The trial, presented today at the San Antonio Breast Cancer Symposium in Texas, suggests that women already at risk for cardiac conditions should limit use of these drugs, researchers said. Tamoxifen, a generic alternative approved in 1977, carries less risk of heart complications, the study found.

Aromasin, AstraZeneca’s Arimidex, and Femara from Novartis, are in a family of medicines known as aromatase inhibitors that work by halting production of estrogen, a hormone that fuels cancer growth. These drugs generated more than $3.5 billion last year, often used after tamoxifen, which prevents tumor cells from using estrogen. Two thirds of all breast-cancers are fed by estrogen, according to the National Institutes of Health.

“It appears that aromatase inhibitors have a significant increase in cardiotoxic side effects, such as heart attack, angina and heart failure,” said Eitan Amir, a senior fellow in oncology and hematology at the Princess Margaret Hospital in Toronto, in a statement. “Switching drugs may reduce the side effects.”

Heart disease is the leading cause of death in women, killing almost 433,000 annually, according to the American Heart Association. An estimated 209,000 women will be diagnosed with breast cancer this year, making it the most common tumor type in women, according to the American Cancer Society. More than 40,000 women die from it each year.

Heart, Fracture Risk

Women who took aromatase inhibitors were 26 percent more likely to have heart disease and 47 percent more likely to suffer a fracture than those on tamoxifen, regardless of how long they took the medicine, an analysis of seven studies used to win approval of the drugs found. The research was conducted in older, post-menopausal women with early breast cancer. Patients on tamoxifen were more likely to develop endometrial cancer and dangerous blood clots in the legs during the studies.

There was a suggestion that women who switched to aromatase inhibitors after starting on tamoxifen were less likely to die from something other than breast cancer than those who started treatment with the medicines. The approach didn’t seem to reduce the risk of serious side effects, the researchers said.

Pfizer, based in New York, reported revenue of $483 million from Aromasin last year. London-based AstraZeneca had sales of $1.92 billion for Arimidex. Novartis, of Basel, Switzerland, generated $1.27 billion from Femara in 2009.