Epidemiological problems associated with studies of the safety of oral contraceptives

Abstract

Much of the needed information on the longterm medical effects of oral contraceptives will be obtained only by epidemiologic studies. Users are almost always self-selected and undergo further screening by their physicians on the basis of their medical histories. Patterns of use may be checkered with periods of discontinuance and changes in formulation prescribed. Furthermore, the drugs used, their formulations, and dose schedules are almost always subject to change. Both prospective and case-control methods can be used for these studies and it may be advantageous to use both for certain problems. The principal difficulties with prospective studies are the large sample sizes required and biases resulting from losses to follow-up. Problems with retrospective studies include potential bias from inappropriate sample selection, difficulties in obtaining histories of illnesses and contraceptive use, and the fact that the procedure does not provide information on the incidence of disease.