Question 1

How much selenium should I take per day?

Accepted Answer

The RDA is just 55 mcg/day for adults (60 in pregnancy, 70 while breastfeeding), and most people who eat a normal diet already get enough — so many people do not need to supplement at all. If you do, typical supplements provide 100–200 mcg, which is already a large share of the way to the tolerable upper limit of 400 mcg/day. Selenium has an unusually narrow window between enough and too much, and it follows a U-shaped curve where both too little and too much are harmful, so more is not better.

Question 2

Can you take too much selenium?

Accepted Answer

Yes, and it is easier than with most minerals. The upper limit is 400 mcg/day — only about 7 times the RDA — and chronic excess causes selenosis: hair loss, brittle nails, garlic breath, GI upset, and at higher exposure nerve damage. A supplement providing 200 mcg is already halfway to the limit before food and multivitamins are counted, and a 200 mcg/day dose was linked to increased type 2 diabetes risk in one trial. The classic overdose scenario is stacking: a multivitamin plus a separate selenium pill plus a few Brazil nuts can quietly cross the limit.

Question 3

How many Brazil nuts equal a selenium dose?

Accepted Answer

Roughly two Brazil nuts provide about the RDA of selenium — that is a real, trial-derived rule of thumb. But the catch is enormous variability: the selenium content of Brazil nuts varies more than a hundredfold depending on where they were grown, so a handful from a high-selenium batch can deliver a supplement-strength dose without you realizing it. If you eat Brazil nuts regularly for selenium, one or two a day is plenty — do not also take a selenium supplement on top.

Question 4

Does selenium help the thyroid or prevent cancer?

Accepted Answer

For the thyroid, selenium (usually 200 mcg) lowers thyroid antibody levels in Hashimoto's in meta-analyses, but that is a change in a lab number, not a proven improvement in symptoms or thyroid function — and it should be done with a doctor, not by over-supplementing. For cancer, the definitive large trial (SELECT) found selenium did not prevent prostate cancer, and reviews find no cancer-prevention benefit, so that once-popular use is not supported. In short, selenium treats deficiency; it is not a general wellness or anti-cancer booster.

Question 5

Who actually needs a selenium supplement?

Accepted Answer

Relatively few people in well-fed populations. Genuine need is mostly limited to people in low-selenium-soil regions, those on long-term IV nutrition, people with severe gut malabsorption, and some cases of Hashimoto's thyroiditis under medical guidance. Most people who eat a varied diet meet the RDA from food alone. If you are unsure, the safest move is not to add a standalone selenium supplement unless a blood test or a doctor indicates you need it.

Selenium Dosage: How Much to Take (and Why More Is Risky)

How much selenium are you actually getting?

The Brazil-nut wildcard

By goal

If you have a reason to supplement

Frequently asked questions

How much selenium should I take per day?

Can you take too much selenium?

How many Brazil nuts equal a dose?

Does it help thyroid or prevent cancer?

Who actually needs it?

Related

Sources