Oral contraceptives and breast cancer. A meta-analysis.

Since the relationship between oral contraceptive use and breast cancer has not been consistent we undertook a meta-analysis of studies published to date. Papers were located by searching the MEDLINE data base, supplemented by a hand search of all the references in the articles recovered. Studies were graded as to quality. Those judged as probably unbiased were included in the analysis. The method of Woolf was used to combine relative risks. Forty-seven studies were collected: 40 case-control and 7 cohort studies. Thirty-nine of these were considered unbiased. The main results observed were: RR was 1.06 (1.02-1.10) for all studies and 1.14 for premenopausal cancer. For premenopausal cancer, higher RRs were observed in women who early used oral contraceptives with a significant linear dose-response effect: 1.25 (1.10-1.44) in OC users before age 25, and 1.17 (1.06-1.30) in users before the first full-term pregnancy. We conclude oral contraceptive use may be a risk factor for premenopausal breast cancer. Limitations to our research are discussed.