Why do inadvertent pregnancies occur in oral contraceptive users? Effectiveness of oral contraceptive regimens and interfering factors.

Inadvertent pregnancies in combined pill users are not uncommon, and are usually due to errors of tablet taking. However, many factors may contribute to 'pill failure'. In this review the endocrine pharmacology of pill use and the changes reported with missed pills have been considered in detail. The influences of other factors including drug interactions have been reviewed and a series of recommendations made for reducing the risk of pregnancy in each of these circumstances.