In the seven-year study, 18 per cent of the men taking the drug finasteride developed prostate cancer, compared with 24 per cent of those taking the placebo, says a report released online yesterday in The New England Journal of Medicine.

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 known cancer preventative is tamoxifen, which, despite some risks, many women take to protect against breast cancer.

"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 warned of serious side-effects, however — including loss of libido and erectile dysfunction, and the possibility that the drug stimulates a more aggressive kind of prostate cancer.

The prostate cancer prevention trial was conducted at 221 sites across the United States.

In 1993, researchers started giving 18,882 healthy men aged 55 or older either 5 milligrams of finasteride — the same dose used to treat enlarged prostates — or a placebo, 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.

Based on the 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.

If every American man started taking the drug, it would prevent an estimated 300,000 prostate cancers over 10 years, officials estimated.

Only skin cancer strikes more men than prostate cancer, and only lung cancer kills more.

"Millions of men may benefit from finasteride's ability to reduce prostate cancer risk," said Leslie Ford, associate director for clinical research at the National Cancer Institute.

`Millions of men may benefit from finasteride's ability to reduce prostate cancer risk.'

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.

"This is potentially a major step forward in the over-all effort to decrease both the morbidity and mortality of this most important cancer in men," said Dr. Arthur Sagalowsky, institutional investigator for the trial at the University of Texas Southwestern Medical Centre.

The study is "qualified good news," but the benefits should be balanced with the potential side effects, said Dr. Pat Folgham, Presbyterian Hospital of Dallas' surgical oncology director.

"What isn't clear is whether the drug was able to suppress or prevent lesser forms of cancer so the ones that actually showed up were the worst ones," said Folgham, principal investigator for the trial at Presbyterian Hospital.

"There's no way of saying whether it causes you to get more aggressive cancer."

But men at high risk because of family history and other factors might want to ask their doctor about the drug, Folgham said.

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

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

"There are benefits and risks to consider," Ford said. "Finasteride may not be right for all men. Fortunately, it's not an emergency decision."

"I wouldn't put a patient on finasteride to prevent prostate cancer based on the results of this study," said Peter Scardino of Memorial Sloan-Kettering Cancer Center in New York. He wrote an editorial that will accompany the study in the July 17 issue of the magazine.

The Canadian Cancer Society estimates that in 2003 some 18,800 men will be diagnosed with prostate cancer, and 4,200 will die from it.