Can you get too much selenium?

The Tolerable Upper Intake Level for selenium is 400 mcg a day for adults.

What this means for you

Yes, and selenium is the nutrient where ordinary food can actually do it. The ceiling is 400 micrograms a day. One brazil nut carries 68 to 91, so four nuts a day sits at the requirement several times over and a generous handful can pass the limit.

Not for everyone. The signs of long-term excess are specific enough to recognise. Hair falling out, nails turning brittle or coming away, a garlic smell on the breath, with skin rash, nausea, diarrhoea, fatigue and irritability also reported. They come on slowly and they reverse when intake drops.

Harm starts well below the ceiling

Four hundred micrograms is the legal top. Something measurable happens at half of it. Pooling 27 randomised trials into a single dose-response curve, from 200 micrograms a day upwards HDL and LDL cholesterol fell and triglycerides rose, and the same pattern appeared once blood selenium went past roughly 150 micrograms per litre. The shift was largest in healthy participants and in trials lasting more than three months, which is to say in exactly the people who take selenium because they feel fine and intend to keep taking it.

The curve was U-shaped. People who began with very little selenium did better on supplements. People who already had enough did worse. That is the whole argument on this page in one sentence.

The skin cancer result

In the supplementation trials pooled for skin cancer, people given selenium had 25 per cent more squamous cell carcinoma than people given placebo, a risk ratio of 1.25 running from 1.04 to 1.49. Basal cell carcinoma did not go up.

Unsettled. The rest of that same analysis does not agree with itself. The melanoma estimate ran from halving the risk to almost tripling it, which is a polite way of saying nobody knows. The observational studies in it pointed the opposite way from the trials for non-melanoma skin cancer. So there is one precise trial finding of harm, one melanoma estimate too wide to use, and observational data disagreeing with both. Nobody should tell you this is settled. What it does do is take selenium capsules out of the harmless category.

The supplement question has a clearer answer than most on this site. Selenium was sold for years on the idea that it prevents cancer, and the trials that tested that found nothing, with high-certainty evidence.

Not for everyone. The same trials found harms. The largest of them reported more hair loss and skin inflammation on selenium, more high-grade prostate cancer in men who already had plenty of it, and randomised trials overall showed a slightly higher risk of type 2 diabetes. That is an unusual combination: no benefit and a measurable cost.

None of this applies to the selenium in an ordinary diet. It applies to capsules, and to brazil nuts eaten as though they were peanuts.

Worth holding both ends in view. Too little selenium rarely causes obvious illness on its own, and where the soil carries almost none it produced Keshan disease, a heart muscle disease, in parts of China. Too much produces the hair and nail signs above. The gap between those two ends is wide for most people and narrow for anyone eating brazil nuts daily.

There is a second harm with better evidence behind it than the hair and nails. Pooling five randomised trials, 200 micrograms of selenium a day raised the risk of type 2 diabetes by 11 per cent. Observational data point the same way: people with blood selenium around 140 micrograms per litre had roughly three and a half times the diabetes risk of people below 45.

Against that, a trial that deliberately gave men 1,600 and 3,200 micrograms a day for about a year found more side effects at the higher dose and no serious toxicity, with liver, kidney and blood tests normal in both groups. So the ceiling is not a cliff. It is the point past which you are taking a risk with no benefit on the other side of the ledger.

Europe moved the ceiling down

Established. In 2023 Europe re-derived its upper level for selenium and set it at 255 micrograms a day for adults, including pregnant and breastfeeding women. The American figure this page has used is 400. The gap comes from what each regulator picked as the first sign of trouble. The American limit was built on selenosis, the brittle hair and nails described in a Chinese population. The European one was built on hair loss alone, chosen as the earliest change anyone can observe. Neither number marks a line where harm begins for one person, and a page quoting one of them without naming whose it is has told you less than it appears to.

Put the food numbers beside that and the distance closes fast. One Brazil nut of 5 grams carries about 96 micrograms. A 28 gram handful carries 537, which clears both ceilings on its own. A cup carries 2,550, more than six times the American limit.

The radium, since somebody always asks

Brazil nuts take natural radium up out of the soil, and the trial that fed people 15 grams of Brazil nut butter a day measured what that adds. Over the 14 days of the trial it came to 6.34 microsieverts a year. Kept up for a full year it works out at 165 microsieverts, against an average of about 300 microsieverts a year from all other food and roughly 2,400 from natural background. The authors called the exposure very low. It is a genuine addition and a small one. The reason to be careful with Brazil nuts is the selenium.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Upper limits apply to everything you take in, from supplements and, for most nutrients, from food as well. Reaching one from whole food alone is uncommon. Supplements are where people cross the line, usually without meaning to.

Full guide: Selenium: a deficiency that kills and a supplement that does not help