What is the most significant drug interaction concern with rifampin?

• A. Potent hepatic enzyme induction that reduces levels of many drugs (e.g., antiretrovirals, OCPs, anticoagulants).
• B. Inhibits metabolism of many drugs.
• C. No significant interactions.
• D. Increases absorption of other TB drugs.

Answer: (A)

Rifampin is a potent inducer of hepatic drug-metabolizing enzymes, especially the cytochrome P450 system (notably CYP3A4) and certain transporters. This induction speeds up the metabolism and clearance of many coadministered drugs, often driving their levels down into subtherapeutic ranges. That makes this the most significant drug interaction concern with rifampin. Clinically, it can compromise antiretroviral therapy by lowering exposure and risking virologic failure and resistance, reduce the effectiveness of hormonal contraceptives leading to unintended pregnancy, and produce inadequate anticoagulation with drugs like warfarin. Other affected drugs include various anticonvulsants, methadone, corticosteroids, and many others. Because the effect results from enzyme induction, it develops over days to weeks after starting rifampin and can persist for a while after stopping, so careful monitoring and possible regimen adjustment are needed. Rifampin does not inhibit metabolism nor does it reliably increase absorption of other TB drugs, which is why those options aren’t the primary concern.