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Study: Recent Use of Birth Control Pills May Increase Breast Cancer Risk

Low-dose estrogen pills appear to be safest

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 new data published in Cancer Research.

“Our results suggest that use of contemporary oral contraceptives 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 at the Fred Hutchinson Cancer Research Center in Seattle, Washington.

“Our results require confirmation and should be interpreted cautiously,” Beaber added. “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 her colleagues found that recent oral contraceptive use increased breast cancer risk by 50% compared with never or former use.

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 mg of norethindrone increased the risk 3.1-fold.

Birth control pills containing low-dose estrogen did not increase breast cancer risk.

Approximately 24%, 78%, and less than 1% 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 her colleagues used electronic pharmacy records to gather information on oral contraceptive use, including drug name, dosage, and duration of medication.

The new study was funded by the National Cancer Institute.

Source: Fred Hutchinson Cancer Research Center; August 1, 2014.

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