RE: “BREAST CANCER BEFORE AGE 45 AND ORAL CONTRACEPTIVE USE: NEW FINDINGS”

Abstract

Miller et als recent study on the relationship between oral contraceptive (OC) use and breast cancer risk found a relative risk of 2.0 in ever-users under 45 years of age. This positive finding stands in contrast which a study conducted 3 years earlier (1983) that used the same case-control technique; in the earlier study the relative risk was 1.0. To determine whether this change in findings reflected new trends in OC use among cases and/or controls the authors compared the crude rates of OC ever-use in both the 1983 and 1986 studies. To control for age cohort the analysis focused on women who were 25-39 years of age in the earlier study compared with the total number of women from the later study. Among cases the rate of ever-use of OCs was equal (69%) in both periods. In contrast among controls the OC ever-use rate was 67% in the 1st study versus 58% in the 2nd study. Thus it would appear that the difference in the odds ratios in the 2 studies reflects different patterns of OC use among controls in 1983 versus 1986. This in turn appears due to the different diagnostic distributions of controls in the 2 years. 14% of controls in the earlier study versus 31% of those in the later study had acute infection as their presenting diagnosis and it cannot be assumed that their use of OCs reflects that found in the general population.