Available evidence concerning oral contraceptives and their potential interaction with reproductive history on breast cancer risk Is reviewed. The relative risks in 15 out of 15 studies were above unity among younger women (i.e., below age 35 and perhaps up to 45) for long-term oral contraceptive use, although apparent heterogeneities emerged in the risk estimates. The overall evidence is reassuring in subsequent age groups, whereas the modifying effects or interactions between oral contraceptives, reproductive factors and breast cancer risk are still largely undefined. Thus we suggest that, besides chance and bias, the apparent discrepancies between various studies should be considered within the framework of the complex time- and age-effects of hormone-related risk factors on breast carcinogenesis.
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