Carbamazepine can reduce olanzapine levels by up to what percent, potentially making them undetectable?

• A. 30%
• B. 10%
• C. 80%
• D. 50%

Answer: (C)

Carbamazepine acts as a strong inducer of hepatic cytochrome P450 enzymes, especially CYP3A4, which speeds up the metabolism of olanzapine. This powerful enzyme induction can markedly lower olanzapine concentrations in the blood, with reductions approaching about 80% and, in some cases, making levels undetectable by standard assays. Clinically, this means coadministration can drastically reduce olanzapine’s effect, so clinicians may need to adjust the antipsychotic dose or choose an alternative strategy and monitor response closely. Smaller reductions (10–50%) are possible with weaker interactions, but the potency of carbamazepine's induction is what makes the potential drop to near-undetectable levels plausible.