A 35-year-old man with a 10-year history of focal seizures (temporal lobe epilepsy) presents with recurrent breakthrough seizures despite adequate dosing of carbamazepine. Serum carbamazepine level is 8 µg/mL (therapeutic range 4–12 µg/mL). He has been compliant with medication. He is also on rifampicin for tuberculosis (recently started 3 months ago). What is the most likely explanation for his loss of seizure control?