Studies comparing the effectiveness of the Yuzpe regimen with a single 600-mg dose of mifepristone for emergency contraception suggest that a single dose of mifepristone appears to be the better option. The Yuzpe regimen requires two doses of 100 mg each of ethinyl estradiol plus 500 mg of levon-orgestrel given 12 hours apart within 72 hours of unprotected intercourse. Both regimens seem equally effective in preventing pregnancy, but women who took mifepristone reported less nausea and vomiting. Lower dosages of mifepristone may be equally effective in providing emergency contraception and should be considered, both to reduce cost and to follow the principle that patients should receive the lowest effective dosage of a drug. The Task Force on Post-ovulatory Methods of Fertility Regulation compared the effectiveness and side effects, including the timing of the next menstrual period, of three different dosages of mifepristone given within five days of unprotected intercourse.

Women eligible for this multinational randomized controlled trial had contacted one of 11 family planning clinics in order to prevent pregnancy. To be included in the study, women had to request contraception within 120 hours of a single act of unprotected intercourse and were required to have regular menstrual cycles, be willing to abstain from intercourse for the remainder of their cycle and have no contraindication to the medication. Women who met the inclusion criteria were randomized to receive either 600 mg, 50 mg or 10 mg of oral mifepristone. Once included in the study, the women were asked to refrain from intercourse or to use barrier methods of contraception until the onset of the next menses. In addition, they were asked to record all symptoms, particularly vaginal bleeding, and any further sexual activity. Follow-up was scheduled one week after onset of their next menses or two weeks after the date of their next anticipated menses, if bleeding did not occur. Pregnancy testing was performed at the follow-up visit in women whose menses had not started, and ultra-sonography was used to confirm and date any suspected pregnancies.

A total of 1,717 women were enrolled in the study; data were collected for 1,684 women. Baseline characteristics were similar for the three groups. Approximately 57 percent of the women reported condom failure, and 41 percent reported that they had not used a contraceptive at the time of intercourse. Between 14 and 18 percent of each group reported a delay of more than 72 hours between coitus and treatment. Twenty-one women were pregnant after treatment, compared with an expected total of 136 without treatment. Of these, one woman was already pregnant at the time of treatment, and six others may have conceived two weeks after treatment. In women who abstained from intercourse for the remainder of the cycle, the pregnancy rates for all three treatment groups was 0.9 percent. Overall, mifepristone prevented 85 percent of the pregnancies that would have occurred without treatment. The only side effect of treatment was delayed onset of menses, which was found to be significantly related to the size of the dose. Delays of more than seven days occurred in 36 percent of those receiving 600 mg, 23 percent of those receiving 50 mg and 18 percent of those receiving 10 mg.

The authors conclude that the lowest dosage of mifepristone is as effective in preventing pregnancy as the larger dosages, making it a more affordable choice. The smaller dosage also resulted in the lowest incidence of side effects, particularly delay in onset of the next menses.