Carbamazepine reduces quetiapine by what percentage?

• A. 50%
• B. 36-71%
• C. 80% - undetectable levels
• D. 10%

Answer: (C)

When a drug acts as a strong enzyme inducer, it ramps up the body's ability to metabolize other drugs. Carbamazepine is a potent inducer of CYP3A4, the main enzyme that metabolizes quetiapine. This makes quetiapine clear from the body faster, so its blood levels drop dramatically. The magnitude of that drop can be quite large, often around 80% and can even bring levels to undetectable in some cases. That’s why this option best reflects the potential severity of the interaction—the quetiapine effect can essentially be wiped out when paired with carbamazepine. In practice, this means poor antipsychotic efficacy may result, so alternatives or dose adjustments are considered under careful supervision, with close monitoring for response.