Objective: To determine whether a 5-mg dose of mifepristone is sufficient to prevent pregnancy.
Design: Clinical study.
Setting: Academic research center.
Subject(s): Healthy, fertile, sexually active female volunteers.
Intervention: Volunteers received a 5-mg dose of mifepristone once weekly, starting on cycle day 2, for up to 6 months. This was their only contraceptive method.
Main outcome measure(s): Number of pregnancies.
Result(s): The treatment resulted in a significant decrease in pregnancy rate without affecting the menstrual cycle or causing disturbing side effects.
Conclusion(s): A low dose of mifepristone, which does not inhibit ovulation, reduces fertility significantly by affecting the endometrium. However, the contraceptive effect needs to be improved for the drug to compete with other contraceptive methods.
PIP: Clinical research has demonstrated that the effect of mifepristone on the endometrium is sufficient to prevent pregnancy. The efficacy of a low dose of mifepristone in preventing implantation was investigated in 18 healthy, fertile women with normal menstrual cycles from Stockholm, Sweden. Study participants received 5 mg of mifepristone once/week, starting on cycle day 2, and were followed for 1-6 months. Three pregnancies occurred in the 63 treatment cycles observed. The mean frequency of sexual intercourse was 2.2 times/week. If ovulation occurred 14 days before the onset of menstruation, at least 1 coital act took place during the period 3 days before and 1 day after ovulation in at least 45 cycles, resulting in a probability of pregnancy of 0.067. Without treatment, 14 pregnancies would have been expected. Although ovulation was not measured objectively, the regular bleeding pattern recorded in all cycles but one makes it unlikely that the contraceptive effect of mifepristone was due to ovulation inhibition. Although the failure rate of 5 mg of mifepristone is unacceptably high for a contraceptive regimen, other treatment schedules merit investigation.