Post-marketing surveillance of oral contraceptives.

Problems arise in the post-market surveillance of the possible long-term effects of oral contraceptives. These drugs are taken by very large numbers of people, in various doses, types and duration. One aspect is considered in this paper, the effects on pregnancy of these drugs taken before conception. Retrospective case trial studies are described together with longitudinal studies on women from their first exposure to a specific oral contraceptive. The difficulties of this approach, the causes of bias, the effect of social habits, and the discontinuity of usage are debated.