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To evaluate outcomes with simultaneous administration of mifepristone and misoprostol for medical abortion at 63 days of gestation or less in the year after its implementation in a British clinic system.We conducted a retrospective cohort study using deidentified data from the electronic booking and complications databases and medical records of women who underwent medical abortion at British Pregnancy Advisory Service. Our primary outcome was treatment success with simultaneous dosing compared with a regimen with a 24- to 48-hour interval between medications. We defined success as complete abortion without surgical evacuation and without continuing pregnancy. To assess relative regimen effectiveness while accounting for self-assignment to simultaneous or interval dosing, we modeled the probability of treatment success using logistic regression with propensity score adjustment for demographic and clinical characteristics. Secondary outcomes were reasons for abortion failure and clinically significant adverse events (hospital admission, blood transfusion, intravenous antibiotic administration).Of 28,901 women treated between May 2015 and April 2016, 85% chose simultaneous dosing. Overall success rates were high with both regimens but lower with simultaneous than with interval dosing (94.5% vs 97.1%, respectively, adjusted relative risk 0.973, 95% CI 0.967–0.979). For both regimens, success rates were lower at higher gestational ages, but the relative effectiveness of simultaneous dosing did not vary significantly with gestational age (P=.268). Surgical intervention rates for continuing pregnancy were lowest at 49 days of gestation or less (1.4% simultaneous vs 0.2% interval, P<.001) and highest at 57–63 days of gestation (5.0% and 2.2%, P<.001). The rate of clinically significant adverse events was 0.2% and did not differ by regimen (P=.972).Simultaneous administration of mifepristone and misoprostol is 97% as effective as a 24- to 48-hour interval at all gestational ages 63 days or less with no increase in the risk of clinically significant adverse events. Pragmatic use of simultaneous dosing is reasonable given the small difference in effectiveness.