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Oral contraceptives and breast cancer.

The question of a possible association between prolonged early use of combined oral contraceptives (COCs) and an increased risk of breast cancer remains unresolved despite the existence of at least 10 large-scale case-control studies and 5 large cohort studies. The marked differences in the findings of different studies especially with regard to COC use before the first full-term pregnancy may reflect differences in COC formulations precise patterns of use or risk factors for breast cancer. The variation in study results may also be explained by a latency effect in which a period of perhaps 15 years or more has to elapse before adverse effects of COCs on the breast become apparent. Even the large-scale epidemiologic investigations currently in progress (e.g. a nationwide study of breast cancer in British women up to 35 years of age a study in the Netherlands and a multicenter study coordinated by the World Health Organization) may not provide conclusive answers since widespread prolonged early use of COCs is a relatively recent phenomenon and breast cancer is not common in young women. Moreover COC formulations have changed over time meaning that any risk found to be associated with preparations used in the 1960s and 1970s may not apply to present-day COCs. Finally there is a need for more basic research on both the normal human breast and on breast cancer especially in relation to the effects of endogenous and exogenous hormones.