A 38-year-old woman with bipolar I disorder has been started on carbamazepine 200 mg twice daily. After 4 weeks, her serum level is 8.2 mcg/mL and she is clinically stable. She also takes an oral contraceptive containing ethinyl estradiol and levonorgestrel. At her follow-up visit 8 weeks later, her carbamazepine level has dropped to 5.1 mcg/mL despite medication adherence, and she reports a breakthrough manic episode. Which of the following best explains these clinical findings?