Written by Gabriele Dennert and the CAM-Cancer Consortium.
Updated October 20, 2013

Selenium prevention

Is it safe?

Adverse events

Early signs of selenium toxicity (garlic breath, hair and nail changes, upset stomach) were observed in participants of a clinical trial who received 1600µg or 3200µg selenised yeast/day for up to 24 months. 24 In the Selenium and Vitamin E Cancer Prevention Trial, 25 which used 200µg/day selenomethionine for 7–12 years, the rate of alopecia (overall incidence: 3% of participants) and mild dermatitis (7% of participants) was higher than in the placebo group (relative risk increase: +28% and +17%, respectively). Neither trial reported more severe adverse effects or signs of chronic toxicity.

Chronic selenium poisoning (selenosis) has been seen in seleniferous areas of North America and China, but has also been attributed to commercially available selenium supplements. 26 Symptoms of selenosis include hair loss, thickened nails, nausea, vomiting, fatigue and paresthesia and paralysis. The EU Scientific Committee on Food considers an upper selenium limit (for adults) of 300µg/day (including supplements) to be acceptable for the avoidance of selenosis. 27 The US Food and Nutrition Board, Institute of Medicine, have set the tolerable upper level of selenium intake to 400µg/day. 11

However, there are concerns that long-term selenium supplementation may increase the risk of developing type 2 diabetes mellitus in selenium-replete populations. The relative risk for male selenium users in the SELECT trial was 1.07 (95% confidence interval (CI) 0.94–1.22), when compared to the placebo group; this meant that there were six additional cases of diabetes in every 1000 selenium users per year (diabetes risk: +7% in the selenium group compared to placebo). 25 In the Nutritional Prevention of Cancer Trial (NPCT), the risk of developing diabetes mellitus in the selenium group was 1.55 (95% CI: 1.03–2.33); this meant that there were four additional cases of diabetes in 1000 selenium users per year (diabetes risk: +55% in the selenium group). 28

Lethal and non-lethal acute poisoning related to the use of selenium as a complementary and alternative medicine has been reported in a number of cases. 26,29 The lethal selenium dose for humans is unknown, but is estimated to be between 0.12g and 1g (120–1000mg). 26 Acute poisoning is characterised by vomiting, garlic breath, abdominal pain, hypersalivation, cardiac arrhythmia, haemolysis, necrosis of the liver, cerebral and pulmonary oedema, coma and death. 27


Chronic overexposure to selenium (selenosis).

According to the NPCT, selenium may increase the risk of non-melanoma cancer recurrence. Selenium supplementation may therefore be problematic, especially for light-skinned people.


Vitamin C can lower the intestinal absorption of selenium. 30

Other problems or complications

Severe complications have been reported because not only consumers, but also healthcare professionals, have confused the measurements ‘milligram’ (mg) and ‘microgram’ (µg). Also, a number of websites mistakenly recommend ‘milligrams’ of selenium instead of ‘micrograms’, the ingestion of which may result in a thousand-fold overdose.

No controlled data are available on the effects of selenium supplements in non-selenium-deficient women during pregnancy or lactation.


Gabriele Dennert, CAM-Cancer Consortium. Selenium prevention [online document]. http://cam-cancer.org/The-Summaries/Dietary-approaches/Selenium-prevention. October 20, 2013.

Document history

Summary first published in November 2010, authored by Gabriele Dennert.

Original summary divided into “Selenium – prevention” and “Selenium – during cancer treatment”, fully revised and updated in October 2013 by Gabriele Dennert.


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