Oral contraceptives and breast cancer. Cause and effect?

Case-control studies dealing with breast cancer and oral contraceptives (OCs) were reviewed against a background of currently accepted rules for conducting and interpreting epidemiologic research. There is a strong consensus that OCs have not increased the risk of breast cancer in women over age 45, even when they have been used for long periods. The controversy is confined to studies of cancer diagnosed in young women. Although an effect of duration of OC use has been observed in some studies, there is neither a latency period nor an effect from recent or current use. There is no obvious relationship to the type of pill and no consistent effect of the dose of estrogen. The effect, if it exists at all, is not confined to any specific subgroup; it has not been related to age at starting OCs. It has been observed in only a few studies, some of which were subsequently discounted. Others have shown no effect or have suggested a reduced risk of breast cancer with increasing duration of use. None of the studies purporting to show an effect of OCs have controlled for the potential confounding effects of duration of lactation, induced abortion, recent pregnancy or a history of diseases that are associated both with reduced use of OCs and reduced risk of breast cancer. Most have inappropriately controlled for benign breast disease. There is an urgent need for a reanalysis of existing studies to take these factors into account. The lack of consistency and the potential for bias and confounding argue strongly against concluding, at this time, that there is a causal relationship between OCs and breast cancer.