Men with high-risk prostate cancer treated with only 18 months of hormone therapy live just as long as those treated for a more standard 36 months, a new study has found.

If the study results are applied in practice, it could mean much shorter treatment, sparing men months of unpleasant side effects, researchers said Tuesday.

“This may well change the standard of care,” said Dr. Bruce J. Roth, a prostate cancer specialist at Washington University in St. Louis. “Three years of hormonal therapy was almost picked randomly, and there’s nothing magical about that duration.”

Dr. Roth was not involved in the study, but he moderated a news conference for the Genitourinary Cancers Symposium, which will take place starting Thursday in Orlando, Fla., and is where the results will be presented.

Hormone therapy is essentially chemical castration, in which drugs are used to block the body’s production of testosterone, which fuels prostate tumor growth.

The side effects, including hot flashes, loss of sexual desire, fatigue and the weakening of bones and muscles, make life “quite miserable,” said Dr. Abdenour Nabid of Sherbrooke University Hospital Center in Sherbrooke, Quebec, who was the lead investigator.

The study involved 630 patients with localized but high-risk prostate cancer who were treated with radiation therapy and hormone therapy. While that description fits only a small portion of the 240,000 new cases of prostate cancer diagnosed each year in the United States, the results would still apply to thousands of men, researchers said.

After a median follow-up of about six and a half years, 77.1 percent of the men who received 36 months of therapy were still alive, as were 76.2 percent of the men treated for 18 months.

While slightly more men receiving the longer treatment were alive at five years, the difference was not statistically significant, and for the patients already followed for 10 years, the survival rates were similar. The death rate specifically from prostate cancer was also the same after 10 years.

There were also no statistically significant differences in the rate of biochemical failure — when the P.S.A. marker rises — or in the spread of cancer to the bone, Dr. Nabid said. The difference between the two therapy durations on the quality of the patients’ lives is still being studied.

Dr. David I. Quinn, medical director of the University of Southern California Norris Cancer Hospital, said the results “will change the approach for men who’ve got the worst localized prostate cancer that we see.” He said the results went against some previous studies that suggested that “more is better.”

But Dr. Michael J. Morris, associate professor at the Memorial Sloan-Kettering Cancer Center, said that 630 patients might be too few to draw “a relatively sweeping conclusion.” A study meant to prove that two treatments are equivalent may need to be much larger, he said.

Dr. Matthew R. Smith, professor of medicine at Massachusetts General Hospital, said it might be “overreaching” to make a conclusion yet because not many trial patients had died. “I think we need longer follow-up,” he said.

Dr. Nabid, the principal investigator, said that patients would be followed for two or three more years but that he was confident the results would hold up.

The trial enrolled patients at 10 hospitals in Quebec from October 2000 to January 2008. The drugs used were bicalutamide and goserelin, sold by AstraZeneca as Casodex and Zoladex, respectively, but now subject to generic competition. AstraZeneca paid for the study.

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By ANDREW POLLACK