A 28-year-old woman with lepromatous leprosy is started on standard multidrug therapy (MDT) with dapsone, rifampicin, and clofazimine. She is also on oral contraceptives (OCP) for family planning. After 3 months of therapy, she reports breakthrough bleeding and becomes pregnant. Which antileprotic drug is responsible for this interaction, and what is the mechanism?

Explanation

## Drug–Drug Interaction: Rifampicin and Oral Contraceptives ### Mechanism of Interaction **Rifampicin is a potent inducer of hepatic cytochrome P450 enzymes**, particularly **CYP3A4** and **CYP2C9**. This leads to: 1. **Increased metabolism** of ethinyl estradiol and progestins 2. **Reduced plasma levels** of OCP hormones 3. **Loss of contraceptive efficacy** → breakthrough bleeding and unintended pregnancy **Key Point:** Rifampicin reduces OCP efficacy by **30–50%**, making it one of the most clinically significant antibiotic–OCP interactions. ### Pharmacokinetic Details $$\text{OCP clearance} \propto \text{CYP3A4 activity}$$ When rifampicin induces CYP3A4: - **Ethinyl estradiol** AUC decreases by ~40% - **Levonorgestrel** AUC decreases by ~50% - **Breakthrough bleeding** occurs within 1–2 cycles - **Pregnancy risk** increases significantly **High-Yield:** Rifampicin is the **only antileprotic drug with significant P450-inducing activity**. Dapsone and clofazimine do not induce or inhibit hepatic enzymes. ### Clinical Management **Tip:** When rifampicin is prescribed to a woman on OCPs: 1. **Counsel on reduced contraceptive efficacy** 2. **Recommend alternative or additional contraception** (barrier methods, IUD, or higher-dose OCP if available) 3. **Continue for duration of rifampicin therapy** (usually 6 months for leprosy) 4. **Resume standard OCP dosing** 1 month after rifampicin discontinuation (allows P450 enzyme induction to reverse) **Clinical Pearl:** The interaction is **bidirectional** — OCPs may also slightly reduce rifampicin levels, but this is clinically less significant. ### Other Drugs with Similar Interactions | Drug Class | Example | Mechanism | OCP Efficacy Impact | |---|---|---|---| | **Antileprotics** | Rifampicin | CYP3A4 induction | ↓↓ (40–50%) | | **Anticonvulsants** | Phenytoin, carbamazepine | CYP3A4 induction | ↓↓ (40–50%) | | **Antiretrovirals** | Nevirapine, ritonavir | CYP3A4 induction/inhibition | ↓↓ or variable | | **Antifungals** | Griseofulvin | CYP3A4 induction | ↓ (20–30%) | | **Antibiotics** | Dapsone, clofazimine | No enzyme effect | No change | **Warning:** Do NOT assume all antileprotics have the same interaction profile — **only rifampicin** is a significant P450 inducer. ### Why This Question Tests High-Yield Knowledge This interaction is **frequently tested in NEET PG** because: - It combines pharmacology (enzyme induction) with clinical relevance (contraceptive failure) - It requires understanding of **drug metabolism** and **CYP450 enzymes** - It has **real-world consequences** in leprosy management in India - It tests **mechanistic understanding**, not just memorization [cite:KD Tripathi 8e Ch 49, Ch 62; Harrison 21e Ch 188]