A 28-year-old woman with newly diagnosed pulmonary tuberculosis is started on standard first-line therapy (RIPE regimen). She is also on oral contraceptive pills (OCP) for contraception. After 3 weeks of anti-TB therapy, she reports breakthrough bleeding and is concerned about contraceptive failure. Which anti-TB drug is responsible for this interaction, and what is the most appropriate counselling?

Explanation

## Drug–Drug Interaction: Rifampicin and OCPs **Key Point:** Rifampicin is a potent inducer of hepatic cytochrome P450 enzymes (CYP3A4, CYP2C9), dramatically increasing the metabolism of ethinylestradiol and progestins in oral contraceptive pills. This reduces OCP plasma concentrations by 30–50%, compromising contraceptive efficacy. ## Mechanism of Interaction ```mermaid flowchart LR A[Rifampicin]:::action --> B[Induces CYP3A4 & CYP2C9]:::outcome B --> C[Increased metabolism of ethinylestradiol & progestin]:::outcome C --> D[Reduced OCP plasma concentration]:::outcome D --> E[Breakthrough bleeding & contraceptive failure]:::urgent ``` **High-Yield:** Rifampicin is the ONLY first-line anti-TB drug with significant enzyme-inducing properties. Isoniazid, pyrazinamide, and ethambutol do NOT significantly affect OCP metabolism. ## Clinical Management **Counselling Points:** 1. **Barrier contraception is essential** during TB treatment and for 4 weeks after completing rifampicin (time for enzyme induction to reverse). 2. **OCP dose escalation** is NOT recommended — it is unreliable and increases side effects. 3. **Alternative contraceptive options:** - Intrauterine devices (copper or levonorgestrel-releasing) - Depot medroxyprogesterone acetate (IM injection) — less affected by enzyme induction - Barrier methods (condoms, diaphragm) - Permanent methods (tubal ligation, vasectomy) **Clinical Pearl:** Breakthrough bleeding is the clinical clue to this interaction. Any woman on OCPs who starts rifampicin-containing therapy should be counselled proactively, even if breakthrough bleeding has not yet occurred. **Mnemonic: RIF-P450** — Rifampicin Induces Cytochrome P450 (and many other drugs: warfarin, theophylline, protease inhibitors, azoles). ## Why Other Options Are Incorrect - **Option 0 (Isoniazid):** Isoniazid is a weak inhibitor of CYP2C19, not an inducer; it does not significantly affect OCP metabolism. - **Option 2 (Pyrazinamide):** Pyrazinamide does not inhibit OCP absorption; GI absorption of OCPs is not the mechanism of this interaction. - **Option 3 (Ethambutol + IM depot only):** Ethambutol does not cause malabsorption; while IM depot is a good option, it is not the *only* alternative, and the mechanism stated is wrong. [cite:KD Tripathi 8e Ch 47 (Antimycobacterial Drugs)]