During the interval 1968-74, 17,032 women aged 25-39 years were recruited to the Oxford-Family Planning Association contraceptive study, more than half of whom were using oral contraceptives. These women have been followed up over the years and breast cancer has been diagnosed in 189 of them. We have analysed the available data in two ways. First, we have calculated standardised breast cancer incidence rates in non-users and users of oral contraceptives according to total duration of use, interval since first use, interval since last use, duration of use before first term pregnancy and duration of use before age 25. Secondly, we have conducted case-control within cohort analyses to examine the possible effects of different types of pill and to search for evidence of a latent effect of oral contraceptive use before first term pregnancy on breast cancer risk. We have found no evidence of any adverse effect of oral contraceptive use on the risk of breast cancer in this study. There was, however, little exposure to the pill before first term pregnancy among the participants and virtually no such exposure at a very young age (i.e. below 20 years). Accordingly, the results of this study strengthen the evidence that oral contraceptive use by mature women does not increase breast cancer risk, but add little to the uncertainty about the effects of early use.
PIP: No evidence of increased risk for breast cancer was found in a study of potential adverse effects of oral contraceptives use. The 1988 study was based on 189 diagnosed cases of breast cancer among 17,032 women aged 25-39 in England and Scotland during 1968-74. More than half of all the women studied were using an oral contraceptive. The study examined breast cancer incidence rates in nonusers and users of oral contraceptives according to total duration of use, interval since 1st use, interval since last use, duration of use before 1st term pregnancy, and duration of use before age 25. The study also attempted to analyze possible effects of different types of pills and search for latent effects of oral contraceptive use before 1st-term pregnancy on breast cancer risk. The study supports other studies showing that the use of oral contraceptives by women during the middle fertile years (ages 25-39) has no adverse effect on breast cancer risk. Because so few women included in the study were younger than age 25, the study's findings were inconclusive regarding the risk of breast cancer associated with early use of oral contraceptives. Regarding the possible effects of different types of pills and the possible latent effects of oral contraceptive use before 1st-term pregnancy, the study was unable to discern any clear pattern or trend.