Quick answer
Selenium causes problems when you get too much of it over time, usually from supplements or Brazil nuts. Early signs are garlic breath and a metallic taste; hair loss and brittle nails are the most common. The adult upper limit is 400 mcg a day from all sources, and Europe's food safety agency set 255 mcg.1,2,3
Key facts
- Organic and inorganic forms of selenium have similar effects when intakes stay too high for a long time.2
- In 2008, a liquid supplement with 200 times its labeled amount of selenium caused severe reactions in 201 people.2
- Europe's 255 mcg limit is based on hair loss seen at 330 mcg a day in a large trial.3
- NIH names cisplatin, a chemotherapy medicine, as one that can lower selenium levels.1
Signs of too much selenium
Getting too much selenium over time (selenosis) can cause garlic breath, a metallic taste in the mouth, hair loss, brittle nails or nail loss, skin rash, nausea, diarrhea, extreme tiredness, irritability and nervous system problems.1,2
Garlic breath and a metallic taste are the earliest signs of too much selenium; hair loss and brittle or lost nails are the most common.2
Selenosis comes from high intake kept up over time. A single very large dose is a different problem, acute toxicity, described below.2
Very high doses
Very high amounts can cause severe stomach and nervous system problems, difficulty breathing, tremors, lightheadedness, flushing, muscle tenderness, kidney or heart failure, heart attacks and, in rare cases, death.1,2
In 2008, 201 people had severe reactions to a liquid dietary supplement that contained 200 times the amount of selenium on its label.2
Cases like these have come from products that held far more selenium than their labels said. Keep selenium supplements away from children, whose upper limits are much lower than an adult's.1,2
Upper limits
The upper limit for selenium is 400 mcg a day for adults and teens 14 and older, including during pregnancy and breastfeeding, counting food, drinks and supplements together. It is lower for children: 90 mcg at ages 1 to 3, 150 mcg at 4 to 8 and 280 mcg at 9 to 13. The limits do not apply when a doctor is giving selenium.1,2
| Who set it | Adult limit | Based on |
|---|---|---|
| US Food and Nutrition Board | 400 mcg | Hair and nail brittleness and loss |
| European Food Safety Authority, 2023 | 255 mcg, including pregnancy and breastfeeding | Hair loss seen at 330 mcg a day in a large trial |
The European agency judged that adults are unlikely to pass its limit unless they take high-dose supplements or eat Brazil nuts regularly. Average US intake from food, 108 mcg a day, leaves room for a multivitamin under the European figure, but not for a 200 mcg product.2,3
What long trials of 200 mcg a day found
Most long-term selenium trials gave 200 mcg a day, nearly four times the adult recommended amount, for years. Two findings from them matter for safety.2
In one trial, adults who took 200 mcg a day for an average of 7.7 years developed type 2 diabetes more often than those on placebo (58 people against 39), most of all those whose blood selenium was already high. A pooled analysis of four trials with 20,294 people found a 9% higher risk that was not statistically significant.4,5
In SELECT, a trial in 35,533 men aged 50 and over, 200 mcg a day did not lower the risk of prostate cancer. In men whose selenium levels were already high, supplements were linked with more high-grade prostate cancer, and the researchers advised men over 55 to avoid selenium supplements above recommended intakes.2
Medicines
Ask your pharmacist or doctor first
Cisplatin, a chemotherapy medicine used for several cancers, can lower selenium levels in the blood and hair, and it is not known whether that matters for health. NIH reports that studies of selenium taken during cisplatin treatment are small, and that a Cochrane review found the evidence insufficient to judge them. Ask your cancer care team before taking selenium during chemotherapy.1,2
Reported in studies
NIH gives cisplatin as one example and says selenium may interact with other medicines too. Tell your doctor and pharmacist about every supplement you take, including multivitamins that list selenium.1
Who should ask first
Questions people ask
Is selenium safe to take daily?
The body needs selenium every day, and 55 mcg is the adult recommended amount. A daily supplement adds to what food already supplies, about 108 mcg on average in the US, so count both against the 400 mcg upper limit.1,2
Can selenium harm the kidneys?
NIH lists kidney failure among the effects of very high, toxic amounts of selenium. If you have kidney disease or are on dialysis, ask your doctor how much selenium is right for you, because dialysis removes some selenium from the blood.1,2
Are the side effects different for men and women?
The signs of too much are the same. The upper limits are the same for both, including during pregnancy and breastfeeding. Long-term trials were mostly in men, and the SELECT researchers advised men over 55 to avoid selenium supplements above recommended intakes.1,2
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Sources
- Office of Dietary Supplements, National Institutes of Health. Selenium: Fact Sheet for Consumers. Updated June 18, 2025. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Selenium: Fact Sheet for Health Professionals. Updated September 4, 2025. Opened 2026-10-07.
- EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA), Turck D, et al. Scientific opinion on the tolerable upper intake level for selenium. EFSA J. 2023;21(1):e07704. Opened 2026-10-07.
- Stranges S, et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med. 2007;147(4):217-223. Opened 2026-10-07.
- Mao S, Zhang A, Huang S. Selenium supplementation and the risk of type 2 diabetes mellitus: a meta-analysis of randomized controlled trials. Endocrine. 2014;47(3):758-763. Opened 2026-10-07.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.