A Cochrane Systematic Review of randomized controlled clinical trials found that taking selenium produces no protective effect against non-melanoma skin cancer or prostate cancer. In addition, there is some indication that taking selenium over a long period of time could have toxic effects. 

These conclusions were reached after researchers scanned the medical literature, looking for trials that studied the effects of taking selenium supplements and observational studies on selenium intake. The researchers locat­ed 49 prospective observational studies and six randomized controlled trials. 

Looking at the data from observational studies gave some indication that people may be marginally more protected from cancer if they had a higher selenium intake than those with a lower intake, and that the ef­fect was slightly greater for men than women. These conclusions have limitations because the data came from a wide variety of trials. When the team of researchers looked at the more carefully conducted randomized control­led trials, any sign of benefit disappeared. 

The results of the Nutritional Prevention of Cancer Trial and the Selenium and Vitamin E Cancer Prevention Trial raised concerns about possible harmful effects from long-term use of selenium supplements. 

Researchers believe that there is a need for more research looking at selenium’s effect on liver cancer and think that it would be worth investigating the possible gender differences that appear to be present in the uncontrolled studies. 

More information is available at www.thecochranelibrary.com [9].