Selenium Dosage: How Much to Take (and Why More Is Risky)
Quick answer
The RDA is just 55 mcg/day, and most people already get enough from food — so many don't need to supplement at all. The tolerable upper limit is only 400 mcg, an unusually narrow window, and both too little and too much are harmful (a U-shaped curve). If you do supplement, label doses commonly run 100–200 mcg — but don't stack a multivitamin, a selenium pill, and Brazil nuts.
Informational summary of published research — not medical advice. Selenium has a narrow safe window; don't stack multiple sources, and check with your doctor before taking 200 mcg long-term.
More is not better — selenium is U-shaped. Too little is a problem, and higher-end intake is linked to a higher rate of type 2 diabetes and offers no proven wellness benefit (Rayman 2012). A 200 mcg/day dose significantly raised diabetes incidence in one trial (Stranges 2007), and the definitive prostate-cancer prevention trial (SELECT) found no benefit. Selenium is a nutrient, not a booster.
How much selenium are you actually getting?
Overdose almost always comes from stacking. Add up your sources and check against the 400 mcg upper limit.
The Brazil-nut wildcard
Brazil nuts are the richest food source of selenium, and "two Brazil nuts ≈ the RDA" is a genuine trial-derived shorthand — in one study, 2 nuts/day raised selenium about as much as a 100 mcg supplement (Thomson 2008). But the selenium content varies over a hundredfold depending on where they grew (Chang 1995), so a handful from a high-selenium lot can quietly deliver a supplement-strength dose. If you eat them for selenium, 1–2 a day is plenty — and don't also take a selenium pill.
What the research shows
- Cancer: the SELECT trial found no benefit, and a Cochrane review of 27,000+ RCT participants agrees (Vinceti 2018).
- Everything else: research on selenium supplements is small and mixed, and this page does not recommend selenium for any condition. Talk to your doctor before starting.
If you have a reason to supplement
200 mcg — only if your doctor directs it. Most people should get selenium from food, not a pill.
See the best selenium ranking — but for most people, two Brazil nuts is the better answer.
Frequently asked questions
How much selenium should I take per day?
RDA is just 55 mcg; most people get enough from food and don't need to supplement. Label doses commonly run 100–200 mcg — but the upper limit is only 400 mcg, and both too little and too much are harmful (U-shaped). More isn't better. Talk to your doctor before starting.
Can you take too much selenium?
Yes, easily. UL is 400 mcg (~7× RDA); excess causes selenosis (hair/nail loss, garlic breath, nerve damage). 200 mcg is already halfway before food, and was linked to a higher rate of type 2 diabetes in one trial. The classic overdose is stacking a multi + selenium pill + Brazil nuts.
How many Brazil nuts equal a dose?
~2 nuts ≈ the RDA (a real rule of thumb), but selenium content varies over 100-fold by origin, so a handful from a high-selenium batch can be a big dose. 1–2/day is plenty; don't add a supplement on top.
Is selenium a treatment for any condition?
No. Selenium is a nutrient, not a treatment. The large SELECT trial found no cancer-prevention benefit, and research on selenium and thyroid measures is small and mixed. Talk to your doctor before taking it for any reason.
Who actually needs it?
Few in well-fed populations — mainly people in low-soil regions, on IV nutrition, or with malabsorption. Most people meet the RDA from food; don't add a standalone pill without a reason, and talk to your doctor first.
Related
Sources
- Rayman MP. "Selenium and human health." Lancet. 2012. PMID: 22381456
- Stranges S, et al. "Effects of long-term selenium supplementation on diabetes incidence." Ann Intern Med. 2007. PMID: 17620655
- Lippman SM, et al. "Effect of selenium and vitamin E on prostate cancer (SELECT)." JAMA. 2009. PMID: 19066370
- MacFarquhar JK, et al. "Acute selenium toxicity associated with a dietary supplement." Arch Intern Med. 2010. PMID: 20142570
- Chang JC, et al. "Selenium content of Brazil nuts from two geographic regions." Chemosphere. 1995. PMID: 7889353
- Thomson CD, et al. "Brazil nuts: an effective way to improve selenium status." Am J Clin Nutr. 2008. PMID: 18258628
- Vinceti M, et al. "Selenium for preventing cancer." Cochrane. 2018. PMID: 29376219