Although some people believe that taking selenium can reduce a person’s risk of cancer, a Cochrane Systematic Review of randomized controlled clinical trials found no protective effect against non-melanoma skin cancer or prostate cancer.

Gabriele Dennert and co-workers, Institute for Transdisciplinary Health Research, Berlin, Germany, searched the literature for trials studying the effects of taking selenium supplements and found 49 prospective observational studies and six randomized controlled trials. The observational studies indicated that a higher selenium intake protects against cancer, but the randomized controlled trials showed no evidence of this. In addition, there was some indication that taking selenium over a long period of time could have harmful effects.

• Selenium for preventing cancer
  G. Dennert, M. Zwahlen, M. Brinkman, M. Vinceti, M. P. A. Zeegers, M. Horneber,
  Cochrane Database Syst. Rev. 2011, 5, CD005195.
  DOI: 10.1002/14651858.CD005195