Oral contraceptives and breast cancer risk.

A large case:control study was conducted among women in the San Francisco Bay area to determine the association between oral contraceptive usage and breast cancer. The study included 1432 breast cancer patients diagnosed between 1973 and 1977 and 2560 matched controls 1 from the medical and 1 from surgical service of the same hospital for each patient. The contraceptive menstrual and reproductive histories of all patients were studied. The relative risk of developing breast cancer was 1.19 for users as compared with nonusers a nonsignificant association. The risk for usage of more than 2 years increased to 1.38 significant at the P=.02 level. The breast cancer patients reported a higher frequency of oral contraceptive use somewhat longer use a more advanced age at 1st use and more use prior to childbirth. Most of these differences were only marginally significant statistically. Changes in the progestogen-estrogen content of pills during the 1965-1968 period were not reflected in the breast cancer incidence. All data are tabulated in detail. Possible explanations for the findings which suggest that long-term oral contraceptive usage either continuous or interrupted may have a biological meaning to breast cancer occurrence are discussed. If the association is 1 of cause and effect the mechanism may be through enhancement of cell growth rather than by tumor induction. (Summary in FRE)