Finasteride, a generic drug used by physicians to treat enlarged prostate and male pattern baldness, was found to significantly reduce the risk of prostate cancer. During the trial of 19,000 men, a slightly higher percentage of those on finasteride developed high-grade cancer than those taking a placebo. This difference shrank in the follow-up analysis, which caused concern and debate in the medical community, and doctors backed away from prescribing the drug.

Multiple studies prompted by this concern ultimately concluded finasteride, by shrinking the prostate and making the PSA test work better, made the tumors easier to find. Nonetheless, in 2011, the Food and Drug Administration added a warning to the label about the increased risk of being diagnosed with high-grade prostate cancer.

A long-term follow-up led by the director of the Cancer Therapy&Research Center confirms that  finasteride  has no impact on lifespan but further reduces the risk of prostate cancer. 

Reducing the risk of prostate tumors by about 30 percent - and low-grade tumors by 43 percent - means thousands of men can avoid a cancer diagnosis and subsequent treatments that significantly affect quality of life, said Ian M. Thompson Jr., M.D., director of the CTRC at The University of Texas Health Science Center at San Antonio.

"If you look at the number of prostate cancers that are diagnosed annually and multiply that by 30 percent, that's the number of cancers we might be able to prevent each year," Thompson said. "That's more than 71,000 men. That's more than 175 jumbo jets full of men who won't get cancer, who won't face treatments with side effects like sexual dysfunction.

"There's nothing like disease prevention. Nothing comes close."

The Prostate Cancer Prevention Trial, funded by a National Cancer Institute grant, began in 1993. It was coordinated by SWOG, an international network of research institutions, and led by Thompson. The men in the finasteride arm had a median age of 62 and took the drug for seven years.

The 18-year follow-up to the study examined survival in both study arms to see if there was an increased risk of death in men who took finasteride. The results show no impact on either overall survival or survival after prostate cancer diagnosis.

"What that tells us is that in men who take finasteride, a third fewer will be diagnosed with prostate cancer." Dr. Thompson said. In today's medical climate, many men with even low-grade tumors are treated unnecessarily, he noted, and those treatments carry a considerable burden for the patient and for society.

"If we can free thousands of men each year from that unnecessary burden," he said, "we could use those resources for other important medical interventions, reducing death and suffering from disease."