A drug used to fight baldness and enlarged prostates also protects against prostate cancer, offering men the first pill proven to cut the risk for a major cancer killer, researchers reported yesterday.

The drug reduced by nearly 25 percent the chances of developing prostate cancer in a massive, federally funded study that was stopped early because the results were so striking, researchers said.

Sold as Propecia for baldness and as Proscar for enlarged prostates, the drug, whose scientific name is finasteride, reduces the cancer risk by lowering levels of a form of the male hormone testosterone that promotes the growth of prostate cells.

"This trial proves that prostate cancer, at least in part, is preventable. It is a huge step forward for cancer research," said Peter Greenwald, director of the division of cancer prevention at the National Cancer Institute, which sponsored the study.

Experts urged caution, however, noting that while the drug appeared to reduce the overall number of cancers, men taking finasteride may be at greater risk for more aggressive tumors.

Finasteride becomes only the second drug shown to reduce the risk for any type of cancer, an intensely sought-after goal in oncology research. The other cancer preventative is tamoxifen, which, despite risks, many women take to protect against breast cancer.

Based on the new findings, if 1,000 63-year-old men started taking finasteride, only 45 would get prostate cancer within the next seven years, compared with 60 who would be expected to develop the cancer without the drug.

"Millions of men may benefit from finasteride's ability to reduce prostate cancer risk," said Leslie Ford, the NCI's associate director for clinical research in the division of cancer prevention.

But specialists acknowledged that the findings were complicated, and as a result men should discuss with their doctor whether to start taking the drug. There are also potential sexual side effects that could discourage them.

"I wouldn't put a patient on finasteride to prevent prostate cancer based on the results of this study," said Peter T. Scardino of the Memorial Sloan-Kettering Cancer Center in New York, who wrote an editorial that will accompany the study in the July 17 issue of the New England Journal of Medicine. The journal released the study early because of its public health implications.

The drug may encourage the growth of more aggressive tumors, but it is also possible that the drug only makes cancers look more aggressive than they really are, Ford said. Another possibility is that the drug primarily prevents the less aggressive cancers, leaving mostly the kind more likely to spread.

"The bottom line is we'll need to do further study," Ford said.

Prostate cancer hits 221,000 men in the United States each year and kills about 29,000. Only skin cancer strikes more men and only lung cancer kills more. Many men can live with prostate cancer for years without treatment. But those who undergo surgery and radiation often experience urinary incontinence and impotence. The walnut-sized prostate gland, which surrounds the urinary passageway, produces fluid components of semen.

Finasteride was approved in 1992 to treat a noncancerous enlargement of the prostate known as benign prostatic hyperplasia. A few years later, after patients started reporting unexpected hair growth, the drug was approved at a much lower dose to slow hair loss, and subsequently became a bestseller.

In the new study, researchers in 1993 started giving 18,882 healthy men 55 or older either 5 milligrams of finasteride -- the same dose used to treat enlarged prostates -- or a placebo, or dummy pill, every day. The study was supposed to continue until May 2004. But on March 3, a panel of experts monitoring the study ordered it halted because it had already produced convincing findings.

Eighteen percent of the men taking finasteride for seven years developed prostate cancer, compared with 24 percent of those taking the placebo -- a 24.8 percent reduction.

"For the first time, we know that it is possible to prevent the clinical expression of prostate cancer," Ford said at a briefing.

But of the men who developed prostate cancer, 6.4 percent of men taking finasteride developed what appeared to be more aggressive tumors, compared with 5.1 percent of those taking the placebo.

Men taking finasteride were also more likely to experience sexual side effects such as a decline in libido and ability to ejaculate, though they were less likely to experience problems urinating.

The findings are likely to prompt millions of men to go through the same difficult decision-making process that many women experience in deciding whether to take tamoxifen and hormones to alleviate menopause symptoms.

Men at increased risk for prostate cancer, such as those with a family history of the disease as well as all African American men, might be more inclined to take the drug, officials said.

Harmon J. Eyre, chief medical officer of the American Cancer Society, said the study "will no doubt prompt a lot of men to start asking their doctors whether they should be on the drug, and we would encourage men to carefully weigh their options, as this information is very new."