Oral contraceptives and breast cancer risk: a case-control study.

The association between breast cancer risk and oral contraceptive use was examined in 401 breast cancer patients and 519 hospital controls interviewed in New York City during 1979-1981. Control subjects were ascertained utilizing variable ratio matching to the cases (2:1 or 1:1) by sex, age, hospital, and time of diagnosis. No evidence of a positive association was found between cancer risk and the duration of use in either parous or nulliparous women. The odds ratios obtained by comparing users to non-users in women under 50 years of age after adjusting for other risk factors were 0.8 (95% CI = 0.4-1.4) for less than five years duration and 0.4 (95% CI = 0.2-0.8) for five or more years duration (P less than 0.05 when tested for decreasing trend). There was also no evidence of effect modification between oral contraceptive use and other breast cancer risk factors (viz. family history, nulliparity, late age at first pregnancy, or abstention from breastfeeding). Our results do not indicate that the use of oral contraceptives increases the risk of breast cancer.