A 82-year-old man with a CFS of 5 is admitted with delirium. He has a background of well-controlled epilepsy on Carbamazepine 400 mg BD. During his delirium workup, his Carbamazepine level is found to be subtherapeutic despite good adherence. He was recently started on Clarithromycin for a chest infection. What is the MOST likely explanation?