Both trials were set up following some evidence that taking supplements might have a positive effect.
But one study of 35 533 men, and a second of 15 000 doctors, found no evidence that cancer rates were any lower in those taking supplements.
Both studies are published in the Journal of the American Medical Association.
A number of trials have suggested that taking vitamin supplements can reduce the risk of cancer by boosting antioxidant levels, although results were equivocal. These latest studies involve very large numbers of people and are though to be more definitive.
In the first study, researchers from University of Texas and the Cleveland Clinic Lerner College of Medicine gave healthy men either the trace mineral selenium, vitamin E, both or a dummy pill.
The team intended to monitor all the participants for at least seven years but the trial was stopped early because the results were so disappointing. The researchers found there were no statistically significant differences in the numbers of men who developed prostate cancer in the four groups. In all cases the proportion of men diagnosed with prostate cancer over a five-year period was 4% to 5%.
In the second study, researchers at Boston's Brigham and Women's Hospital tested the impact of regular vitamin E and C supplements on cancer rates among 14 641 male doctors.
Over eight years, taking vitamin E had no impact at all on rates of either prostate cancer, or cancer in general. Vitamin C had no significant effect.
According to researchers this is yet more evidence that supplements are no substitute for a healthy diet and some studies have shown that supplements may even increase the risk of cancer.
Eating a diet high in fruit and vegetables is still the best way to obtain vitamins and minerals.
