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The drug, Aromasin (exemestane), has been shown to reduce the odds of breast cancer by 65 percent, but it also worsens bone density by about three times in older women who are taking it, Canadian researchers report.
"The drug did affect bone density at the hip and spine," said lead researcher Dr. Angela Cheung, a senior scientist at the University Health Network in Toronto. "It does not affect everyone; about 65 percent of women have some bone loss."
The fear of bone loss is not a reason not to take the drug, Cheung said. "You really need to pay attention to your bone health when you take this medication, especially for preventing breast cancer."
Exemestane is an aromatase inhibitor and works by suppressing the female hormone estrogen. These drugs are standard treatment for postmenopausal women with early stage hormone-receptor-positive breast cancer.
It had been speculated that exemestane, a third-generation aromatase inhibitor, might result in less bone loss than other similar drugs and might even stimulate bone formation.
For the new study, Cheung's team looked at bone loss among the more than 4,500 women who took part in a trial that compared exemestane with a placebo.
Among women taking the drug, the risk of developing breast cancer was lowered 65 percent, compared with women taking a placebo.
Among the 351 women in whom bone loss was studied, the researchers found that after two years there was an 8 percent loss of cortical bone in women taking exemestane, compared with 1 percent in the placebo group.
Cortical bone is the outer shell of bone that provides most of the bone support, and its loss accounts for about 80 percent of fractures in older people, the researchers noted.
The findings were published in the Feb. 6 online edition of The Lancet Oncology.
Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City, was somewhat cautious about the new research. She said that "the study needs longer follow-up to see if there is an increased risk of fracture."
"This study doesn't mean that we should stop using these drugs," she said. "We certainly rely on aromatase inhibitors more than tamoxifen in postmenopausal women, because the survival benefit has been proven."
The benefit of the drug outweighs that risk for most women, she said. However, if there is a family history of osteoporosis it may not be the best choice, Bernik said.
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