A 28-year-old woman presents 60 hours after unprotected intercourse. She is on phenytoin for epilepsy and has a negative urine pregnancy test. She asks for emergency contraception. Which is the most appropriate next step?

Explanation

## Emergency Contraception in Drug-Induced Enzyme Induction **Key Point:** Phenytoin is a potent CYP3A4 inducer that significantly reduces the bioavailability and efficacy of levonorgestrel. In women on enzyme-inducing drugs, the copper IUD becomes the preferred emergency contraceptive. **High-Yield:** Enzyme-inducing drugs that reduce hormonal contraceptive efficacy: - **Anticonvulsants:** Phenytoin, carbamazepine, phenobarbital, oxcarbazepine - **Antibiotics:** Rifampicin (most potent), rifabutin - **Antiretrovirals:** Ritonavir, efavirenz - **Others:** St. John's Wort, griseofulvin **Clinical Pearl:** When a woman on enzyme-inducing drugs requires emergency contraception, hormonal methods (levonorgestrel, ulipristal) are unreliable. The copper IUD is the gold standard because it works via a non-hormonal mechanism (spermicide + endometrial inflammation) and is unaffected by drug interactions. ## Emergency Contraception Options in Drug Interactions | Method | Phenytoin Effect | Efficacy | Recommendation | |--------|------------------|----------|----------------| | Levonorgestrel | ↓↓ (CYP3A4 induction) | Reduced ~50% | NOT recommended | | Ulipristal acetate | ↓ (CYP3A4 substrate) | Reduced | NOT recommended (less affected than levonorgestrel, but still suboptimal) | | Copper IUD | None (non-hormonal) | 99% | **FIRST-LINE** | | Mifepristone | ↓ (CYP3A4 substrate) | Reduced | Not widely available; not first-line | **Mnemonic:** **RICE** — **R**ifampicin, **I**soniazid, **C**arbamazepine, **E**nzyme inducers reduce hormonal EC efficacy; use **IUD** instead. ## Why Copper IUD Is the Best Next Step 1. **Drug-independent mechanism:** Works via spermicide and endometrial inflammation, not affected by phenytoin. 2. **Timing:** Still within the 5-day window for copper IUD efficacy (60 hours = 2.5 days). 3. **Highest efficacy:** 99% contraceptive efficacy, superior to any hormonal method in this context. 4. **Dual benefit:** Provides both emergency contraception and long-term contraception (can remain in situ for 10 years). 5. **Evidence-based:** NFSG and WHO recommend copper IUD as first-line for women on enzyme-inducing drugs. [cite:Park 26e Ch 10; KD Tripathi 8e Ch 66]