It's a huge disappointment in the field of cancer prevention.
The largest trial of its kind - known as "SELECT" - hoped Vitamin E and selenium would prevent prostate cancer. But CBS News has learned patients are being told to stop taking the pills - because they don't seem to work, CBS News medical correspondent Dr. Jon LaPook reports. Researchers will continue to monitor the patients in the trial.
"Taking selenium or Vitamin E at the doses that we used in the SELECT trial for an average of five years did not prevent prostate cancer," said Dr. Eric Klein.
The study of more than 35,000 men in their 50s and older began back in 2001.
Doctors at more than 400 sites gave combinations of Vitamin E, the mineral selenium and a placebo.
The supplements failed to prevent prostate cancer - and there were two worrisome trends.
Patients taking Vitamin E alone had a slightly higher rate of prostate cancer.
Men who took selenium alone were more likely to develop diabetes. The researchers believe the increases may just be due to chance.
"The safety issues are of more minor concern and bear further follow-up," Klein said.
Vitamin E and selenium are antioxidants that exist in relatively small amounts in food. Previous studies suggested that higher doses might help prevent prostate cancer.
That's why urologists like Dr. Aaron Katz had high hopes for the trial.
"I'm disappointed with the study. I'm very concerned about the results of the trial," Katz said. "I would have hoped this would have been the way to prevent cancer in this country."
Over the next week, the 35,000 men in the study will be notified of the results and told to stop the supplements.
Katz spoke to one of the participants Monday.
"He's concerned. You've got to be concerned," Katz said. "You're hearing the results of a national trial that you participated in, and are now finding that perhaps I may have increased my risk of developing the disease."
This is a major disappointment, but it is also progress, LaPook reports. Because it's also important to know what does not prevent cancer.