Lead investigator Lynn Rosenberg, an associate director of the Slone Epidemiology Center and professor of epidemiology at Boston University Medical Center, says the researchers used data from the Black Women's Health Study.
The researchers tracked 53,848 participants in the BWHS for 12 years, during which time 789 cases of breast cancer developed on which information on receptor status was obtained. The incidence of estrogen receptor negative cancer was 65 percent greater among women who had ever used oral contraceptives than among non-users of contraceptives.
The study, published online in the journal Cancer Epidemiology Biomarkers and Prevention, found the increase in risk was greatest for women who had used oral contraceptives within the previous five years and whose use had lasted 10 or more years.
In addition, the increase in risk was greater for estrogen receptor negative than for estrogen receptor positive breast cancer -- estrogen receptor positive tumors have a better prognosis than estrogen receptor negative breast cancers.