Pills with ethynodiol diacetate -- a progestin -- appeared to increase the risk of breast 2.6 fold, Beaber said.
Risks seemed lower with moderate-dose estrogen pills -- those with 30 to 35 micrograms of ethinyl estradiol or 50 micrograms mestranol were linked to a 1.6 times higher risk of breast cancer.
How can a woman tell if she's taking a formulation linked to a higher risk? "The specific doses and types of hormones used in oral contraceptives are included in packaging information," Beaber said.
The study, funded by the National Cancer Institute, was published Aug. 1 in the journal Cancer Research.
Beaber stressed that the study results need to be confirmed before any recommendations can be made to women. The results are based on data about recent oral contraceptive use who were diagnosed with breast cancer and nearly 22,000 healthy women who served as the comparison group. The women were all between the ages of 20 and 49.
The researchers used electronic pharmacy records to gather information on prescriptions filled and information on formulas. The study looked at the years 1990 through 2009.
The researchers evaluated the risks of breast cancer in women who had taken birth control pills in the past year compared to former or never users. They then looked at risk with the specific formulas of birth control pills.
The study results suggest that the lower-dose estrogen pills, which became popular in the 1990s, are not a problem, said Dr. Courtney Vito, a breast surgeon and assistant clinical professor of surgical oncology at the City of Hope Comprehensive Cancer Center in Duarte, Ca.
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