CHICAGO - Birth control pills may raise the already heightened risk of breast cancer faced by women with a strong family history of the disease, a study suggests.
Among sisters and daughters of women with breast cancer, users of the pill were three times more likely than nonusers to get the disease.
And if at least five family members had breast or ovarian cancer, pill users faced an 11-fold risk, the researchers reported in Wednesday's Journal of the American Medical Association.
Because granddaughters of women with breast cancer did not appear to be significantly more prone, the researchers suggested the risks were associated with older forms of the pill made before 1975. Older forms of the pill contained higher hormone levels.
But an accompanying editorial said it is unclear whether the risk is linked only to older pills. Instead, the granddaughters, whose average age was 43, may have simply been too young for breast cancer to have developed.
Birth control pills have long been thought to increase slightly women's risk of breast cancer, though recent research has suggested the risk disappears after women stop taking them. The data have been less clear on the pill's effect on women with a strong family history of the disease.
Thomas Sellers, a Mayo Clinic Cancer Center epidemiologist, said his study, which included 3,396 blood relatives of 426 women diagnosed with breast cancer between 1944 and 1952, is the first multigenerational study to address the question.
Relatives were questioned and medical records reviewed between 1991 and 1996.
The findings suggest the pill-linked risks are in women with the BRCA genetic mutations associated with breast and ovarian cancer, though genetic testing was not involved in the study. Sellers said he hopes the next phase of the research will answer that question as well as whether newer pills containing low doses of estrogen and progestin are safer.
The study raises troubling questions for women with the genetic mutations because birth control pills are one of the few proven methods of reducing the risk of ovarian cancer, Dr. Wylie Burke of the University of Washington wrote in the JAMA editorial.
The study "argues for avoidance of oral contraceptives, but at the price of forgoing an attractive option for reducing ovarian cancer risk," Burke said. Ovarian cancer is harder to detect and often more deadly than breast cancer.
"These trade-offs underscore the need for careful attention to what is known and what is not known when women at high risk consider their options," he said.
Journal of the American Medical Association: http://jama.ama-assn.org
National Cancer Institute: http://cancernet.nci.nih.gov/cancer-types/breast-cancer.shtml