Some 15,000 men aged 50 and older participated in the study, which included an eight-year follow-up period, but neither vitamin appeared to appreciably reduce their cancer risk, according to the studies appearing in the January 7 issue of the Journal of the American Medical Association (JAMA).
The findings are disappointing news for the more than half of American adults take vitamin supplements - many in the hope of warding off illness.
They appear to refute earlier observational studies that linked use of vitamins E and C with reduced risk of certain forms of cancers, including cancer of the prostate.
One of the two studies - the Selenium and Vitamin E Cancer Prevention Trial (SELECT) - found that vitamin E or selenium supplements, whether taken alone or in combination, appear not to reduce the risk of prostate cancer, which is the second leading cause of cancer death in the United States.
"It may be time to give up the idea that the protective influence of diet on prostate cancer risk can be emulated by isolated dietary molecules given alone or in combination to middle-aged and older men," Peter Gann of the University of Illinois at Chicago reflected in a JAMA editorial.
SELECT researchers studied the supplements' effects over seven years on some 35,533 men, aged 50 years or older.
The researchers said that "large-scale, randomized trials" still must be conducted on the use of vitamin supplements and cancer.
Until that next generation of trials, "physicians should not recommend selenium or vitamin E or any other antioxidant supplements to their patients for preventing prostate cancer," said Gann.