CHICAGO - A prostate cancer study that could change how doctors treat some patients found that widely used hormone-blocking drugs did not improve survival chances for older men whose disease hadn't spread.

In fact, men given the drugs alone were slightly more likely to die of prostate cancer during the next six years than men who'd gotten only medical monitoring.

The study involved nearly 20,000 Medicare patients with prostate cancer that hadn't spread. A surprising 41 percent got only drug treatment, showing that the therapy has become a popular alternative to surgery and radiation, the study authors said.

Other experts said the study gives doctors important information about how to treat older men with slow-growing disease that hasn't spread beyond the prostate. However, the study didn't look at whether hormone-blocking drugs alone benefit younger men or compare that treatment with radiation or surgery.

Randomized studies have shown that the drugs can benefit men with more aggressive disease when used along with surgery or radiation. But research is sparse on using hormone-blockers alone or in patients with localized cancer, such as those in their study, the authors said.

The drugs block production of testosterone, which feeds cancer cells. They are sometimes given in addition to surgery or radiation; using them alone is a less traditional but increasingly used approach, particularly among older men whose prostate cancer hasn't spread, the study authors said.

The drugs are typically given in a doctor's office and can have serious side effects, including increased risk for diabetes, heart diseases, impotence and bone loss.

Patients often think that any treatment is better than nothing, said lead author Dr. Grace Lu-Yao of the Robert Wood Johnson Medical School in New Jersey.

"What we are saying is doing something may not always be the best choice, because given the overall picture, this doesn't really give you any proven benefit," she said.

She said the researchers hope the study will prompt doctors to avoid hormone-blocking drugs alone in older men whose disease hasn't spread.

The results appear in today's Journal of the American Medical Association.