Side effects are causing women to stop taking pills that can help keep breast cancer from returning, and women who stop taking the pills risk early death, according to several reports released on Thursday.
One study found that women in Scotland were not taking the breast cancer pill tamoxifen as directed, and a second one found that a new type of breast cancer pill was causing mysterious aches and pains in women.
The women who did not take tamoxifen as directed were 16 percent more likely to die, researchers told a meeting of breast cancer experts in San Francisco.
“We need to acknowledge that this is something that is truly affecting our patients,” Dr. Julie Gralow of the University of Washington School of Medicine told reporters in a telephone briefing.
Tamoxifen transformed breast cancer therapy when it was shown to reduce the risk of cancer coming back by close to 50 percent. But the pill, sold by AstraZeneca under the brand name Nolvadex and also available generically, raises the risk of death from strokes and endometrial cancer.
A newer class of drugs, the aromatase inhibitors, appear to be safer, but they cannot be taken by women who are not yet through menopause.
Dr. Alastair Thompson of the University of Dundee in Scotland and colleagues looked at the records of 2,080 women treated for breast cancer between 1993 and 2002.
Ten percent of the women failed to fill 30 percent or more of their tamoxifen prescriptions. These women were 16 percent more likely to die over the study period than women who took the pills as directed.
Although tamoxifen is shown to work best when taken for five years, the women in the study took the drug for an average of just under 2 1/2 years.
“It could well be a question of side effects,” Thompson told reporters. “I think most women who take tamoxifen know that it does have very substantial and life-affecting side effects, even if just hot flashes, which can make life a misery.”
In another study, Dr. Lynn Henry of the University of Michigan Comprehensive Cancer Center found that 13 percent of women taking an aromatase inhibitor asked to stop because of muscle aches.
Overall, 23 percent of 100 women enrolled in a trial comparing two different aromatase inhibitors dropped out because of side effects, Henry told the Breast Cancer Symposium, sponsored by five cancer care societies, including the American Society of Clinical Oncology.
“The vast majority of patients are switching to something else. They understand that they still need to have their breast cancer treated,” Henry told reporters.
They may need to switch to another aromatase inhibitor or tamoxifen, she said.
Aromatase inhibitors include anastrozole, made by AstraZeneca under the brand name Arimidex, and exemestane, made by Pfizer Inc. under the brand name Aromasin.
Breast cancer is the second-leading cause of cancer death among U.S. women, after lung cancer. It kills 500,000 people globally every year.