" A total of 974 men with a history of either a basal cell or squamous cell carcinoma were randomized to either a daily supplement of 200 microgram of selenium or a placebo. Patients were treated for a mean of 4.5 years and followed for a mean of 6.5 years. The selenium treatment was associated with a significant (63%) reduction in the secondary endpoint of prostate cancer incidence. ."
(Clark LC; Dalkin B; Krongrad A; Combs GF Jr; Turnbull BW; Slate EH; Witherington R; Herlong JH; Janosko E; Carpenter D; et al. Decreased incidence of prostate cancer with selenium supplementation: results of a double-blind cancer prevention trial. British Journal of Urology, 1998 May, 81(5):730-4.)
There was no effect of selenium on the existing skin cancers in this trial. Detractors use this fact to pretend that Nutraceuticals in general and selenium in particular are useless for cancer prevention.