A 28-year-old woman from Delhi with newly diagnosed pulmonary tuberculosis is started on standard first-line therapy. She is also on oral contraceptives for menstrual regulation. After 6 weeks of antitubercular therapy, she reports breakthrough bleeding and is concerned about contraceptive failure. She is otherwise compliant and tolerating therapy well. Which antitubercular drug is responsible for this drug–drug interaction?

## Rifampicin-Induced Oral Contraceptive Failure ### Mechanism of Interaction **Key Point:** Rifampicin is a potent **inducer of hepatic cytochrome P450 enzymes** (especially CYP3A4, CYP2C9, and CYP2C19), which accelerates the metabolism of ethinylestradiol and progestins in oral contraceptives. ### Clinical Consequence **High-Yield:** This increased metabolism leads to: - Subtherapeutic contraceptive hormone levels - Breakthrough bleeding - Increased risk of unintended pregnancy - Effect begins within 1–2 weeks of rifampicin initiation ### Management Strategy ```mermaid flowchart TD A[Woman on OCP starts rifampicin]:::outcome A --> B{Need contraception during TB treatment?}:::decision B -->|Yes| C[Switch to non-hormonal method]:::action C --> D[IUD, condoms, or barrier methods]:::action B -->|If hormonal needed| E[Increase OCP dose by 50%]:::action E --> F[Use high-dose OCP 50 mcg EE]:::action A --> G[Counsel about breakthrough bleeding risk]:::action G --> H[Reassure: reversible after rifampicin cessation]:::outcome ``` **Clinical Pearl:** Breakthrough bleeding is NOT a sign of treatment failure — it is an expected pharmacokinetic interaction. Reassurance and alternative contraception are the cornerstones of management. ### Other Drugs Affected by Rifampicin Induction | Drug Class | Examples | Clinical Consequence | |------------|----------|---------------------| | Anticoagulants | Warfarin | ↓ INR, loss of anticoagulation | | Antiarrhythmics | Quinidine, digoxin | ↓ Levels, loss of efficacy | | Antifungals | Ketoconazole, itraconazole | ↓ Levels, treatment failure | | Corticosteroids | Prednisolone, dexamethasone | ↓ Levels, loss of efficacy | | Antiretrovirals | Protease inhibitors, NNRTIs | ↓ Levels, resistance risk | | Calcium channel blockers | Diltiazem, verapamil | ↓ Levels, loss of control | **Mnemonic:** **CWAK** — Contraceptives, Warfarin, Antiretrovirals, Ketoconazole (and other azoles) are the high-yield drug interactions to remember for rifampicin. ### Why Other First-Line Drugs Do NOT Cause This Interaction **Key Point:** Only rifampicin is a potent CYP450 inducer among first-line antitubercular drugs. - **Isoniazid**: Weak CYP450 inhibitor (opposite effect) - **Pyrazinamide**: Minimal enzyme interaction - **Ethambutol**: No significant enzyme interaction ### Duration of Effect **High-Yield:** Enzyme induction by rifampicin: - Begins: 1–2 weeks after starting - Peaks: 3–4 weeks - Resolves: 2–3 weeks after stopping rifampicin Patients must continue alternative contraception for **at least 4 weeks after completing TB therapy** to allow enzyme activity to normalize. [cite:KD Tripathi 8e Ch 49; Harrison 21e Ch 165]