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Ingredient guide

How long does selenium take to work?

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

Blood selenium starts rising within weeks of a daily supplement and leveled off by about week 10 in one trial. The selenoproteins that do selenium's work rise only in people short of it: in adults with very low intakes, selenoprotein P was at its full level after 40 weeks once total intake reached about 49 mcg a day.1,2,3

Key facts

  • Selenium in plasma or serum mostly reflects recent intake, while hair and nails reflect intake over months or years.3
  • In adults aged 50 to 64 in the United Kingdom, 50 to 200 mcg a day raised plasma selenium to a steady level by week 10.1
  • In adults who already had enough selenium, 16 weeks of about 200 to 600 mcg a day did not raise glutathione peroxidase activity or selenoprotein P.4
  • Blood selenium rose in step with the dose for selenomethionine and selenium yeast, but not for sodium selenite.4

What working means for a nutrient

A nutrient does its job by filling a need. If your diet already covers it, as it does for most people in the United States, there may be nothing for a supplement to change. Researchers track selenium in three ways: its level in the blood, the activity of the selenoproteins that use it, and its build-up in tissues over time.3,5

Blood selenium: weeks

In a 12-week trial in 119 adults aged 50 to 64, daily selenium yeast raised blood selenium for about 10 weeks before it leveled off.1

In that trial, average plasma selenium started at 95.7 ng/mL and settled at about 118, 152 and 177 ng/mL on 50, 100 and 200 mcg a day. A bigger daily amount gave a higher plateau, not a faster finish: all three groups had leveled off by week 10.1

In a 16-week trial, selenomethionine and selenium yeast raised blood selenium in step with the dose while sodium selenite did not, and urine measurements suggested selenomethionine was absorbed better.4

That is why the authors of that 16-week trial concluded that blood selenium is a good check on selenomethionine intake but not on selenite. A blood test may not reflect a selenite product, even though the body absorbs selenium from selenite.3,4

Selenoproteins: only if you were low

Supplements do not seem to raise selenoprotein P or glutathione peroxidase activity unless a person is short of selenium.3,4

The longest of these trials ran for 40 weeks in China, in adults whose food supplied only about 14 mcg of selenium a day. Glutathione peroxidase activity reached its full level at about 35 mcg a day in total, and selenoprotein P, which needed more selenium, at about 49 mcg a day, both judged at week 40.2

In the UK trial, where people already ate about 55 mcg a day, selenoprotein P rose with every dose within 12 weeks, while glutathione peroxidase in platelets did not change. People who start with more selenium have less room to move.1

What longer trials measured

Trials that ran for months looked for changes in body function, and in healthy people they mostly found none. In 368 healthy adults aged 60 to 74, six months of 100 to 300 mcg a day raised blood selenium but left thyroid function unchanged. In 54 healthy men, 48 weeks of 300 mcg a day did not change sperm concentration or motility.3

So a rise in blood selenium shows the supplement is being absorbed. It does not by itself show a change in how the body works, especially in people who were not short of selenium to begin with.3,4

If you take it for years

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.6,7

Hair loss is an early, well-established sign of long-term excess. Europe's food safety agency set its upper limit of 255 mcg a day after hair loss appeared at 330 mcg a day in a large trial, and the US limit is 400 mcg a day from all sources.5,8

Questions people ask

Will I feel a difference when I take selenium?

Not in any way these studies measured. They tracked blood selenium and selenoproteins, which you cannot feel, and in people who already had enough selenium the selenoproteins did not change.3,4

Which form raises blood selenium fastest?

In a 16-week trial, selenomethionine and selenium yeast raised blood selenium in proportion to the dose, while sodium selenite did not. Urine measurements in the same trial suggested selenomethionine was also absorbed better.4

How long does it take to recover from too much selenium?

The sources here do not give a timeline. Selenium is cleared mainly through urine, and NIH lists hair loss and nail loss among the signs of chronic excess. If you notice them, stop extra selenium and see your doctor.3

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Sources

  1. Hurst R, et al. Establishing optimal selenium status: results of a randomized, double-blind, placebo-controlled trial. Am J Clin Nutr. 2010;91(4):923-931. Opened 2026-10-07.
  2. Xia Y, et al. Optimization of selenoprotein P and other plasma selenium biomarkers for the assessment of the selenium nutritional requirement: a placebo-controlled, double-blind study of selenomethionine supplementation in selenium-deficient Chinese subjects. Am J Clin Nutr. 2010;92(3):525-531. Opened 2026-10-07.
  3. National Institutes of Health, Office of Dietary Supplements. Selenium: Fact Sheet for Health Professionals. Updated September 4, 2025. Opened 2026-10-07.
  4. Burk RF, et al. Effects of chemical form of selenium on plasma biomarkers in a high-dose human supplementation trial. Cancer Epidemiol Biomarkers Prev. 2006;15(4):804-810. Opened 2026-10-07.
  5. Office of Dietary Supplements, National Institutes of Health. Selenium: Fact Sheet for Consumers. Updated June 18, 2025. Opened 2026-10-07.
  6. 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.
  7. 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.
  8. 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.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.