Use of oral contraceptives and risk of breast cancer in young women.

Many studies have shown that oral contraceptive (OC) use increases a young woman's risk of breast cancer, although some studies suggest that the risk may be limited to recent use. The objective of this study was to determine what particular aspects of OC use could be important for breast cancer development at an early age in the cohort of women who had the opportunity to use OCs all of their reproductive life. The cases were first diagnosed with breast cancer at age 40 or younger between 1983 and 1988, and identified by the Los Angeles County Cancer Surveillance Program. Control subjects were individually matched to participating cases on birth date (within 36 months), race (white), parity (nulliparous versus parous), and neighborhood of residence. Detailed OC histories were obtained during in-person interviews with subjects. In general the risk estimates were small, and not statistically significant. Compared to no use, having used OCs for 12 years or more was associated with a modest non-significant elevated breast cancer risk with an odds ratio (OR) of 1.4 (95% confidence interval (CI) = 0.8-2.4). Long-term (12 years or more) users of high-dose estrogen pills had a non-significant 60% higher breast cancer risk than never users (CI = 0.9-3.2). Early use was associated with slightly higher ORs among young women (age < or =35), and among parous women. Recent use was associated with somewhat higher ORs among parous women and women above age 36. Analyses by stage, body weight, and family history yielded similar results. This study is consistent with a modest effect of early OC use on breast cancer risk in young women.