Last Updated: 2003-06-24 13:00:36 -0400 (Reuters Health)

WASHINGTON (Reuters) - A popular baldness drug that works by lowering male hormone levels also seems to prevent prostate cancer, a major killer of men, U.S. researchers said Tuesday.

The drug, finasteride, worked so well that the researchers stopped their trial so that all the men could take the drug. It seemed to reduce prostate cancer by 25 percent.

The drug used in the study is sold as Proscar, made by Merck & Co., Inc. Finasteride was at first developed to stop the benign enlargement of the prostate that comes with normal aging.

It became best-selling baldness drug when it was also found to stop the thinning of the hair that many men see with age.

Prostate cancer is the most common cancer in men, after skin cancer, and will affect nearly 221,000 men in the United States this year. About 29,000 men will die of the disease.

"Millions of men may benefit from finasteride's ability to reduce prostate cancer risk," Dr. Leslie Ford of the National Cancer Institute, who helped lead the study, said in a statement.

Writing in the New England Journal of Medicine, Ford and colleagues said they gave 9,000 men either finasteride or a placebo. The study was supposed to last for 10 years or until 2004.

They found that 18 percent of the men who took finasteride, or 803 men, developed prostate cancer. About 24 percent of men who took placebo, or 1,147 men, developed prostate cancer.

Many of the men with cancer had normal prostate specific antigen levels -- a standard blood test for prostate cancer, and also seemed normal with a standard digital rectal exam, the NCI said.

But the men who got the drug who did develop cancer developed a more aggressive form, the study found.

They found that 6.4 percent of men on finasteride -- 280 men out of 4,368 in the study -- had high-grade tumors. For men on placebo, 5.1 percent had high-grade cancers.

It was not clear if this would mean the cancer would be harder to treat, the NCI said.

"The reason men on finasteride had more high-grade tumors is currently unknown, but the researchers are studying several possibilities," the institute said in a statement.

"The drug affects the appearance of prostate cancer cells, and this may lead to a false estimate of tumor grade, which is determined visually by a pathologist."