Selenium lowers your cancer risk

In randomized trials, selenium supplements have not lowered the risk of cancer. A 2018 Cochrane review pooled the trials at low risk of bias, 19,475 people, and found the same rate of cancer with selenium as with placebo (risk ratio 1.01). Cancer deaths, reported by one of those trials in 17,444 people, did not differ (1.02). The largest trial, SELECT, enrolled 35,533 men in the United States, Canada and Puerto Rico and tested 200 micrograms of selenium a day against vitamin E, both together, and placebo; prostate cancer was no less common with selenium alone (hazard ratio 1.04), and none of the other cancers it tracked differed.

In two trials, some groups given selenium fared worse than those on placebo. In an analysis of a sample of SELECT's men whose toenail selenium was measured at the start, those at or above the 60th percentile who were given selenium (alone or with vitamin E) had a 91% higher risk of high-grade prostate cancer, while those below it saw no effect. In a Danish pilot trial in 491 older adults, those given 300 micrograms a day for five years, a dose below the adult upper limit of 400 micrograms, had more deaths from any cause than those on placebo over the whole follow-up (hazard ratio 1.59); during the five years of treatment the difference was not significant. Most of the people in the trials were men, so the trials say little about women. The idea that selenium protects comes from cohort studies, in which people with higher selenium levels have lower cancer death rates, and from one earlier US trial whose benefit appeared only in men and in people with lower starting selenium.

That earlier trial, the Nutritional Prevention of Cancer trial, gave 200 micrograms a day of selenium yeast to 1,312 people in the eastern United States who had had skin cancer. It was designed to test skin cancer recurrence; in its analyses of other cancers, total cancer was lower with selenium (hazard ratio 0.75) and prostate cancer about halved (0.48). The effect was confined to men (0.67) and to people whose starting plasma selenium was below 121.6 ng/ml, the lower two thirds of that trial's participants, while those in the top third had a higher rate that was not significant (1.20). SELECT did not repeat this. With longer follow-up, prostate cancer was slightly more common with selenium, a difference that was not significant (hazard ratio 1.09), about 0.8 extra cases per 1,000 men per year. A 2026 analysis that followed SELECT's men for years after the supplements stopped found lower rates of melanoma and colorectal cancer, a slightly higher rate of lung cancer, and higher rates of Hodgkin lymphoma, multiple myeloma and the nerve disease ALS, these last three based on few cases. A 2023 UK trial in 270 people newly diagnosed with early bladder cancer found that selenium did not delay recurrence (hazard ratio 0.92); it stopped early because too few people joined.

The cohort studies point the other way from the trials. A 2025 review of cohort studies found that each step up in measured selenium levels (one standard deviation) went with 15% lower cancer mortality. These studies measure selenium status rather than giving supplements, so people with higher levels may differ in other ways, and the trials that assigned selenium at random did not reproduce the pattern.

Selenium has a known toxicity. The US Institute of Medicine set the adult upper limit at 400 micrograms a day, and chronic excess (selenosis) most often shows as brittle hair and nails and their loss, with stomach upset, skin rash, garlic breath, tiredness, irritability and nervous system problems. In SELECT, type 2 diabetes was slightly more common with selenium, a difference that was not significant (relative risk 1.07). A 2022 evidence review for the US Preventive Services Task Force rated the evidence for selenium low, insufficient or absent for every outcome, and the Task Force concluded that the evidence is insufficient to weigh the benefits and harms of selenium and most other single-nutrient supplements for preventing cancer.

The trials were mainly publicly funded, and in each of the four selenium trials described here supplement companies supplied the tablets; a selenium yeast supplier also helped fund the Danish trial. Whether selenium fits alongside cancer treatment or other medicines is a conversation with the reader's clinician.