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Research · Supplements

Selenium

In trials with 19,475 people, selenium did not change cancer incidence. Heart events and mortality did not change either. In Hashimoto's thyroiditis it lowered thyroid antibodies, and reviews disagree on TSH. Selenium improved mild Graves' eye disease in one trial. A US trial found more type 2 diabetes on 200 µg a day.

Add to Bio HackChecked 2026-11-09
Status
Dietary supplement in the US.
Also called
selenomethionine, L-selenomethionine, sodium selenite, selenium yeast, selenized yeast, Se, Brazil nuts
Taken for
cancer prevention, thyroid health, Hashimoto's thyroiditis, Graves' eye disease, heart health, antioxidant, immunity, fertility, lymphedema

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 help2

Reported risks3

What people take it for, and what was found

Shown to help2

Limited

Mild Graves' eye disease

One trial found better quality of life and slower eye disease progression over 6 months in people with mild Graves' eye disease.

The numbers and sources
Effect
After 6 months, 100 µg twice a day improved quality of life, reduced eye involvement and slowed progression against placebo. The Clinical Activity Score fell more on selenium. Results held at 12 months [8].
Population
159 people with mild Graves' orbitopathy [8].
Sources
[8]
Caveats
One trial. The abstract gives P values without effect sizes. Funded by the University of Pisa and the Italian ministry for education and research [8].
Limited

Lymphedema

Small trials found less lymphedema and fewer skin infections in people after cancer treatment, two of them giving selenium by vein.

The numbers and sources
Effect
A Cochrane review found one RCT of selenium for secondary lymphedema, with 60 participants at the start. It reported fewer recurrent erysipelas infections on selenium than on placebo [9]. In 26 women with stage II to III lymphedema after breast cancer, five intravenous doses of 500 µg sodium selenite within 2 weeks shifted 83.3% from stage III to II at follow-up, against 10.0% on saline (p 0.002), and lowered the extracellular water ratio [10]. In 20 patients after oral cancer surgery, 1,000 µg sodium selenite a day by mouth or vein for 3 weeks reduced postoperative lymphedema [11]. A 2022 systematic review of lymphedema drug trials found one small RCT suggesting selenium might help [12].
Population
Cancer patients with secondary lymphedema [9]. 26 women with breast cancer-related lymphedema [10]. 20 patients after oral tumor surgery [11].
Sources
[9], [10], [11], [12]
Caveats
The Cochrane authors note drawbacks in study quality and reporting, and that the result cannot be generalized [9]. All trials are small, and two gave sodium selenite intravenously [10][11].

Mixed results1

Moderate

Hashimoto's thyroiditis

Mixed results: thyroid antibodies fell in people with Hashimoto's in both reviews, while reviews disagree on whether TSH changed.

The numbers and sources
Effect
Thyroid peroxidase antibodies fell in both reviews. In people on levothyroxine, the weighted mean difference after 3 months was -271 (95% CI -366 to -175) [5]. Across 29 cohorts, the SMD was -0.96 (95% CI -1.36 to -0.56) [6]. On TSH, sources disagree. In people not on thyroid hormone, a 2017 review found no change in TSH, quality of life or ultrasound findings [7]. A 2024 review found TSH lower (SMD -0.21, 95% CI -0.43 to -0.02, 869 people) [6].
Population
Adults with chronic autoimmune thyroiditis, 16 trials [5], 9 trials [7] and 35 studies [6].
Sources
[5], [6], [7]
Caveats
Antibody levels are a lab marker. The 2016 review rated evidence quality low [5], and the 2024 review moderate, with I² of 90% for antibodies [6].

Not shown to help2

Strong

Cancer prevention

Not shown to lower cancer rates, including prostate, colorectal, lung and bladder cancer, in large trials. Low-bias trials suggested more melanoma.

The numbers and sources
Effect
In low-bias RCTs, selenium did not change cancer incidence (RR 1.01, 95% CI 0.93 to 1.10, 3 studies, 19,475 people). Prostate (RR 1.01), colorectal (RR 0.99), lung (RR 1.16) and bladder cancer did not differ either. Low-bias trials suggested more melanoma [1]. In SELECT, prostate cancer did not significantly differ on selenium (HR 1.09, 99% CI 0.93 to 1.27) [2].
Population
10 RCTs with 27,232 participants [1]; 34,887 men in SELECT [2].
Sources
[1], [2]
Caveats
The Cochrane authors graded the evidence high certainty for most sites. In SELECT, men with the highest baseline selenium had more high-grade prostate cancer on supplements [1].
Moderate

Heart attack, stroke and mortality

Not shown to lower death, heart attack, stroke or other cardiovascular events in people, with events mostly from two US trials.

The numbers and sources
Effect
Selenium did not change death from any cause (RR 0.97, 95% CI 0.88 to 1.08), cardiovascular death (RR 0.97, 95% CI 0.79 to 1.2) or all cardiovascular events (RR 1.03, 95% CI 0.95 to 1.11) [3]. A Cochrane review of antioxidants found no effect of selenium on mortality (RR 0.97, 95% CI 0.91 to 1.03, 17 trials) [4].
Population
12 trials with 19,715 people, mostly the US SELECT and NPC trials [3]; 17 low-bias selenium trials [4].
Sources
[3], [4]
Caveats
Clinical events came mostly from two US trials [3].

Doses studied

SELECT and the NPC trial used 200 µg a day [2][13]. The Graves' trial used 100 µg twice a day [8]. Lymphedema trials used 500 µg sodium selenite intravenously five times over 2 weeks [10] and 1,000 µg a day by mouth or vein for 3 weeks [11]. In a poisoning outbreak from a mislabeled product, the median dose consumed was 41,749 µg a day [14]. EFSA sets a tolerable upper intake level of 255 µg a day for adults, including during pregnancy and lactation [15].

Risks

Reported risks2

LimitedStudied in menHow often is not known

Hair loss and dermatitis

One trial found hair loss and mild dermatitis more common on selenium in men.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
In SELECT, alopecia (RR 1.28, 95% CI 1.01 to 1.62) and grade 1 to 2 dermatitis (RR 1.17, 95% CI 1.0 to 1.35) were more common on selenium [3].
Population
Men in SELECT [3].
Sources
[3]
Caveats
One trial.
LimitedHow often is not known

Poisoning from a mislabeled supplement

Case reports: 201 people who took a supplement with 200 times its labeled selenium had diarrhea, fatigue, hair loss and nail changes.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
A liquid supplement contained 200 times its labeled selenium. Of 201 cases in 10 states, diarrhea (78%), fatigue (75%), hair loss (72%), joint pain (70%) and nail changes (61%) were common. At 90 days or more, 52% still had nail discoloration or loss and 29% hair loss [14].
Population
201 people in an outbreak investigation, 1 hospitalized [14].
Sources
[14]
Caveats
A case series from one product.

Mixed results1

LimitedHow often is not known

Type 2 diabetes

One trial found more type 2 diabetes on 200 µg a day, 12.6 against 8.4 cases per 1,000 person-years. Larger analyses found no significant rise.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Over 7.7 years, type 2 diabetes developed in 58 people on 200 µg a day and 39 on placebo (12.6 against 8.4 cases per 1,000 person-years, HR 1.55, 95% CI 1.03 to 2.33). Risk was highest in people with the highest baseline selenium (HR 2.70) [13]. In SELECT the increase was not significant (RR 1.07, 99% CI 0.94 to 1.22) [16], and in the Cochrane review it was 1.06 (95% CI 0.97 to 1.15) [3].
Population
1,202 people from US dermatology clinics in low-selenium areas [13]; 35,533 men in SELECT [16].
Sources
[3], [13], [16]
Caveats
Diabetes was a secondary outcome in the NPC trial, and diagnoses were self-reported [13].

Claims tested and not found

  1. That selenium prevents cancer. Low-bias trials found no change in cancer incidence or in any common site [1].
  2. That selenium protects the heart. Cardiovascular events and deaths did not differ from placebo [3].
  3. That selenium prevents type 2 diabetes. In the NPC trial, more people on selenium developed diabetes [13].

Sources

  1. Vinceti M, Filippini T, Del Giovane C, et al. Selenium for preventing cancer. Cochrane Database of Systematic Reviews. 2018. https://doi.org/10.1002/14651858.CD005195.pub4
  2. Klein EA, Thompson IM, Tangen CM, et al. Vitamin E and the Risk of Prostate Cancer. JAMA. 2011. https://doi.org/10.1001/jama.2011.1437
  3. Rees K, Hartley L, Day C, et al. Selenium supplementation for the primary prevention of cardiovascular disease. Cochrane Database of Systematic Reviews. 2013. https://doi.org/10.1002/14651858.CD009671.pub2
  4. Bjelakovic G, Nikolova D, Gluud LL, et al. Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database of Systematic Reviews. 2012. https://doi.org/10.1002/14651858.CD007176.pub2
  5. Wichman J, Winther KH, Bonnema SJ, et al. Selenium Supplementation Significantly Reduces Thyroid Autoantibody Levels in Patients with Chronic Autoimmune Thyroiditis: A Systematic Review and Meta-Analysis. Thyroid. 2016. https://doi.org/10.1089/thy.2016.0256
  6. Huwiler VV, Maissen-Abgottspon S, Stanga Z, et al. Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. Thyroid. 2024. https://doi.org/10.1089/thy.2023.0556
  7. Winther KH, Wichman JEM, Bonnema SJ, et al. Insufficient documentation for clinical efficacy of selenium supplementation in chronic autoimmune thyroiditis, based on a systematic review and meta-analysis. Endocrine. 2017. https://doi.org/10.1007/s12020-016-1098-z
  8. Marcocci C, Kahaly GJ, Krassas GE, et al. Selenium and the Course of Mild Graves' Orbitopathy. New England Journal of Medicine. 2011. https://doi.org/10.1056/NEJMoa1012985
  9. Dennert G, Horneber M. Selenium for alleviating the side effects of chemotherapy, radiotherapy and surgery in cancer patients. Cochrane Database of Systematic Reviews. 2006. https://doi.org/10.1002/14651858.CD005037.pub2
  10. Han HW, Yang EJ, Lee SM. Sodium Selenite Alleviates Breast Cancer-Related Lymphedema Independent of Antioxidant Defense System. Nutrients. 2019. https://doi.org/10.3390/nu11051021
  11. Zimmermann T, Leonhardt H, Kersting S, et al. Reduction of postoperative lymphedema after oral tumor surgery with sodium selenite. Biological Trace Element Research. 2005. https://doi.org/10.1385/BTER:106:3:193
  12. Walker J, Tanna S, Roake J, Lyons O. A systematic review of pharmacologic and cell-based therapies for treatment of lymphedema (2010-2021). Journal of Vascular Surgery: Venous and Lymphatic Disorders. 2022. https://doi.org/10.1016/j.jvsv.2021.09.004
  13. Stranges S, Marshall JR, Natarajan R, et al. Effects of Long-Term Selenium Supplementation on the Incidence of Type 2 Diabetes. Annals of Internal Medicine. 2007. https://doi.org/10.7326/0003-4819-147-4-200708210-00175
  14. MacFarquhar JK, Broussard DL, Melstrom P, et al. Acute Selenium Toxicity Associated With a Dietary Supplement. Archives of Internal Medicine. 2010. https://doi.org/10.1001/archinternmed.2009.495
  15. European Food Safety Authority. Overview on Tolerable Upper Intake Levels as derived by the Scientific Committee on Food (SCF) and the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Version 11. 2025. https://www.efsa.europa.eu/sites/default/files/assets/UL_Summary_tables.pdf
  16. Lippman SM, Klein EA, Goodman PJ, et al. Effect of Selenium and Vitamin E on Risk of Prostate Cancer and Other Cancers. JAMA. 2009. https://doi.org/10.1001/jama.2008.864

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