Carbamazepine causes autoinduction of its own metabolism through which cytochrome P450 enzyme, and what is the clinical consequence?
- A CYP2D6 inhibition; after 4–6 weeks, co-administered drugs metabolised by CYP2D6 accumulate
- B CYP1A2 induction; theophylline levels rise requiring dose reduction
- C CYP3A4 induction; after 4–6 weeks of therapy, dose must be increased to maintain the same serum level ✓
- D CYP2C19 inhibition; phenytoin levels fall below therapeutic range
Correct answer: C. CYP3A4 induction; after 4–6 weeks of therapy, dose must be increased to maintain the same serum level
Explanation
Carbamazepine is a potent inducer of CYP3A4, including for its own metabolism (autoinduction). Over 3–6 weeks, CYP3A4 activity increases, accelerating carbamazepine clearance. Serum carbamazepine levels fall despite a constant dose, and the dose must be titrated upward to maintain efficacy. This is a classic example of time-dependent pharmacokinetics. CYP2D6 is not induced by carbamazepine; CYP1A2 induction would lower theophylline levels (not raise them); CBZ interacts with CYP2C19 substrates differently.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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