A 39-year-old male with bipolar I disorder is started on carbamazepine 200 mg twice daily. At 2 weeks, his serum level is 8.2 mcg/mL (therapeutic range 4-12 mcg/mL) with good symptom control. At his 6-week follow-up on the same dose, his serum level has dropped to 4.8 mcg/mL and he reports re-emergence of mood cycling and irritability. Which of the following best explains this clinical scenario?