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Women with chronic medical conditions are often encouraged to time their pregnancies carefully; thus, many seek reassurance that their medications will not diminish the efficacy of their contraceptives (or vice versa). Carbamazepine causes birth defects and reduces the efficacy of oral contraceptives by inducing liver enzymes (see U.S. Medical Eligibility Criteria for Contraceptive Use). To investigate whether carbamazepine also adversely affects serum etonogestrel levels in women using subdermal contraceptive implants, researchers conducted a crossover study in 10 healthy women who had used an implant for a median of 23 months.
Median etonogestrel concentrations differed significantly before versus after carbamazepine coadministration (158.…