Why is rifampin a potent inducer of liver enzymes and what is a common drug interaction?

• A. It inhibits CYP enzymes
• B. It reduces metabolism of many drugs
• C. It induces CYP enzymes, increasing metabolism of drugs like warfarin and oral contraceptives
• D. It has no effect on liver enzymes

Answer: (C)

Rifampin acts as a potent liver enzyme inducer by activating nuclear receptors (like PXR) that boost the production of cytochrome P450 enzymes, especially CYP3A4, and related drug transporters. With more of these enzymes available, many drugs are metabolized faster, so their blood levels drop. A common and clinically important interaction is with warfarin: rifampin speeds up warfarin metabolism, lowering its anticoagulant effect and often requiring a higher dose or a different regimen. Another well-known interaction is with oral contraceptives: increased metabolism can reduce contraceptive effectiveness, risking breakthrough bleeding or pregnancy. Because enzyme induction builds over days to weeks, these interactions typically appear after some time of therapy and can linger after rifampin is stopped. The other statements misstate the mechanism—rifampin does not inhibit enzymes or reduce their activity, and it does affect liver enzymes.