Women who recently used birth control pills containing high-dose estrogen and a few other formulations had an increased risk for breast cancer, whereas women using some other formulations did not, according to data published in Cancer Research, a journal of the American Association for Cancer Research.
"Our results suggest that use of contemporary oral contraceptives [birth control pills] in the past year is associated with an increased breast cancer risk relative to never or former oral contraceptive use, and that this risk may vary by oral contraceptive formulation," said Elisabeth F. Beaber, PhD, MPH, a staff scientist in the Public Health Sciences Division of Fred Hutchinson Cancer Research Center in Seattle, Washington.
"Our results require confirmation and should be interpreted cautiously," added Beaber. "Breast cancer is rare among young women and there are numerous established health benefits associated with oral contraceptive use that must be considered. In addition, prior studies suggest that the increased risk associated with recent oral contraceptive use declines after stopping oral contraceptives."
In a nested case-control study of 1,102 women diagnosed with breast cancer and 21,952 controls, Beaber and colleagues found that recent oral contraceptive use increased breast cancer risk by 50 percent, compared with never or former use. All study participants were at Group Health Cooperative in the Seattle-Puget Sound area. Patients received a cancer diagnosis between 1990 and 2009.
Birth control pills containing high-dose estrogen increased breast cancer risk 2.7-fold, and those containing moderate-dose estrogen increased the risk 1.6-fold. Pills containing ethynodiol diacetate increased the risk 2.6-fold, and triphasic combination pills containing an average of 0.75 milligrams of norethindrone increased the risk 3.1-fold.
Birth control pills containing low-dose estrogen did not increase breast cancer risk.
About 24 percent, 78 percent, and less than 1 percent of study controls who were recent oral contraceptive users filled at least one prescription in the past year for low-, moderate-, and/or high-estrogen dose oral contraceptives, respectively, according to Beaber.
Unlike most previous studies that depended on women's self-report or recall, which may cause bias, Beaber and colleagues used electronic pharmacy records to gather detailed information on oral contraceptive use including drug name, dosage, and duration of medication.