Statistical evidence about the mechanism of action of the Yuzpe regimen of emergency contraception.
OBJECTIVE
To determine whether published statistical evidence about the effectiveness of the Yuzpe regimen of emergency contraception provides insight about its mechanism of action.


METHODS
We searched the literature for studies that present information on the effectiveness of the Yuzpe regimen, on the probability of conception by menstrual cycle day, or on the occurrence of ovulation in women treated with the regimen. Searches of the electronic databases MEDLINE, POPLINE, EMBASE, and BIOSIS were supplemented by scrutiny of the bibliographies of all papers identified through the electronic search.


METHODS
We identified a review of the effectiveness of the Yuzpe regimen based on all seven clinical trials that present the number of women treated on each cycle day and the outcome of each treatment; this review also provided external estimates of the probability of conception by cycle day of unprotected intercourse from two other clinical studies. We identified three clinical studies of ovulation after treatment with the Yuzpe regimen. We included all identified studies in our analysis.


RESULTS
We compared 40 estimates of the actual effectiveness of the Yuzpe regimen with the maximum theoretical effectiveness that could be obtained if the regimen worked only by preventing or delaying ovulation. In the overwhelming majority of these comparisons, the former exceeded the latter.


CONCLUSIONS
The Yuzpe regimen could not be as effective as it appears to be if it worked only by preventing or delaying ovulation.
Office of Population Research, Princeton University, New Jersey 08544, USA. trussell@princeton.edu
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J
Trussell
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Raymond
D002986:Clinical Trials as Topic; D003267:Contraception / Q000379:methods*; D003277:Contraceptives, Oral, Combined / Q000494:pharmacology*; D003281:Contraceptives, Postcoital / Q000494:pharmacology*; D005260:Female; D005306:Fertilization / Q000187:drug effects*; D006801:Humans; D010060:Ovulation / Q000187:drug effects*
D003277:Contraceptives, Oral, Combined; D003281:Contraceptives, Postcoital