What does the research show about caffeine?

Caffeine

An umbrella review of 21 meta-analyses finds caffeine improves endurance, strength, muscular endurance, power and speed over placebo, with larger effects for endurance. Doses studied cluster at 3 to 6 mg per kg. Caffeine shortened sleep by 45 minutes in pooled trials and raised blood pressure about 2 mmHg. Pure powders have caused deaths.

Add to Bio HackChecked 2026-11-09

Status: Dietary supplement and food ingredient in the US. The FDA treats some pure or highly concentrated caffeine supplements as adulterated (2018 guidance).
Also called: caffeine anhydrous, 1,3,7-trimethylxanthine, pre-workout, caffeine pills, caffeine gum, energy drinks, guarana
Taken for: endurance, strength, power, focus, alertness, staying awake, fat loss, pre-workout energy

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

Shown to help5

StrongShown to improve endurance time-trial power by about 3% and completion time by about 2% in mostly young, trained men. Responses varied.
StrongShown to improve maximal strength and jump power slightly in mostly resistance-trained young men. Lower body strength alone did not reach significance.
StrongShown to improve attention, reaction time and driving lane control in sleep-deprived adults, compared with sleep loss without caffeine.
StrongShown to improve attention accuracy and reaction time slightly in rested adults, more at doses of 200 mg or more.
LimitedLimited evidence: across 13 trials, higher caffeine intake went with more weight and fat loss, with very high variation between trials.

Reported risks5

StrongShown to cut total sleep by about 35 to 45 minutes and reduce deep sleep in adults, with effects from doses taken hours before bed.How often is not known
ModerateShown to raise blood pressure by about 2 mmHg systolic and 2 mmHg diastolic in adults, more in men and at higher doses.How often is not known
ModerateShown to raise racing heart, anxiety, insomnia, gut upset and headache in athletes, with several rising with dose.How often is not known
ModerateShown to raise heart rate and blood pressure and lengthen QTc in most energy drink studies. Case reports describe arrhythmias in 39 adolescents.How often is not known
ModerateFDA states pure or highly concentrated caffeine products have been linked to at least two US deaths, and seizures can occur with rapid intake of about 1,200 mg.How often is not known

What people take it for, and what was found

Shown to help5

Strong

Endurance performance

Shown to improve endurance time-trial power by about 3% and completion time by about 2% in mostly young, trained men. Responses varied.

The numbers and sources

Effect: Across 46 placebo-controlled studies, caffeine improved time-trial mean power output by 3.03% (± 3.07, effect size 0.23) and completion time by 2.22% (± 2.59, effect size 0.41). Two studies reported slower times and five reported lower power on caffeine [1]. An umbrella review of 21 meta-analyses rated the endurance evidence moderate quality and found effects generally larger for aerobic than anaerobic exercise [2].
Population: Mostly young, trained men [1][2].
Sources: [1], [2], [3]
Caveats: Individual responses varied, including some slower performances. Most trials were in young men, and the umbrella review notes few data in women and older adults [2]. A 2026 meta-analysis of 19 trials in 235 women found a small cycling gain (SMD 0.21, 95% CI 0.11 to 0.29) [4]. In 17 studies, people given a placebo they believed was caffeine also performed better (d 0.49, 95% CI 0.28 to 0.69), on low-quality evidence [5]. Not every pooled estimate gave a definite direction across its 95% prediction interval [2]. Reference [3] is an ISSN position stand, a narrative consensus rather than a systematic review.

Strong

Muscle strength and power

Shown to improve maximal strength and jump power slightly in mostly resistance-trained young men. Lower body strength alone did not reach significance.

The numbers and sources

Effect: Across 10 studies, 1RM strength improved by SMD 0.20 (95% CI 0.03 to 0.36) and vertical jump power by SMD 0.17 (95% CI 0.00 to 0.34). Upper body strength improved (SMD 0.21, 95% CI 0.02 to 0.39); lower body did not reach significance (SMD 0.15, 95% CI -0.05 to 0.34) [6]. The umbrella review rated strength, muscular endurance and anaerobic power evidence moderate quality [2].
Population: Mostly resistance-trained young men [2][6].
Sources: [2], [6]
Caveats: Effects are small. The authors flag that blinding was rarely checked, and caffeine is often detectable by its feel [6].

Strong

Attention and alertness under sleep loss

Shown to improve attention, reaction time and driving lane control in sleep-deprived adults, compared with sleep loss without caffeine.

The numbers and sources

Effect: Across 45 publications in sleep-deprived or sleep-restricted people, caffeine improved attention response time (g 0.86), attention accuracy (g 0.68, 95% CI 0.48 to 0.88), executive function (g 0.35, 95% CI 0.15 to 0.55), reaction time (g 1.11, 95% CI 0.75 to 1.47) and driving lane control (g 1.67, 95% CI 1.32 to 2.02) [7]. In 7 placebo-controlled trials in military personnel under sleep loss, caffeine improved vigilance and response performance (SMD 0.73, 95% CI 0.50 to 0.96), complex cognitive performance (0.54, 95% CI 0.16 to 0.93) and military task performance (0.45, 95% CI 0.19 to 0.71) [8].
Population: Sleep-deprived or restricted adults, 327 effect estimates [7]. Military personnel in 7 randomized trials [8].
Sources: [3], [7], [8]
Caveats: The pooled effects are against a sleep-deprived baseline, not a rested one. The abstract's confidence interval for attention response time (0.53 to 0.83) does not contain its point estimate (0.86), which suggests a typo in the source.

Strong

Attention when rested

Shown to improve attention accuracy and reaction time slightly in rested adults, more at doses of 200 mg or more.

The numbers and sources

Effect: Across 31 trials (1,455 participants) in the general population, caffeine improved attention accuracy (g 0.27) and reaction time (g 0.28). Doses of 200 mg or more improved both more than lower doses. Reaction time kept improving with dose, while accuracy rose and then fell [9]. In 14 studies (250 participants) aged 20 or younger, caffeine improved cognitive task performance (SMD 0.41, 95% CI 0.28 to 0.55) [10].
Population: Adults from the general population [9]. Children, adolescents and young adults up to age 20 [10].
Sources: [9], [10]
Caveats: Effects were small. The youth meta-analysis rests on small-sample studies [10]. Habitual caffeine intake did not change the effect in either review [9][10].

Limited

Fat loss

Limited evidence: across 13 trials, higher caffeine intake went with more weight and fat loss, with very high variation between trials.

The numbers and sources

Effect: Across 13 RCTs (606 participants), the pooled dose-response coefficient favored reductions in weight (beta 0.29, 95% CI 0.19 to 0.40), BMI (beta 0.23) and fat mass (beta 0.36). The authors estimate each doubling of intake was associated with 22% more weight reduction, 17% more BMI reduction and 28% more fat mass reduction [11].
Population: Adults in 13 RCTs [11].
Sources: [11]
Caveats: Heterogeneity was 91% to 94% across outcomes. The method models effect per doubling of intake on a log scale rather than reporting kilograms lost against placebo.

Doses studied

The endurance meta-analysis found effects at 3 to 6 mg per kg body mass [1]. The ISSN position stand reports benefits at 3 to 6 mg per kg, possible effects as low as 2 mg per kg, a high incidence of side effects at around 9 mg per kg, and 60 minutes before exercise as the most common timing in trials [3]. For scale, the FDA cites 400 mg a day as an amount not generally associated with negative effects in most adults [12]. A pooled sleep analysis used a 217.5 mg standard pre-workout serve as a reference amount [13].

Risks

Reported risks5

StrongHow often is not known

Shorter and lighter sleep

Shown to cut total sleep by about 35 to 45 minutes and reduce deep sleep in adults, with effects from doses taken hours before bed.

The numbers and sources

How common: How often is not known: the sources give no rate.
Effect: Across 24 studies, caffeine reduced total sleep time by 45 minutes and sleep efficiency by 7%, lengthened sleep onset by 9 minutes and wake after sleep onset by 12 minutes, and reduced deep sleep by 11.4 minutes. The authors modeled that 107 mg of coffee caffeine 8.8 hours before bed, or 217.5 mg 13.2 hours before bed, was the point after which total sleep time fell [13]. Across 22 crossover trials (956 healthy adults) measured by polysomnography, caffeine cut total sleep time by 34.67 minutes, sleep efficiency by 4.74 points and slow-wave sleep by 1.01 points, and lengthened sleep onset by 8.35 minutes. REM sleep did not change [14]. In 10 studies of 128 athletes taking caffeine before late afternoon or evening training, sleep efficiency fell 4.87 points (95% CI 2.29 to 7.45), and athletes consistently reported worse sleep [15].
Population: Adults in 24 controlled studies [13]. Healthy adults in sleep lab crossovers [14]. Athletes [15].
Sources: [13], [14], [15]
Caveats: The cutoffs are model outputs from pooled data and vary with dose and individual metabolism [3]. In the athlete review, total sleep time fell 32.47 minutes but not significantly (95% CI -69.93 to 4.99), and the sleep efficiency result was not robust to removing single studies [15].

ModerateHow often is not known

Blood pressure

Shown to raise blood pressure by about 2 mmHg systolic and 2 mmHg diastolic in adults, more in men and at higher doses.

The numbers and sources

How common: How often is not known: the sources give no rate.
Effect: Across 8 RCTs (11 effect sizes), caffeine raised systolic pressure by 1.94 mmHg (95% CI 0.52 to 3.35) and diastolic by 1.66 mmHg (95% CI 0.75 to 2.57). Rises were larger in men, with dose, and in trials longer than 9 weeks; diastolic rose at doses above 400 mg a day [16].
Population: Adults 18 and older in RCTs [16].
Sources: [16]
Caveats: Only 8 trials. The ISSN position stand names anxiety and sleep disruption as the side effects that vary most between people [3].

ModerateHow often is not known

Headache, racing heart, gut upset and anxiety

Shown to raise racing heart, anxiety, insomnia, gut upset and headache in athletes, with several rising with dose.

The numbers and sources

How common: How often is not known: the sources give no rate.
Effect: Across 48 placebo-controlled trials in 940 athletes (mean dose 4.9 mg per kg), caffeine raised the odds of self-reported tachycardia (log odds ratio 1.47, more than 4 times the odds), anxiety (1.29), insomnia (1.20), abdominal discomfort (1.12), increased urine output (1.04), nervousness (0.82) and headache (0.74). Tachycardia, headache, abdominal discomfort and anxiety rose with dose [17]. A systematic review of 27 studies in healthy people found anxiety symptoms rose with caffeine dose [18].
Population: Athletes aged 15 and older, 63% male [17]. Healthy individuals [18].
Sources: [17], [18]
Caveats: Side effects were self-reported and often secondary outcomes [17]. The anxiety review's search ran to March 2021 and its statistical methods were not fully reported [18].

ModerateHow often is not known

Heart effects of energy drinks and high doses in teenagers

Shown to raise heart rate and blood pressure and lengthen QTc in most energy drink studies. Case reports describe arrhythmias in 39 adolescents.

The numbers and sources

How common: How often is not known: the sources give no rate.
Effect: In a systematic review of 37 studies (1,597 participants, mean age 22.6), energy drinks raised heart rate in 60.9% of studies, systolic pressure in 53.8%, diastolic pressure in 61.5%, and prolonged the QTc interval in 63.2% [19]. A review of 12 reports, 9 of them case reports, described 39 adolescents aged 13 to 18 with caffeine-associated arrhythmias or ECG changes after energy drinks, caffeine tablets or powder, ranging from sinus tachycardia to ventricular tachycardia and QTc prolongation [20].
Population: People aged 13 and older drinking energy drinks [19]. Adolescents [20].
Sources: [19], [20]
Caveats: Energy drinks contain ingredients besides caffeine [19]. The percentages are shares of studies, not of people [19]. Case reports cannot give a rate [20].

ModerateHow often is not known

Pure and highly concentrated caffeine powders

FDA states pure or highly concentrated caffeine products have been linked to at least two US deaths, and seizures can occur with rapid intake of about 1,200 mg.

The numbers and sources

How common: How often is not known: the sources give no rate.
Effect: The FDA states pure or highly concentrated caffeine products have been linked to at least two US deaths and treats some as adulterated [21]. It estimates seizures can occur with rapid intake of about 1,200 mg, less than half a teaspoon of pure caffeine [12]. A Maryland medical examiner series found 8 caffeine deaths from 1999 to 2009, average postmortem concentration 140.4 mg/L, none from caffeine-containing foods [22].
Population: US consumers [12][21]; 8 adult deaths [22].
Sources: [12], [21], [22]
Caveats: Case series and regulatory statements, not trials. Two of the 8 deaths were suicides and the others involved large amounts of tablets or other medications [22].

Claims tested and not found

That more caffeine gives more performance. The ISSN stand reports that around 9 mg per kg brings a high incidence of side effects without being required for the performance effect seen at 3 to 6 mg per kg [3].
That caffeine taken in the afternoon leaves sleep untouched. Pooled trials place the point of measurable sleep loss at 8.8 hours before bed for a cup of coffee [13].

Sources

Southward K, Rutherfurd-Markwick KJ, Ali A. The Effect of Acute Caffeine Ingestion on Endurance Performance: A Systematic Review and Meta-Analysis. Sports Medicine. 2018. https://doi.org/10.1007/s40279-018-0939-8
Grgic J, Grgic I, Pickering C, et al. Wake up and smell the coffee: caffeine supplementation and exercise performance, an umbrella review of 21 published meta-analyses. British Journal of Sports Medicine. 2019. https://doi.org/10.1136/bjsports-2018-100278
Guest NS, VanDusseldorp TA, Nelson MT, et al. International society of sports nutrition position stand: caffeine and exercise performance. Journal of the International Society of Sports Nutrition. 2021. https://doi.org/10.1186/s12970-020-00383-4
Bittencourt de Santa Ritta G, Brito Soares LM, Abreu ACP, et al. Addressing the Evidence Gap: Ergogenic Effects of Caffeine on Cycling Performance in Females. A Systematic Review and Three-Level Meta-Analysis. International Journal of Sport Nutrition and Exercise Metabolism. 2026. https://doi.org/10.1123/ijsnem.2026-0022
Hurst P, Hanssen J, Marticorena F, et al. Placebo effects of caffeine on sport performance: a systematic review and meta-analysis. Journal of the International Society of Sports Nutrition. 2026. https://doi.org/10.1080/15502783.2026.2747142
Grgic J, Trexler ET, Lazinica B, et al. Effects of caffeine intake on muscle strength and power: a systematic review and meta-analysis. Journal of the International Society of Sports Nutrition. 2018. https://doi.org/10.1186/s12970-018-0216-0
Irwin C, Khalesi S, Desbrow B, et al. Effects of acute caffeine consumption following sleep loss on cognitive, physical, occupational and driving performance: A systematic review and meta-analysis. Neuroscience and Biobehavioral Reviews. 2020. https://doi.org/10.1016/j.neubiorev.2019.12.008
Wang R, Wang W, Meng L, et al. Acute caffeine supplementation as a nutrition-based strategy to mitigate sleep-loss-related cognitive and operational performance impairments in military personnel: a systematic review and meta-analysis. Frontiers in Nutrition. 2026. https://doi.org/10.3389/fnut.2026.1893033
Kløve K, Petersen A. A systematic review and meta-analysis of the acute effect of caffeine on attention. Psychopharmacology. 2025. https://doi.org/10.1007/s00213-025-06775-1
Niu J, Li H, Wang Z, et al. Acute caffeine intervention and cognitive performance in populations aged 20 years and younger: a systematic review and meta-analysis. Frontiers in Nutrition. 2026. https://doi.org/10.3389/fnut.2026.1888658
Tabrizi R, Saneei P, Lankarani KB, et al. The effects of caffeine intake on weight loss: a systematic review and dos-response meta-analysis of randomized controlled trials. Critical Reviews in Food Science and Nutrition. 2018. https://doi.org/10.1080/10408398.2018.1507996
U.S. Food and Drug Administration. Spilling the Beans: How Much Caffeine is Too Much? Consumer Update. 2024. https://www.fda.gov/consumers/consumer-updates/spilling-beans-how-much-caffeine-too-much
Gardiner C, Weakley J, Burke LM, et al. The effect of caffeine on subsequent sleep: A systematic review and meta-analysis. Sleep Medicine Reviews. 2023. https://doi.org/10.1016/j.smrv.2023.101764
Chang YH, Cheng YC, Cheng WJ. Age- and dose-specific effects of caffeine on sleep: A meta-analysis of controlled crossover trials. Sleep Medicine. 2025. https://doi.org/10.1016/j.sleep.2025.106874
Kocak A, Georgousopoulou E, Knight-Agarwal CR, et al. The Effect of Consuming Caffeine Before Late Afternoon/Evening Training or Competition on Sleep: A Systematic Review with Meta-Analysis. Sports. 2025. https://doi.org/10.3390/sports13090317
Abbas-Hashemi SA, Hosseininasab D, Rastgoo S, et al. The effects of caffeine supplementation on blood pressure in adults: A systematic review and dose-response meta-analysis. Clinical Nutrition ESPEN. 2023. https://doi.org/10.1016/j.clnesp.2023.09.923
Negaresh R, Al-Riyami SAA, Ghahfarrokhi MM, et al. Caffeine Use in Sport: A Systematic Review and Meta-analysis of Acute Side Effects and Implications for Athlete Health and Safety. Sports Medicine. 2026. https://doi.org/10.1007/s40279-026-02441-4
Nascimento AO, Silva CEC, Silva ML, Cunha KC. The Effects of Caffeine on Anxiety Behavior in Healthy Individuals: A Systematic Review of the Literature. Stress and Health. 2026. https://doi.org/10.1002/smi.70139
Mandato J, Kola R, Tyson T, et al. The Effects of Energy Drinks on the Cardiovascular System: A Systematic Review. Current Cardiology Reports. 2025. https://doi.org/10.1007/s11886-025-02293-w
Oeffl N, Koestenberger M, Sallmon H, Kurath-Koller S. Caffeine and the Young Heart: A Systematic Review of Arrhythmogenic Risks in Children and Adolescents. Pediatric Cardiology. 2026. https://doi.org/10.1007/s00246-026-04348-3
U.S. Food and Drug Administration. Guidance for Industry: Highly Concentrated Caffeine in Dietary Supplements. 2018. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/guidance-industry-highly-concentrated-caffeine-dietary-supplements
Banerjee P, Ali Z, Levine B, et al. Fatal caffeine intoxication: a series of eight cases from 1999 to 2009. Journal of Forensic Sciences. 2014. https://doi.org/10.1111/1556-4029.12387

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