However, these important benefits could be overshadowed by an increased risk of breast cancer among pill users, especially if the risk persists after stopping the pill. Unfortunately, a clear understanding of the relationship is not yet available, even though there have been more than 50 investigations into the issue. Any association is likely to be small, and perhaps confined to particular subgroups of women such as those who use the pill at a young age for prolonged periods. Although some of the studies had a rather limited ability to detect a modest effect when considered alone, greater statistical power can be achieved by combining data from all studies in a meta-analysis. One such overview is currently underway in Oxford, and its findings are eagerly awaited.
Any overview is only as good as the data available to it.
Thus, the meta-analysis will be extremely useful for looking at the overall risks associated with pill use but less informative about the risk in young users. More research is clearly needed into this fundamental question. Nearly all of the evidence relating to the effects of different contraceptives comes from cohort or case-control studies. When interpreting the results of such observational studies, it is important to remember that association does not necessarily mean causation. Alternative explanations, such as bias, confounding or chance should be sought for any observed association.