title: Minimum effectiveness of the levonorgestrel regimen of emergency contraception.
abstract: The standard method for estimating the effectiveness of emergency contraceptive pills (ECPs) uses external data to calculate the proportion of expected pregnancies averted by the treatment. Because these data may not be applicable to ECP study populations, this approach could result in substantial overestimation of effectiveness. We used data from two published randomized trials of the levonorgestrel and Yuzpe ECP regimens to calculate the minimum effectiveness of the levonorgestrel regimen. Conservatively assuming that the Yuzpe regimen was entirely ineffective in these trials, we estimate that the levonorgestrel regimen prevented at least 49% of expected pregnancies (95% confidence interval: 17%, 69%). Because physiologic data suggests that the Yuzpe regimen does, in fact, have some efficacy, the effectiveness of the levonorgestrel regimen is likely to be higher than our minimum estimate.
authors.affiliation: Family Health International, PO Box 13950, Research Triangle Park, NC 27709, USA. eraymond@fhi.org
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Raymond
authors.forename: Douglas
authors.initials: D
authors.lastname: Taylor
authors.forename: James
authors.initials: J
authors.lastname: Trussell
authors.forename: Markus J
authors.initials: MJ
authors.lastname: Steiner
mesh_terms: D000328:Adult; D003280:Contraceptives, Oral, Synthetic / Q000008:administration & dosage* / Q000627:therapeutic use; D003282:Contraceptives, Postcoital, Hormonal / Q000008:administration & dosage* / Q000627:therapeutic use; D005260:Female; D006801:Humans; D016912:Levonorgestrel / Q000008:administration & dosage* / Q000627:therapeutic use; D011247:Pregnancy; D016032:Randomized Controlled Trials as Topic; D016896:Treatment Outcome
chemical_list: D003280:Contraceptives, Oral, Synthetic; D003282:Contraceptives, Postcoital, Hormonal; D016912:Levonorgestrel