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

The relationship between the risk of breast cancer and oral contraceptive use was investigated in a case-control study conducted in France between 1983 and 1987 in five public hospitals. Some 464 cases aged 25 to 56 years and 542 matched controls were interviewed about their history of the use of oral contraceptives (OC). Results are given for the entire population and for the subgroup of 358 and 379 premenopausal cases and controls. The multivariate relative risk estimate, for ever user, was 1.5 (p less than 0.01) in the whole group as well as in the premenopausal subgroup (p less than 0.02). However, there was no evidence that the effect varied appreciably according to duration of use, age at first use, use before first full-term pregnancy (FFTP) and time since first or last use. The risk was not altered for any particular brand of OC. We conclude that, because of the widespread attention given to the relationship between OC use and breast cancer, information bias might be responsible for part of the excess in risk observed among OC ever users.