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The effect of carbamazepine on the etonogestrel contraceptive implant

Aaron Lazorwitz, MD, University of Colorado, 2015

Project abstract

The etonogestrel (ENG) contraceptive implant is one of the most effective methods currently available and uptake in the US is increasing. We found no published data regarding the pharmacokinetic and pharmacodynamic effects of common anti-epileptic drugs such as carbamazepine, a known cytochrome P-450 enzyme inducer, on the ENG implant. The US Centers for Disease Control 2010 Medical Eligibility Criteria rates the concomitant use of the contraceptive pill, patch, or ring and carbamazepine as class 3, but use of the contraceptive implant and carbamazepine as class 2. This may lead providers to believe that the contraceptive implant is a more effective option for women using carbamazepine, despite no known evidence to support this. We aim to explore the contraceptive effectiveness of the ENG implant with concomitant use of carbamazepine. This study of healthy, reproductive age women with an ENG implant in place will compare their baseline ENG serum levels to their ENG serum level after three weeks of taking a titrated dose of carbamazepine. We expect to find at least a 40% decrease in ENG levels. Such a decrease is clinically concerning if it results in sub-therapeutic (<90pg/ml) eng levels that could lead to failed ovulation suppression. participants will also undergo transvaginal ultrasound to investigate potential pharmacodynamic evidence of impaired ovulation. given that unintended pregnancies impose emotional, social, and financial costs, it is imperative to better understand drug-drug interactions that may decrease contraceptive efficacy.

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