A 28-year-old woman with newly diagnosed pulmonary tuberculosis is started on standard first-line therapy. She is also on oral contraceptive pills (OCP) for menstrual regulation. After 3 weeks of antitubercular therapy, she reports breakthrough bleeding and irregular menstrual cycles. She is concerned about contraceptive failure. Which antitubercular drug is responsible for this drug interaction, and what is the mechanism?

Explanation

## Rifampicin-Induced OCP Failure **Key Point:** Rifampicin is a potent inducer of hepatic cytochrome P450 enzymes (particularly CYP3A4, CYP2C9, and CYP2C19). It dramatically increases the metabolism of oral contraceptives, reducing their plasma concentration and efficacy — a classic high-yield drug interaction in NEET PG. **High-Yield:** Breakthrough bleeding and irregular menses are the clinical hallmarks of OCP failure due to rifampicin. The patient is at risk of unintended pregnancy. ### Mechanism of Interaction ```mermaid flowchart TD A[Rifampicin administered]:::action --> B[Induces hepatic CYP3A4 and other P450 enzymes]:::outcome B --> C[Increased metabolism of ethinylestradiol and progestin]:::outcome C --> D[Reduced plasma concentration of OCP]:::outcome D --> E{OCP level below therapeutic threshold?}:::decision E -->|Yes| F[Breakthrough bleeding, cycle irregularity]:::outcome E -->|Yes| G[Contraceptive failure, unintended pregnancy risk]:::urgent H[Patient counseling + alternative contraception]:::action -.-> G ``` **Clinical Pearl:** The induction effect of rifampicin begins within 2–3 weeks and persists for up to 2 weeks after discontinuation. This is why breakthrough bleeding occurred at 3 weeks in this case. ### Management Recommendations | Intervention | Rationale | |--------------|----------| | **Increase OCP dose** | Higher ethinylestradiol (≥50 μg) may overcome induction, but not always reliable | | **Switch to alternative contraception** | Intrauterine device (IUD), barrier methods, or depot medroxyprogesterone acetate (DMPA) are preferred | | **Counsel on dual contraception** | Barrier methods + OCP during TB treatment | | **Continue TB therapy** | Do NOT discontinue rifampicin; TB control takes priority | **Mnemonic — Rifampicin Induction:** **RIFAMPICIN = Really Induces Fast Antibiotic Metabolism, Particularly In Cytochrome P450 Induction Network** ### Other Drugs Affected by Rifampicin Induction - Warfarin (reduced anticoagulation) - Corticosteroids (reduced efficacy) - Protease inhibitors (reduced antiviral effect) - Methadone (withdrawal symptoms) - Theophylline (reduced bronchodilation) [cite:KD Tripathi 8e Ch 48; Harrison 21e Ch 205]