patients, and one small outpatient trial found faster symptom improvement but no difference in time to resolution.
Compared with patients not given famotidine [9]; placebo [10]
The numbers and sources
- Effect
- In hospitalized patients, famotidine use was not associated with the composite of death, intubation or intensive care (HR 0.63, 95% CI 0.35 to 1.16) or with death (HR 0.67, 95% CI 0.26 to 1.73) [9]. In outpatients with mild to moderate illness, time to symptom resolution did not differ, but a 50% drop in symptom scores came at 8.2 days on famotidine and 11.4 days on placebo [10].
- Population
- 4 observational studies with 46,435 patients, 3,110 on famotidine [9]; 55 non-hospitalized adults, median age 35, randomized [10].
- Sources
- [9], [10]
- Caveats
- The pooled studies were observational with heterogeneity of 84% to 88% [9]. The trial was a small phase 2 study [10].
Acute bronchitis and chest infections
Not shown to improve overall recovery from acute bronchitis, with cough about half a day shorter, and amoxicillin did not shorten bad symptoms in a 2,061-person trial.
The numbers and sources
- Effect
- The share clinically improved did not differ (RR 1.07, 95% CI 0.99 to 1.15). Cough was less frequent at follow-up (RR 0.64, a number needed to treat of 6) and lasted 0.46 days less. Adverse effects were more frequent (RR 1.20), one extra for every 24 treated [4]. In adults with a chest infection not suspected to be pneumonia, amoxicillin 1 g three times daily did not shorten moderately bad symptoms (HR 1.06, 95% CI 0.96 to 1.18). New or worse symptoms were 15.9% against 19.3% (number needed to treat 30), and nausea, rash or diarrhea one extra per 21 treated, including in people 60 or older [5].
- Population
- 5,099 people in 17 trials [4]; 2,061 adults in 12 European countries [5].
- Sources
- [4], [5]
- Caveats
- Frail older people with several conditions may have been left out of trials [4].
Common cold and green nasal discharge
Not shown to help the common cold or persisting colored nasal discharge against placebo, while adults given antibiotics for a cold had more side effects.
Compared with placebo [6]
The numbers and sources
- Effect
- Lack of cure or persistent cold symptoms did not differ (RR 0.83, 95% CI 0.60 to 1.14). Persisting purulent rhinitis did not differ (RR 0.73, 95% CI 0.47 to 1.13). Adverse effects were more frequent on antibiotics for colds (RR 1.8), in adults (RR 2.62) but not children (RR 0.91), and for purulent rhinitis (RR 1.46) [6].
- Population
- Up to 1,147 people in 6 common cold trials and 723 in 4 purulent rhinitis trials [6].
- Sources
- [6]
- Caveats
- Trials used various antibiotics and outcome measures, and some participants may have had chest or sinus infections [6].
Supplements
Colds and upper respiratory infections (yeast beta-glucan)
Shown to lower the odds of colds and shorten them in healthy people taking yeast beta-glucan, across few trials with high heterogeneity.
The numbers and sources
- Effect
- In 13 trials in healthy people, yeast beta-glucan lowered the odds of an upper respiratory infection (OR 0.345, 95% CI 0.192 to 0.620), the number of episodes (SMD -0.315) and their duration (SMD -0.312) compared with placebo [9]. A review of 34 trials of fungal beta-glucans at 2.5 to 1,000 mg a day for up to 6.5 months reported fewer respiratory infection symptoms, but immune cell and molecular findings were inconsistent [10].
- Population
- Healthy subjects [9]. Healthy people and patients [10].
- Sources
- [9], [10]
- Caveats
- Heterogeneity was high and the trials were few [9]. A review of commercial beta-glucan products found an average of 33 participants per arm, and more than half of trials that reported funding were sponsored by beta-glucan producers [11].
Late-onset sepsis in preterm infants
Shown to lower late-onset sepsis confirmed by culture about 17% in preterm infants in hospital, with shorter stays.
The numbers and sources
- Effect
- In 31 studies (8,040 preterm infants of 36 weeks or less), probiotics lowered culture-proven late-onset sepsis (RR 0.83, 95% CI 0.72 to 0.95). Hospital stays were shorter, and mortality trended lower without reaching significance [42].
- Population
- Preterm infants in hospital [42].
- Sources
- [42]
- Caveats
- One meta-analysis. Probiotic sepsis and the FDA warning on preterm products are listed under Risks.
COVID-19 in hospital
Shown to lower death, ventilation and worsening in patients with COVID-19 when added to standard care, about 61% fewer deaths, in small trials.
The numbers and sources
- Effect
- Across 23 RCTs (1,407 patients), turmeric added to standard care lowered all-cause death (RR 0.39, 95% CI 0.23 to 0.67, 8 RCTs, moderate certainty), need for intubation or mechanical ventilation (RR 0.35, 95% CI 0.17 to 0.72) and clinical deterioration (RR 0.36, 95% CI 0.22 to 0.59) [40].
- Population
- Patients with mild to severe COVID-19, hospitalized in 17 of 23 trials. Nano-curcumin at 160 to 240 mg a day was the most common form [40].
- Sources
- [40]
- Caveats
- Nine trials were at high risk of bias. Results held when those were excluded [40]. The trials were small.
Length of a cold, zinc started at onset
Shown to shorten colds about 2.4 days when started at onset, in low-certainty evidence from children and adults.
The numbers and sources
- Effect
- In treatment trials, zinc shortened colds by a mean 2.37 days (95% CI -4.21 to -0.53, I² 97%, 8 studies, 972 participants, low certainty) [1]. In 7 lozenge trials using more than 75 mg a day, colds were 33% shorter (95% CI 21% to 45%). Acetate and gluconate lozenges did not differ significantly, and 192 to 207 mg a day did not outperform 80 to 92 mg a day [2].
- Population
- 34 trials with 8,526 children and adults [1]; 575 adults with natural colds [2].
- Sources
- [1], [2]
- Caveats
- Heterogeneity was very high and most trials were at unclear or high risk of bias [1]. The single-author lozenge analysis [2] selected trials by dose.
Colds
Limited evidence: in children with nephrotic syndrome, astragalus granules were tied to fewer upper respiratory infections. A Cochrane review found no placebo-controlled trial for preventing them in children.
The numbers and sources
- Effect
- A Cochrane review screened 120 full-text papers on oral astragalus to prevent frequent respiratory infections in children and found no randomized trial comparing astragalus alone with placebo, so it could not assess benefit or safety. 44 studies were excluded for having no placebo [10]. In two studies pooled for children with nephrotic syndrome, upper respiratory tract infections occurred in 23.9% given astragalus granules with prednisone against 42.9% on prednisone alone (RR 0.56, 95% CI 0.33 to 0.93), with evidence quality rated low [11].
- Population
- Children up to 14 years in community settings [10]. Children with nephrotic syndrome taking prednisone [11].
- Sources
- [10], [11]
- Caveats
- Evidence quality was low, and the comparison was against prednisone alone with no placebo [11]. No trial in adults with colds was located.
Immune markers
One trial found 1.5 g a day raised natural killer cell activity and other immune markers in 79 healthy men. Infections were not measured.
The numbers and sources
Cold and upper respiratory symptoms
Small trials found fewer cold days in air travelers, 57 against 117 on placebo. A review rated the evidence uncertain.
The numbers and sources
- Effect
- A meta-analysis of 180 participants reported a large reduction in upper respiratory symptoms with elderberry [1]. A systematic review of 5 randomized trials found elderberry may reduce cold duration and severity, with uncertain evidence [2]. In 312 air travelers, cold days totaled 57 on elderberry and 117 on placebo, and summed symptom scores 247 against 583 [3].
- Population
- 180 participants [1], 5 RCTs [2], 312 economy-class passengers flying from Australia [3].
- Sources
- [1], [2], [3]
- Caveats
- The meta-analysis abstract gives no effect size or trial count. It was funded by the Franklin Institute of Wellness [1]. The air travel trial was funded by Iprona AG, which supplied the capsules and was partly involved in design [3].
Common cold
One trial found 24 colds on garlic against 65 on placebo over 12 weeks. Days to recovery were similar.
The numbers and sources
- Effect
- A Cochrane review found only one eligible trial. Its 146 participants on a daily garlic supplement for 12 weeks reported 24 colds against 65 on placebo, and 111 against 366 days of illness. Days to recovery were similar, 4.63 against 5.63 [9].
- Population
- 146 adults in one trial [9].
- Sources
- [9]
- Caveats
- One trial with self-reported colds. The review authors found the evidence insufficient [9].
Colds
One trial found fewer colds and fewer cold days over 4 months on a North American ginseng extract in adults prone to colds.
The numbers and sources
- Effect
- In 279 adults with at least 2 colds in the prior year, an extract of North American ginseng taken for 4 months lowered colds per person (0.68 vs 0.93), the share with 2 or more colds (10.0% vs 22.8%) and days with cold symptoms (10.8 vs 16.5) [8].
- Population
- Adults aged 18 to 65 in Edmonton, Canada [8].
- Sources
- [8]
- Caveats
- One trial of one polysaccharide-rich extract. 44 people who never started treatment were excluded from analysis [8].
Energy and immunity
One trial found a fruit and vegetable juice powder cut days with moderate or severe cold symptoms by 20% in hospital staff. No trial measuring energy was located.
The numbers and sources
- Effect
- In 529 healthcare workers given encapsulated fruit and vegetable juice powder (Juice Plus+) or placebo daily for 8 months, days with moderate or severe cold symptoms over 6 months averaged 7.6 against 9.5 (p = 0.023). Total days with any cold symptom did not differ, 29.4 against 30.7 (p = 0.616) [11].
- Population
- 529 hospital staff in Berlin aged 18 to 65, mostly nurses, 80% women [11].
- Sources
- [11]
- Caveats
- One trial of one encapsulated fruit and vegetable product, not a greens drink powder. Symptoms were self-reported in diaries, and the effect was limited to more severe symptom days [11]. No trial of AG1 or other greens blends measured energy, fatigue or infections.
Preventing colds and respiratory infections
Mixed results: a Cochrane review found no single prevention comparison significant, while pooled analyses suggested about 10% to 35% fewer infections.
The numbers and sources
- Effect
- A Cochrane review of 24 trials (4,631 participants) found none of 12 prevention comparisons reached significance alone, and a post hoc pooling suggested a 10% to 20% relative risk reduction [1]. A 2019 meta-analysis found a risk ratio of 0.78 (95% CI 0.68 to 0.88) for infection [2]. A third meta-analysis of 6 studies (2,458 participants) reported a risk ratio of 0.649 (95% CI 0.545 to 0.774) for recurrent infections and 0.503 for complications such as pneumonia and ear infections [3].
- Population
- Otherwise healthy children and adults [1][2][3].
- Sources
- [1], [2], [3]
- Caveats
- Products differed widely in species, plant part and extraction [1]. The Cochrane authors call potential effects of questionable clinical relevance [1]. Review [2] lists selective reporting and protocol deviations. Review [3] used a fixed-effect model and its abstract does not report funding.
Respiratory infections in children
Mixed results: a meta-analysis found fewer infections and less antibiotic use in children, while the largest single trial found no difference in duration or severity.
The numbers and sources
- Effect
- A 2025 meta-analysis of 9 RCTs (1,518 children on Echinacea purpurea, 1,651 on placebo) found shorter treatment duration for upper respiratory infections (SMD -0.19, 95% CI -0.30 to -0.09), fewer episodes (RR 0.81, 95% CI 0.75 to 0.87) and less antibiotic use (RR 0.18, 95% CI 0.13 to 0.25). Ear infection episodes fell (RR 0.56, 95% CI 0.44 to 0.73), and ear infection treatment duration did not change (p 0.10) [5]. A 2025 systematic review of 5 RCTs rated the evidence high certainty for shorter symptoms and fewer antibiotics and moderate certainty for preventing infection [6]. In one trial of 407 children aged 2 to 11 with 707 upper respiratory infections, duration, severity, peak severity and days of fever did not differ between Echinacea purpurea and placebo [7].
- Population
- Children in 9 RCTs [5] and 5 RCTs [6]; 407 healthy children in one trial [7].
- Sources
- [5], [6], [7]
- Caveats
- The duration effect is small (SMD -0.19). Products varied in plant part and dosage form, and the funnel plot showed bias linked to heterogeneity [5]. The 5-trial review found some concerns of bias in randomization, allocation concealment, blinding and selective reporting, and did not pool results [6]. The largest single trial found no difference [7].
Sepsis in preterm infants
Mixed results: pooled trials found less late-onset sepsis in preterm infants, while the largest trial, of 2,203 infants, found no difference.
The numbers and sources
- Effect
- In 12 trials (5,425 infants), lactoferrin added to feeds lowered late-onset sepsis (RR 0.82, 95% CI 0.74 to 0.91, number needed to treat 25, low certainty) and shortened hospital stay by 2.38 days [1]. A 2025 meta-analysis of 10 trials found fewer infants with culture-proven or probable late-onset sepsis (OR 0.60, 95% CI 0.42 to 0.86) [2]. In the largest trial, 2,203 very preterm infants in 37 UK hospitals, late-onset infection occurred in 29% on lactoferrin and 31% on sucrose (adjusted RR 0.95, 95% CI 0.86 to 1.04) [3].
- Population
- Preterm infants in hospital [1][2]. Infants born before 32 weeks, given 150 mg/kg a day [3].
- Sources
- [1], [2], [3]
- Caveats
- The pooled analyses and the largest trial disagree. The Cochrane authors note publication bias and small studies of poor method that may inflate the effect [1]. Both were funded by the UK National Institute for Health Research [1][3].
Respiratory infections
Mixed results: one review found fewer respiratory infections in children but not adults, and another found no difference in children.
The numbers and sources
- Effect
- Across 25 studies, respiratory infection incidence fell in infants and children (OR 0.78, 95% CI 0.61 to 0.98) but not in adults (OR 1.00, 95% CI 0.76 to 1.32). In adults, IL-6 fell by 24.9 pg/mL, while CRP and NK cell activity did not change [4]. A 2025 meta-analysis of 8 trials in people under 18 found no significant difference in upper respiratory infections (OR 0.61, 95% CI 0.27 to 1.40) [2].
- Population
- 20 studies in adults and 5 in children [4]. 8 trials in children [2].
- Sources
- [4], [2]
- Caveats
- The two reviews differ on children [4][2]. The authors of the first describe the clinical studies as limited. The review was supported by an Australian government grant in partnership with Freedom Foods Group Nutritionals [4].
Upper respiratory infections
Mixed results: one review found fewer respiratory infections and shorter episodes, while a later review found no significant drop in infections.
The numbers and sources
- Effect
- Across 23 trials (6,950 children, adults and older people), probiotics reduced the number with at least one infection (RR 0.76, 95% CI 0.67 to 0.87, low certainty) and at least three infections (RR 0.59, moderate certainty), shortened episodes by 1.22 days (95% CI -2.12 to -0.33), and reduced antibiotic use (RR 0.58) [15]. A 2026 review of 32 RCTs found no significant pooled drop in infection incidence (risk difference -0.07, 95% CI -0.23 to 0.09), while some strains shortened symptoms and cut antibiotic use [16]. Across 13 RCTs of Lacticaseibacillus rhamnosus GG, respiratory episodes fell (MD -0.14, 95% CI -0.27 to -0.01) [17].
- Population
- People in community, care facilities, schools and hospitals [15]. RCTs of oral or topical probiotics [16]. People given LGG for prevention or treatment [17].
- Sources
- [15], [16], [17]
- Caveats
- Most outcomes were low certainty [15]. Heterogeneity in [16] was very high (I² 93%) and certainty very low. The reviews disagree on incidence.
Catching colds and how long they last
Not shown to prevent colds in the general population, though colds were 8% shorter in adults. Risk halved in people under heavy physical stress.
The numbers and sources
- Effect
- In general community trials, regular vitamin C did not change the risk of catching a cold (RR 0.97, 95% CI 0.94 to 1.00). In marathon runners, skiers and soldiers on subarctic exercises, the risk halved (RR 0.48, 95% CI 0.35 to 0.64). Regular supplementation shortened colds by 8% in adults (95% CI 3% to 12%) and 14% in children. Taken only once a cold had started, vitamin C showed no consistent effect on duration or severity [1].
- Population
- 29 trial comparisons with 11,306 participants for incidence, 10,708 of them in community trials and 598 under heavy physical stress. 31 comparisons covering 9,745 cold episodes for duration [1].
- Sources
- [1]
- Caveats
- Trials used at least 0.2 g a day. The therapeutic trials were few, 7 comparisons [1].
Flu duration
Small trials found mixed results: one saw flu symptoms ease about 4 days sooner, and a later trial of 87 patients saw no difference.
The numbers and sources
- Effect
- In 60 Norwegian adults with flu symptoms for 48 hours or less, 15 mL of elderberry syrup four times a day for 5 days relieved symptoms about 4 days sooner than placebo, with less rescue medicine [4]. In a 2020 trial of 87 emergency room patients with PCR-confirmed flu, days until symptoms were none or mild were 5.3 on elderberry and 4.9 on placebo (p 0.57), and days to full resolution were 8.6 and 8.7 [5]. The systematic review rated the flu duration evidence uncertain [2].
- Population
- 60 adults aged 18 to 54 [4]. 87 patients aged 5 and older [5].
- Sources
- [4], [2], [5]
- Caveats
- The two trials disagree. The 2020 trial was powered above 0.90 to detect a 2-day difference and was funded by the Wendel Family Foundation and Cleveland Clinic [5]. The abstract of [4] does not report funding.
Immune function
One small trial found changes in lab immune markers in older women, without measuring infections or illness.
The numbers and sources
- Effect
- In 39 older women who completed 8 weeks of 2,000 mg a day of dry extract, stimulated lymphocyte proliferation rose, Th17 cells fell and Th2 cells rose, with changes in several immune gene expressions against placebo [5]. In cancer trials, T-cell percentages rose modestly [1].
- Population
- 60 older women enrolled, 39 completed [5]. Cancer patients [1].
- Sources
- [5], [1]
- Caveats
- Lab markers, not infection or illness rates. High dropout in [5].
Infections other than colds
Limited evidence: one trial found fewer infections in children with sickle cell anemia, while another found no change in deaths in infants with sepsis.
The numbers and sources
- Effect
- In children with sickle cell anemia, 20 mg zinc sulfate a day for 6 months led to 80 infections against 124 on placebo (305.7 against 480.7 per 100 person-years, IRR 0.62, 95% CI 0.45 to 0.86) [11]. In infants hospitalized with suspected sepsis, 10 mg a day for 14 days with antibiotics did not change death in hospital (4.1% against 4.9%, RR 0.83, 95% CI 0.60 to 1.15) or by 12 weeks (9.0% against 8.6%, RR 1.05, 95% CI 0.84 to 1.32) [12]. In adults hospitalized with COVID-19, ICU admission or death occurred in 5.7% on 90 mg zinc acetate a day and 23.5% on standard care alone (OR 0.21, 95% CI 0.03 to 0.96) [13].
- Population
- 100 children aged 1 to 4 in Uganda, all on hydroxyurea [11]; 3,153 infants aged 3 to 59 days in India and Nepal [12]; 71 adults in one Spanish hospital [13].
- Sources
- [11], [12], [13]
- Caveats
- Each result comes from one trial in a different population. The sepsis trial was underpowered because events were fewer than expected, and vomiting was slightly more common on zinc [12]. The COVID-19 trial was small and open-label [13].
How long a cold lasts
Not shown to shorten colds in people with new colds. Only 1 of 7 treatment trials found echinacea better than placebo.
The numbers and sources
- Effect
- Of 7 treatment trials reporting cold duration, only 1 found echinacea better than placebo [1]. A 2019 meta-analysis found a mean difference of -0.45 days, with a confidence interval crossing zero [2]. In 719 patients aged 12 to 80, blinded echinacea shortened colds by a nonsignificant 0.53 days (95% CI -1.25 to 0.19) and lowered global severity by a nonsignificant 28 points [4].
- Population
- People with new-onset colds [1][2][4].
- Sources
- [1], [2], [4]
- Caveats
- The 719-person trial had more variability than expected, which limited power to detect small effects. It was funded by the NIH's National Center for Complementary and Alternative Medicine [4].
Immune function in athletes
Not shown to change lymphocyte or leukocyte counts in athletes.
The numbers and sources
- Effect
- Glutamine did not change lymphocyte or leukocyte counts. Neutrophils fell at doses above 200 mg per kg (WMD -605.77), and glutamine dipeptide raised post-exercise blood glucose by 0.51 mmol/L [1].
- Population
- Athletes [1].
- Sources
- [1]
- Caveats
- Cell counts are not infection rates. The neutrophil confidence interval as printed (-1200.0 to 52.1) crosses zero, which conflicts with its reported p-value of 0.03 [1].
COVID-19
Not shown to lower death, ventilation or hospital stay in patients with COVID-19 in randomized trials. A review adding cohort studies found lower mortality.
The numbers and sources
- Effect
- Across 9 RCTs, vitamin C did not change death from any cause (RR 0.92, 95% CI 0.83 to 1.02), need for ventilation (RR 0.98, 95% CI 0.87 to 1.11) or length of hospital stay (MD -0.63 days, 95% CI -3.04 to 1.78) [3]. Across 13 studies, short-term mortality (RR 0.92, 95% CI 0.72 to 1.17) and in-hospital mortality (RR 1.05, 95% CI 0.95 to 1.16) did not change [4]. A review that also pooled cohort and retrospective studies found lower mortality (OR 0.64, 95% CI 0.51 to 0.80) [5].
- Population
- Patients with COVID-19, 9 RCTs [3]; 13 studies with 12,062 participants [4]; 22 studies with 3,429 hospitalized patients [5].
- Sources
- [3], [4], [5]
- Caveats
- Sources disagree. The review that found lower mortality pooled cohort and retrospective studies [5]. Certainty ranged from very low to moderate [4].
Respiratory infections
Not shown to clearly lower respiratory infections in people. An earlier review found one point fewer, which lost significance in a 2025 update.
The numbers and sources
- Effect
- Across 37 trials with primary outcome data (48,488 participants aged 0 to 95), 61.3% on vitamin D and 62.3% on placebo had one or more acute respiratory infections (OR 0.92, 95% CI 0.86 to 0.99, I² 35.6%) [12]. A 2025 update by the same group added 3 trials with 16,085 participants. Across 40 studies (61,589 participants), the effect was no longer statistically significant (OR 0.94, 95% CI 0.88 to 1.00, P 0.057, I² 26.4%), and age, baseline vitamin D status, dosing frequency and dose size did not modify it [13]. In 17 pediatric RCTs (18,372 children), infections did not fall significantly (RR 0.82, 95% CI 0.64 to 1.06), and daily doses of 1,000 IU or less had RR 0.42 (95% CI 0.27 to 0.66) [14]. A Cochrane review of children up to 5 found RR 0.95 (95% CI 0.91 to 1.00) for the share making infection-related healthcare visits, and no change in visits per child [15]. Across 6 RCTs (15,677 participants), tuberculosis infection (OR 0.95, 95% CI 0.79 to 1.14) and active tuberculosis (OR 0.77, 95% CI 0.56 to 1.05) did not differ from placebo [16]. In 1,747 adults newly diagnosed with COVID-19, a 4-week course did not change healthcare visits or death (28% against 29%, OR 0.97, 95% CI 0.75 to 1.24) [17].
- Population
- 46 double-blind RCTs, 75,541 participants [12]; 40 placebo-controlled studies, 61,589 participants [13]; children in 17 RCTs [14] and 107 RCTs from pregnancy to age 5 (31,521 participants) [15]; people without active tuberculosis [16]; adults in the US and Mongolia [17].
- Sources
- [12], [13], [14], [15], [16], [17]
- Caveats
- The 2021 and 2025 analyses come from the same group and share most trials. The 2021 absolute difference was one percentage point [12]. The 2025 funnel plot was asymmetric (Egger P 0.002) [13]. The Cochrane result rests on low-certainty evidence [15].
Catching a cold, zinc taken daily
Not shown to prevent colds in children and adults taking zinc daily.
The numbers and sources
- Effect
- Zinc made little or no difference to the risk of developing a cold (RR 0.93, 95% CI 0.85 to 1.01, 9 studies, 1,449 participants, low certainty). When colds occurred, their length did not differ (MD -0.63 days, 95% CI -1.29 to 0.04, moderate certainty) [1].
- Population
- 15 prevention trials in children and adults [1].
- Sources
- [1]
- Caveats
- Trials mixed common cold and upper respiratory infection definitions.
Preventing colds
Limited evidence: cold episodes did not differ significantly from placebo in air travelers, and a review found elderberry may not lower cold risk.
The numbers and sources
- Effect
- In 312 air travelers, cold episodes were 12 on elderberry and 17 on placebo, a difference that was not significant (p 0.4) [3]. The systematic review found elderberry may not reduce the risk of developing a cold [2].
- Population
- Air travelers [3], 5 RCTs [2].
- Sources
- [3], [2]
- Caveats
- Few prevention trials.
Immunity and COVID-19
One trial found no difference in COVID-19 cases or symptomatic illness against placebo in older US adults, in a secondary analysis.
The numbers and sources
- Effect
- In the final COSMOS year (2020), COVID-19 incidence did not differ between multivitamin and placebo (HR 0.93, 95% CI 0.81 to 1.07). Among 338 cases with symptom data, symptomatic illness did not differ significantly (OR 0.70, 95% CI 0.44 to 1.11). In the per-protocol analysis, the odds ratio was 0.60 (95% CI 0.37 to 0.99) [11].
- Population
- 18,205 adherent COSMOS participants as of January 2020 [11].
- Sources
- [11]
- Caveats
- A secondary analysis with self-reported infections. The per-protocol result is secondary.
- Why limited
- This rests on one trial, of 21,442 people. Strength counts independent studies that agree, so one trial is limited at any size.
Infections and candida treated by mouth
Not studied in people. Mice with a body-wide candida infection survived more often when fed oregano oil, and in the lab it stopped candida growth.
No study in people. This rests on animal studies, lab studies.
The numbers and sources
- Effect
- In culture, oregano oil at 0.25 mg/mL completely inhibited Candida albicans growth. In mice with systemic candidiasis, oregano oil given by mouth daily for 30 days was followed by 80% survival with no candida in the kidneys, while mice fed olive oil alone died within 10 days [7]. A 2025 systematic review of lab studies of four oregano species found antifungal activity against Candida, Aspergillus and Penicillium species, linked to carvacrol, thymol and related compounds [8].
- Population
- Mice, 6 per group, and Candida albicans cultures [7]. Lab studies of fungal strains [8].
- Sources
- [7], [8]
- Caveats
- One small mouse study and lab work. Results in culture do not show what happens in the body. No trial of oral oregano oil for candida or other infections in people was located.
Therapies
Immune markers in blood
Mixed results: in 6 trials, manipulation did not change lymphocyte levels against sham, while some immune markers changed briefly in people without symptoms.
Compared with sham manipulation [13]; a lecture series [13]; blood draw only [13]
The numbers and sources
- Effect
- Spinal manipulation was not associated with changes in lymphocyte levels or physiological markers in people with low back pain or without symptoms. It was associated with short-term changes in selected immune biomarkers in people without symptoms. The reviewers found no clinical evidence that manipulation changes immune system outcomes [13].
- Population
- 529 participants in 6 high- and acceptable-quality trials [13].
- Sources
- [13]
- Caveats
- The reviewers call the clinical relevance of the short-term biomarker changes unknown [13].
Flu and flu-like illness, Oscillococcinum
Mixed results: Oscillococcinum did not prevent flu-like illness more than placebo, and two low-quality trials found slightly more symptom relief at 48 hours.
Compared with placebo [10]
The numbers and sources
- Effect
- For prevention, flu-like illness did not differ from placebo (RR 0.48, 95% CI 0.17 to 1.34, 2 trials). For treatment, symptom relief at 48 hours was 7.7 percentage points more common than with placebo (RR 1.86, 95% CI 1.27 to 2.73), smaller at 3 days (RR 1.27), and not different at 4 days (RR 1.11) or 5 days (RR 1.06). The reviewers found insufficient good evidence for robust conclusions [10].
- Population
- 6 trials: 327 young to middle-aged adults in two prevention trials in Russia, and 1,196 teenagers and adults in four treatment trials in France and Germany [10].
- Sources
- [10]
- Caveats
- Trial reporting was poor and many methods had unclear risk of bias. Only 2 treatment trials could be fully analyzed [10].
Stress hormones, blood pressure and immune cells after one session
Mixed results: a single session lowered blood pressure and heart rate in pooled trials, and cortisol fell in one small trial but not in pooled trials.
Compared with control conditions in randomized studies the source abstract does not describe [12]; light touch [18]
The numbers and sources
- Effect
- Single sessions reduced state anxiety, blood pressure and heart rate, and did not reduce cortisol [12]. In healthy adults, 45 minutes of Swedish massage produced a large drop in arginine vasopressin and a small drop in blood cortisol against light touch. Circulating lymphocytes rose (effect sizes 0.14 to 0.43) and stimulated cytokine production fell [18].
- Population
- 37 randomized studies [12]. Healthy adults aged 18 to 45 [18].
- Sources
- [12], [18]
- Caveats
- The trial calls its data preliminary, and these were changes over one session [18]. Lymphocyte counts in blood do not show fewer infections.
Colds and respiratory infections in children
Not shown to prevent or cure respiratory infections in children. Every trial at low risk of bias found no benefit over placebo.
Compared with identical placebo or self-selected conventional treatment [11]
The numbers and sources
- Effect
- Non-individualized products had little preventive effect (OR 1.14, 95% CI 0.83 to 1.57). Individualized prevention had little effect on antibiotic use (OR 0.79, 95% CI 0.35 to 1.76). Short-term cure (OR 1.31 favoring placebo, 95% CI 0.09 to 19.54) and long-term cure (OR 0.99) were too uncertain to judge. All studies at low risk of bias showed no benefit, while trials at uncertain or high risk of bias reported benefits [11].
- Population
- 8 trials with 1,562 children aged 0 to 16 with upper respiratory tract infections [11].
- Sources
- [11]
- Caveats
- Evidence was low or very low in quality, with high attrition and selective reporting. Three trials were funded by homeopathy manufacturers [11].
Preventing infections or boosting immunity
Not studied in people. A systematic review found no clinical study of spinal manipulation preventing infectious disease or improving its outcomes.
No study in people. This rests on how it acts in the body.
The numbers and sources
- Effect
- Reports posted by a chiropractic trade association in March 2020 claimed adjustments affect the immune system. More than 150 researchers from 11 countries reviewed the reports and found no valid clinical evidence for the claim [14]. A systematic review searching to April 2020 found no clinical studies of manipulation for preventing infectious disease or improving outcomes in people with one [13].
- Population
- No study population. Two association reports reviewed [14], and 13 studies screened [13].
- Sources
- [14], [13]
- Caveats
- The claim rests on a proposed link between the spine, the nervous system and immunity, not on any trial in people [14].
Immunity in healthy people
Not studied in people as a drip. In experimental sepsis, high-dose IV vitamin C acted on immune signaling, which no trial in healthy people has tested.
No study in people. This rests on how it acts in the body.
The numbers and sources
- Effect
- A systematic review describes proposed antioxidant and immune-modulating actions of high-dose IV vitamin C, including down-regulation of inflammatory STAT1 and PD-L1 pathways in experimental sepsis. Its clinical evidence came from sepsis and cancer care [9]. No trial located tested a drip for preventing infections in healthy people.
- Population
- No study population. A review of mechanistic and clinical studies [9].
- Sources
- [9]
- Caveats
- Mechanisms seen in experimental sepsis do not show what a drip does for a healthy immune system.
Practices
Sickness absence and immune markers
One trial of 3,018 adults found 29% less sickness absence with a daily cold shower finish for 30 days. Days of illness did not differ.
The numbers and sources
- Effect
- Adults randomized to end a daily shower with 30, 60 or 90 seconds of cold water had 29% less sickness absence from work than controls (incident rate ratio 0.71, P = 0.003). Illness days showed no significant group effect [17]. In 60 adults randomized to daily cold or hot showers for 90 days, the cold group showed higher immunoglobulin levels and higher interleukin-2 and interleukin-4 at 90 days, with no difference in interferon-gamma or TNF-alpha [18]. Pooled trials found no significant effect on immune function immediately or 1 hour after immersion [14].
- Population
- 3,018 Dutch adults aged 18 to 65 without severe comorbidity [17]. 60 healthy Egyptian adults [18]. Healthy adults in 11 trials [14].
- Sources
- [17], [18], [14]
- Caveats
- Sickness absence was self-reported by people who knew their group, and people felt ill as often in both groups [17]. Blood immune markers are not infections avoided [18].
- Why limited
- This rests on one trial, of 3,018 people. Strength counts independent studies that agree, so one trial is limited at any size.
Pneumonia and respiratory disease
Linked to fewer hospital diagnoses of pneumonia and other respiratory disease in Finnish men who used a sauna 2 or more times a week.
The numbers and sources
- Effect
- Over 25.6 years, 379 of 1,935 men had a hospital diagnosis of chronic obstructive pulmonary disease, asthma or pneumonia. Against one session a week or fewer, hazard ratios were 0.73 (95% CI 0.58 to 0.92) at 2 to 3 sessions and 0.59 (95% CI 0.37 to 0.94) at 4 or more [17]. For pneumonia alone, in 2,210 men with 375 cases, the fully adjusted hazard ratios were 0.72 (95% CI 0.57 to 0.90) and 0.63 (95% CI 0.39 to 1.00) [18].
- Population
- Finnish men aged 42 to 61 [17][18].
- Sources
- [17], [18]
- Caveats
- Observational, and both analyses use the same cohort. Only men were studied.
Common colds
One trial found fewer colds over 6 months in 25 people who took regular saunas than in 25 who did not. Duration and severity did not differ.
The numbers and sources
- Effect
- The sauna group had significantly fewer episodes of common cold over 6 months. In the last 3 months the incidence was roughly half that of controls. Mean duration and severity of colds did not differ between groups [19].
- Population
- 50 volunteers, 25 taking regular saunas and 25 abstaining, followed for 6 months [19].
- Sources
- [19]
- Caveats
- One small trial from 1990. The abstract gives no exact counts and does not say that allocation was randomized or that anyone was blinded.
Oral health, energy and immunity
Not studied in people. Social media videos credit mouth taping with better sleep and oral health, and with more energy and immunity.
No study in people. This rests on what users report.
The numbers and sources
- Effect
- A review of TikTok videos about mouth taping found the most common proposed advantages were improved sleep and oral health. The same review reports claims of increased energy, immunity and dental health, and found few of them had been evaluated [5].
- Population
- The first 50 TikTok videos meeting the review's criteria [5].
- Sources
- [5]
- Caveats
- Claims in videos are not measurements. No study located tested mouth taping for oral health, dental health, energy or immunity.
Daily life
Colds and vaccine response
Limited evidence: sleeping under 6 hours was linked to more colds after viral exposure in one study and to weaker vaccine antibody responses when sleep was measured objectively.
Compared with people sleeping more than 7 hours a night [16]; people not sleeping under 6 hours a night [17]
The numbers and sources
- Effect
- After rhinovirus nasal drops, adults whose wrist-measured sleep was under 5 hours had higher odds of a clinical cold (OR 4.50, 95% CI 1.08 to 18.69), as did those sleeping 5 to 6 hours (OR 4.24). Those sleeping 6.01 to 7 hours did not (OR 1.66, 95% CI 0.40 to 6.95) [16]. Objectively measured short sleep was linked to a lower antibody response to vaccination (effect size 0.79, 95% CI 0.40 to 1.18). Self-reported short sleep was not (0.29, 95% CI -0.04 to 0.63). The link was significant in men and not in women [17].
- Population
- 164 healthy adults aged 18 to 55, quarantined [16]. 304 adults with objective sleep measures and 504 with self-reported sleep [17].
- Sources
- [16], [17]
- Caveats
- Sleep was observed, not assigned, so other differences between short and long sleepers may explain the results. Intervals were wide [16]. Antibody levels are not infections prevented.
Food and drink
Respiratory infections
Shown to lower respiratory infections by about 19% with probiotic fermented dairy against placebo, in one meta-analysis of 22 trials across ages.
Compared with placebo [4]
The numbers and sources
- Effect
- Probiotic fermented dairy lowered respiratory tract infections overall (RR 0.81, 95% CI 0.74 to 0.89), in children (RR 0.82), adults (RR 0.81, 95% CI 0.66 to 1.00) and older people (RR 0.78). Upper respiratory infections fell (RR 0.83), the common cold fell (RR 0.68, 95% CI 0.49 to 0.96), and lower respiratory infections fell marginally (RR 0.78, 95% CI 0.60 to 1.01) [4].
- Population
- 10,190 participants in 22 randomized trials [4].
- Sources
- [4]
- Caveats
- The products were dairy foods with added probiotic strains, so this may not apply to other fermented foods. The adult estimate's interval reaches 1.00.
Lab markers
Viral infections, blood cancers and death after a low count on a routine test
Limited evidence: one Danish primary care cohort linked low counts to more viral infections, blood cancers and deaths, more strongly the lower the count.
The numbers and sources
- Effect
- Neutropenia, defined as below 1,500 cells/µL, was seen in about 1% of people. Odds ratios for viral infections were 2.32 (95% CI 1.84 to 2.91) at 1,500 to 1,800, 2.80 (2.20 to 3.57) at 1,000 to 1,500 and 4.77 (3.22 to 7.07) below 1,000. For blood cancers they were 3.23 (2.35 to 4.45), 8.69 (6.58 to 11.47) and 46.03 (33.98 to 62.35). Neutropenia was particularly associated with HIV, acute leukemias and myelodysplastic syndromes, and not with autoimmune disorders or solid cancers. For severe neutropenia the authors report absolute risks of 40% for blood cancers and above 50% for death from any cause over 4 years [14].
- Population
- More than 370,000 people with a routine blood count in Danish primary care, linked to national health registers and followed for 4 years [14].
- Sources
- [14]
- Caveats
- Observational, in people whose doctor had ordered a blood count. A low count can be the first sign of an illness already present, so the count marks the risk and need not cause it [14].
Pneumonia in older adults in nursing homes
Limited evidence: one study of Boston nursing home residents linked serum zinc of 70 µg/dL or above to fewer and shorter pneumonia episodes.
The numbers and sources
- Effect
- Compared with residents whose serum zinc was below 70 µg/dL, those with final concentrations of 70 µg/dL or above had a lower incidence of pneumonia, almost 50% fewer new antibiotic prescriptions, a shorter duration of pneumonia and fewer days of antibiotic use (2.6 against 3.9 days), with P at or below 0.004 for all. Baseline serum zinc of 70 µg/dL or above was associated with lower all-cause mortality (P = 0.049) [6].
- Population
- Residents of 33 nursing homes in Boston who took part in a 1-year vitamin E trial, 578 with baseline and 420 with final serum zinc. All received daily essential vitamins and minerals, including zinc, at 50% of the US dietary allowance (RDA) [6].
- Sources
- [6]
- Caveats
- An observational analysis within a trial of another nutrient. Infection itself lowers serum zinc [1], so low zinc may mark illness and need not cause it.
Infections in people with the Duffy-null genotype
Limited evidence: one study of two population samples found the Duffy-null genotype lowered neutrophil counts and was not linked to more infections.
The numbers and sources
- Effect
- In UK Biobank participants of Black African or Caribbean ethnicity, the mean count was 2,820 cells/µL with the Duffy-null genotype and 4,430 without it. About 21% of people with the genotype had a count below 2,000, against 1.1% of the others. The genotype was not associated with a higher risk of viral or bacterial infection in UK Biobank or in a Danish sample. Among people with the genotype, 17.4% of those with a count of 1,500 to 2,000 had an infection against 18.8% of those with a count of 2,000 to 7,500 (RR 0.94, 95% CI 0.81 to 1.10) [15].
- Population
- UK Biobank, including 7,450 people with the Duffy-null genotype, and the Danish iPSYCH case-cohort sample [15].
- Sources
- [15]
- Caveats
- Observational. Only 323 people with the genotype had a count of 1,000 to 1,500, and the authors state that the small numbers below 1,500 preclude firm conclusions about infection risk at those counts [15].