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In women who use combination oral contraceptives, concurrent use of efavirenz (a non-nucleoside reverse transcriptase inhibitor for treating AIDS) attenuates serum levels of progestin (Journal Watch Womens Health Apr 28 2011). Now, two case reports of unintended pregnancies among users of subdermal contraceptive implants who were also taking efavirenz indicate that efavirenz's induction of cytochrome p450 function might reduce the effectiveness of such implants.