Carbamazepine is a potent inducer of CYP3A4 and also autoinduces its own metabolism. A patient with bipolar disorder stabilised on carbamazepine is started on oral contraceptives. Which clinical consequence is most likely?

• A Carbamazepine toxicity due to inhibited CYP3A4
• B Increased carbamazepine levels due to OCP-mediated CYP inhibition
• C Oral contraceptive failure due to increased ethinyl estradiol metabolism ✓
• D Serotonin syndrome due to combined monoamine activity

Explanation

Carbamazepine strongly induces CYP3A4, the primary enzyme metabolising ethinyl estradiol in combined oral contraceptives. This induction accelerates estrogen catabolism, resulting in subtherapeutic OCP levels and contraceptive failure. Patients on carbamazepine need higher-dose contraceptives or non-hormonal methods. Carbamazepine does not inhibit CYP3A4 and OCPs are not clinically significant CYP3A4 inhibitors.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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