Comparison of three single doses of mifepristone as emergency contraception: a randomised controlled trial

This is an analysis of the Australian component of a large World Health Organization multicentre dose-finding study of mifepristone for emergency contraception and the first clinical study of this controversial drug in Australia.

To compare the effectiveness and side-effects of three single doses of mifepristone taken within 120 h after unprotected coitus as emergency contraception.

Double-blind, randomised controlled trial.

One hundred fifty healthy women with regular menstrual cycles who requested emergency contraception. Participants were allocated randomly to one of the three doses (10, 50 and 600 mg). The primary outcome was confirmed pregnancy, and secondary outcome measures included side-effects and delay in the onset of the next menses.

Pregnancy rates for mifepristone 10, 50 and 600 mg were 2.0, 2.1 and 2.1%, respectively, with no significant difference between groups. No major side-effects occurred, except an unpredictable delay in the onset of the next menses. Mifepristone 600 mg caused a significantly longer delay in the onset of the next menses than either the 10 or the 50 mg dose.

Lowering the dose of mifepristone from 600 to 10 mg did not significantly impair its effectiveness as an emergency contraceptive, and caused less delay in the onset of the next menses. Therefore, a dose as low as 10 mg may be preferable to 600 mg for emergency contraception. This is very much lower than the dose required to terminate a pregnancy.