How much selenium should you take a day?

The European labelling reference is 55 micrograms. EFSA places the tolerable upper limit at 255 micrograms a day for adults, so the margin between what is needed and the ceiling is narrower than for almost any other mineral.

Is too much selenium dangerous?

The critical adverse effect EFSA uses to set the limit is alopecia. In addition, a trial with 200 micrograms a day for almost eight years found more cases of type 2 diabetes (HR 1.55), especially in those who already had high plasma selenium levels.

Does selenium prevent cancer?

The SELECT trial, with over 35,000 men and 200 micrograms a day, found no reduction in prostate cancer risk (HR 1.09, not significant). That same trial is the one EFSA later uses to establish the adverse effect level.

What about Brazil nuts?

They are the most concentrated food source, so much so that EFSA explicitly names them, alongside high-dose supplements, as the way an adult can exceed the upper limit. If you eat them daily, adding a supplement makes no sense.

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Selenium · Moderate evidence

What selenium is for and how much to take

The short answer. Selenium is a trace element needed for the thyroid and several antioxidant enzymes, and very little is needed: the European labelling reference is 55 micrograms a day. What sets this mineral apart is how narrow the margin is between what is needed and what is too much. EFSA sets the tolerable upper limit at 255 micrograms a day. And two trials with 200 micrograms a day, a dose sold over the counter, found more type 2 diabetes in one and no benefit on prostate cancer in the other.

There are studies in favour, but few, with small samples or measuring markers instead of outcomes. No figure on this page is attributed to a specific product: the effect belongs to the ingredient and every number carries its source next to it.

01 What it does and how much

Each row is a measured effect, with the figure the study gives and the source number. Where the evidence gives no figure, the column says so instead of making up a percentage.

Effects of selenium with their source
Effect	How much	Detail
Nutrient reference value	55 µg	The European labelling figure. A tiny amount compared with almost any other mineral in the pill box. [1]
Tolerable upper limit	255 µg/day	Set by EFSA for adults, including pregnant and breastfeeding women, taking alopecia as the critical adverse effect from a level of 330 micrograms observed in a large trial. [2]
Type 2 diabetes with 200 µg/day	HR 1.55	12.6 cases per 1000 person-years with selenium versus 8.4 with placebo, after a mean follow-up of 7.7 years. The risk was higher in those who already started with high plasma selenium levels. [3]
Prostate cancer with 200 µg/day	HR 1.09	In the SELECT trial, with no significant difference: it prevented nothing. It is the trial EFSA later uses to derive the adverse effect level for the limit. [4]
Brazil nuts	an easy way to overdo it	EFSA considers exceeding the limit unlikely except in two cases: regular users of high-dose supplements and regular consumers of Brazil nuts. [2]

02 How to take it

Dose. The labelling reference is 55 micrograms. The tolerable upper limit is 255 micrograms a day.
When. With meals. No time of day is backed.
How long before you notice it. As a trace element, there is only something to expect if you start from a deficiency, which is not the norm in Europe.

Details that change the result

Stacking sources without noticing is easy: a multivitamin, a thyroid formula and a handful of Brazil nuts can all push in the same direction.
EFSA notes that selenium intake from the natural content of foods is not a cause for concern. The problem is high-dose supplements.
For children’s supplements EFSA asks for caution and use according to individual need.
The usual form is selenomethionine, which is the one used in the trials cited.

03 When it makes sense and when it does not

Worth it if

With a documented deficiency, assessed by a doctor with a blood test.
As part of a multivitamin at reference doses, adding up what other sources provide.

Not worth it if

At 200 micrograms a day out of habit: it is the dose of the two trials that found no benefit and did find a signal of more type 2 diabetes.
Combining several products that contain it without counting the total.
Eating Brazil nuts daily and adding a supplement on top.

04 Sources

Every figure above comes from here. They are position stands from scientific societies, meta-analyses and peer-reviewed reviews: if a claim cannot hang from one of these lines, it is not on the page. References are cited in their original form.

Reglamento (UE) 1169/2011 sobre la informacion alimentaria facilitada al consumidor, Anexo XIII: valores de referencia de nutrientes. Selenio, 55 ug. Es un valor de ETIQUETADO, no una dosis con efecto demostrado. https://eur-lex.europa.eu/legal-content/ES/TXT/?uri=CELEX%3A32011R1169
EFSA NDA Panel. Scientific opinion on the tolerable upper intake level for selenium. EFSA Journal. 2023;21(1):7704. Limite superior tolerable de 255 ug/dia para adultos, con la alopecia como efecto critico y un nivel mas bajo con efecto adverso observado de 330 ug/dia procedente del ensayo SELECT; se senala el riesgo de superarlo en consumidores habituales de complementos de dosis alta o de nueces de Brasil. doi:10.2903/j.efsa.2023.7704
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. 1202 personas, 200 ug/dia, seguimiento medio de 7,7 anos: 12,6 casos por 1000 personas-ano con selenio frente a 8,4 con placebo (HR 1,55; IC 95 % 1,03-2,33). doi:10.7326/0003-4819-147-4-200708210-00175
Klein EA et al. Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2011;306(14):1549-1556. Con 200 ug/dia de selenio, 575 casos de cancer de prostata frente a 529 con placebo (HR 1,09; IC 99 % 0,93-1,27; sin significacion). doi:10.1001/jama.2011.1437

This is not medical advice. It is what some studies say about an ingredient, not a protocol for you. With an illness, medication or pregnancy involved, the decision belongs to a healthcare professional who knows you.

Questions about selenium

Answered with the data on this page

How much selenium should you take a day?: The European labelling reference is 55 micrograms. EFSA places the tolerable upper limit at 255 micrograms a day for adults, so the margin between what is needed and the ceiling is narrower than for almost any other mineral.
Is too much selenium dangerous?: The critical adverse effect EFSA uses to set the limit is alopecia. In addition, a trial with 200 micrograms a day for almost eight years found more cases of type 2 diabetes (HR 1.55), especially in those who already had high plasma selenium levels.
Does selenium prevent cancer?: The SELECT trial, with over 35,000 men and 200 micrograms a day, found no reduction in prostate cancer risk (HR 1.09, not significant). That same trial is the one EFSA later uses to establish the adverse effect level.
What about Brazil nuts?: They are the most concentrated food source, so much so that EFSA explicitly names them, alongside high-dose supplements, as the way an adult can exceed the upper limit. If you eat them daily, adding a supplement makes no sense.

And now, which to buy

Prices, kept apart from the evidence

This page explains what selenium does and deliberately names no brand. The comparison is the other half: 33 products from 5 stores from €0.037 per capsule, sorted by price and by how verifiable their certification is.

See the selenium comparison

Other guides

What each supplement does, with its sources