A 28-year-old woman with newly diagnosed pulmonary tuberculosis is started on standard first-line therapy (HRZE). She is also on oral contraceptives for menstrual regulation. After 1 month, she reports breakthrough bleeding and is concerned about contraceptive failure. What is the mechanism of this drug–drug interaction and which anti-TB agent is primarily responsible?

Explanation

## Mechanism of Interaction **Rifampicin is a potent inducer of hepatic cytochrome P450 enzymes** (especially CYP3A4), which metabolizes ethinyl estradiol and progestins in oral contraceptives. Induction accelerates contraceptive metabolism, reducing plasma concentrations below therapeutic levels and causing contraceptive failure. ## Key Point: **Rifampicin-induced enzyme activity reduces oral contraceptive efficacy by 40–50%, leading to breakthrough bleeding and unintended pregnancy risk.** This is a critical drug–drug interaction in TB treatment. ## High-Yield Facts | Aspect | Details | |--------|----------| | **Primary culprit** | Rifampicin (RIF) — most potent P450 inducer among first-line agents | | **Mechanism** | Upregulation of CYP3A4, CYP2C9, CYP2C19 | | **Clinical effect** | Reduced OCP levels → breakthrough bleeding → contraceptive failure | | **Onset** | Within days to weeks of RIF initiation | | **Duration** | Persists during RIF therapy; enzyme activity normalizes 2–3 weeks after RIF cessation | | **Management** | Switch to alternative contraception (barrier, IUD, implant) or increase OCP dose (not standard; alternative method preferred) | ## Comparison of First-Line Agents | Agent | P450 Effect | Drug Interaction Risk | |-------|-------------|----------------------| | **Rifampicin** | Strong inducer | Very high — affects many drugs | | **Isoniazid** | Weak inhibitor (at high doses) | Minimal with OCPs | | **Pyrazinamide** | No significant effect | Minimal | | **Ethambutol** | No significant effect | Minimal | ## Clinical Pearl: **Breakthrough bleeding during TB treatment in a woman on OCPs is a red flag for rifampicin-induced contraceptive failure.** Counsel all women of reproductive age on this interaction at TB diagnosis. ## Mnemonic: **RIF = Rapid Induction of P450** (remember: RIF is the rifampicin culprit) ## Tip: In NEET PG, questions on TB drug interactions commonly test rifampicin's enzyme-inducing properties. This is a high-yield, frequently tested concept.