Mechanisms of action of mifepristone when used for emergency contraception.

An emergency contraceptive method is used after coitus but before pregnancy occurs. The use of emergency contraception is largely underutilized worldwide. Recently, treatment with 10 mg mifepristone as a single dose has emerged as one of the most effective hormonal methods for emergency contraception, with very low side effects. However, the mechanism of action of mifepristone in humans when used for contraceptive purposes and especially for emergency contraception remains largely unknown. The objective of this review is to summarize available data on the effect of mifepristone on female reproductive functions relevant to emergency use of the compound. Taken together, available data from studies in humans indicate that the contraceptive effect of mifepristone used as a single low dose for emergency contraception is mainly due to impairment of ovarian function, either by blocking or postponing the luteinizing hormone surge, rather than to inhibiting of implantation.