A 35-year-old woman on long-term phenytoin therapy for epilepsy presents requesting contraceptive advice. She is currently using a combined oral contraceptive (COC) containing 35 µg ethinyl estradiol and norethisterone. Her last menstrual period was 6 weeks ago, and a urine pregnancy test is negative. She reports breakthrough bleeding and irregular periods over the past 3 months. What is the most appropriate next step in management?

Explanation

## Clinical Context Phenytoin is a potent **cytochrome P450 enzyme inducer** (CYP3A4, CYP2C9, CYP2C19). It accelerates the hepatic metabolism of oestrogen and progestin, reducing contraceptive efficacy and causing breakthrough bleeding and contraceptive failure. ## Key Point: **Enzyme-inducing antiepileptic drugs (EIAEDs) significantly reduce COC efficacy.** Breakthrough bleeding is a clinical sign of inadequate hormone levels due to increased metabolism. ## Mechanism of Interaction ```mermaid flowchart TD A[Phenytoin ingestion]:::action --> B[CYP3A4/2C9/2C19 induction]:::outcome B --> C[Accelerated metabolism of EE and progestin]:::outcome C --> D[Reduced serum hormone levels]:::outcome D --> E{Contraceptive efficacy}:::decision E -->|Inadequate| F[Breakthrough bleeding + unintended pregnancy risk]:::urgent E -->|Adequate| G[Reliable contraception]:::outcome ``` ## Management Options for EIAED Users | Approach | Details | Recommendation | |----------|---------|----------------| | **Higher-dose COC** | 50 µg EE (or 30 µg desogestrel) | WHO Category 2 — acceptable with counselling | | **LARC** | Levonorgestrel IUD, copper IUD, etonogestrel implant | **WHO Category 1 — preferred** (not affected by enzyme induction) | | **Barrier methods** | Condoms, diaphragm | Unreliable as sole method | | **Continue standard COC** | 30–35 µg EE | **Inadequate** — high failure risk | ## Why Higher-Dose COC or LARC? 1. **Higher-dose COC (50 µg EE)** — increases serum hormone levels to compensate for increased metabolism. Requires counselling on thromboembolism risk. 2. **LARC (levonorgestrel IUD or etonogestrel implant)** — **preferred option** because: - Not affected by enzyme induction (local or depot delivery) - Failure rate < 1% (comparable to sterilisation) - No need for daily compliance - No increased VTE/stroke risk ## Clinical Pearl: **Breakthrough bleeding in a woman on COC + EIAED is a red flag for contraceptive failure.** Do not reassure and wait — act immediately to prevent unintended pregnancy. ## High-Yield: **Enzyme-inducing drugs that reduce COC efficacy:** - **Antiepileptics:** phenytoin, carbamazepine, phenobarbital, oxcarbazepine - **Antibiotics:** rifampicin (most potent), rifabutin - **Antiretrovirals:** ritonavir, efavirenz - **Anticonvulsants:** St. John's wort (herbal) **Mnemonic: "CRAP-S"** — Carbamazepine, Rifampicin, Anticonvulsants (phenytoin, phenobarbital), Protease inhibitors (ritonavir), St. John's wort.