A 30-year-old woman with bipolar I disorder is stable on quetiapine 400 mg at bedtime and an oral contraceptive. She is diagnosed with epilepsy and her neurologist starts carbamazepine 200 mg BID. Two weeks later, she presents with worsening insomnia, racing thoughts, and irritability. The PMHNP suspects a drug interaction. Which of the following best explains this clinical scenario?