An increasingly popular prostate cancer treatment also makes bones brittle and may be responsible for over 3,000 fractures each year in the United States, researchers reported on Wednesday.

Hormonal ablation treatment for prostate cancer blocks the effects of the male hormone testosterone, either with drugs or by castration. Until now, doctors have regarded the treatment as relatively harmless, Vahakn Shahinian, chief author of the new research, told AMN Health.

He said that given the new findings, doctors and patients should be aware of the risks of the therapy, which seems to slow the growth of prostate tumors but may not always help patients live longer.

The findings, reported in Thursday’s New England Journal of Medicine, add another layer of complexity to the already-difficult question of how best to treat prostate cancer, which strikes 220,000 U.S. men each year.

Sometimes a tumor is growing so slowly it’s hard to know whether treatment is necessary.

Neutralizing testosterone is one option. More than 88,000 men a year get the drug treatment, and the number has been “exploding,” Shahinian said.

But the Shahinian team, which studied the records of nearly 51,000 cancer patients, found 19.4 percent of men who survived at least five years after anti-testosterone therapy broke a bone. In other patients, the rate was 12.6 percent.

“Our findings, along with those of smaller clinical series, underscore that such treatment is not benign” and there is no hard evidence it helps patients live longer, the researchers warned.

They also said doctors should be testing whether drugs designed to prevent brittle bone disease can counteract the side effects of anti-testosterone treatment.