A 32-year-old woman on combined oral contraceptive pills (ethinyl estradiol + desogestrel) is diagnosed with epilepsy and started on phenytoin. After 2 months, she becomes pregnant. The contraceptive failure with phenytoin differs from rifampicin failure in which important respect?
- A Phenytoin increases sex hormone-binding globulin (SHBG) synthesis, reducing free hormone levels
- B Phenytoin reduces ethinyl estradiol levels by CYP3A4 induction, similar to rifampicin, but also has direct teratogenic effects ✓
- C Phenytoin impairs gastrointestinal absorption of OCPs
- D Phenytoin is a weaker CYP inducer than rifampicin and contraceptive failure is not clinically significant
Correct answer: B. Phenytoin reduces ethinyl estradiol levels by CYP3A4 induction, similar to rifampicin, but also has direct teratogenic effects
Explanation
Phenytoin induces CYP3B4, accelerating ethinyl estradiol metabolism and reducing contraceptive efficacy, similar to rifampicin. The critical additional point is that phenytoin is itself a known human teratogen (fetal hydantoin syndrome), making contraceptive failure particularly dangerous. Rifampicin is a potent CYP3B4 inducer but is not teratogenic. Carbamazepine, phenobarbital, and topiramate also induce CYP3B4 and reduce OCP efficacy. The teratogenic risk of enzyme-inducing antiepileptics combined with contraceptive failure is a key counseling point.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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