Hormone-based drugs that help treat aggressive prostate cancer may be dangerous for some men with heart disease, U.S. researchers reported on Tuesday.

They found the drugs, which block tumor-fueling surges of testosterone, help most men who get them. But those with more than one form of heart disease or diabetes had a higher risk of death when they got the drugs along with radiation therapy.

Dr. Anthony D'Amico of Brigham & Women's Hospital and the Dana-Farber Cancer Institute in Boston and colleagues studied the cases of more than 5,000 men diagnosed with prostate cancer between 1997 and 2006.

The men received radiation treatment -- either external beam radiation, or little radioactive pellets called brachytherapy -- plus one of several drugs to dampen testosterone production.

More than 400 died over five years of follow-up, the researchers reported in the Journal of the American Medical Association.

Men who suffered from heart failure or had a heart attack were more than twice as likely to die as those without heart disease or who had just one symptom, such as high blood cholesterol. The researchers said 26 percent of the men with heart failure or who had a heart attack died, compared to 11 percent of others.

This was just five percent of the men, so the findings explain why overall, hormone therapy helps prostate cancer patients, D'Amico's team wrote. But men with serious heart disease and their doctors need to know the risk, they added.

Prostate cancer is the second most common cancer in men worldwide after lung cancer, killing 254,000 a year globally.

Many prostate tumors are slow-growing and take years to cause harm, and some doctors are beginning to advocate less aggressive treatment for some patients.