## Monitoring and Adverse Effects of First-Line Anti-TB Drugs ### Isoniazid (INH) **Key Point:** Isoniazid causes peripheral neuropathy through vitamin B~6~ depletion. This is **preventable** with pyridoxine (vitamin B~6~) supplementation, especially in patients at high risk (malnutrition, diabetes, alcoholism, HIV). ### Rifampicin **High-Yield:** Rifampicin has two major clinical implications: 1. **Orange discoloration** of urine, sweat, saliva, and tears — this is harmless but important for patient counseling 2. **Potent CYP450 inducer** — increases metabolism of many drugs (oral contraceptives, warfarin, protease inhibitors, statins) **Clinical Pearl:** Patients on rifampicin may need dose adjustments of concurrent medications or alternative contraception. ### Pyrazinamide (PZA) **Key Point:** Pyrazinamide causes **hyperuricemia** by competing with uric acid for renal tubular secretion. This can precipitate acute gout attacks in susceptible individuals. **Mnemonic:** PZA = **P**yrazinamide = **P**urine metabolism disruption = **P**recipitates gout ### Ethambutol (EMB) **Warning:** Ethambutol is NOT the safest drug and DOES require monitoring. The critical adverse effect is **optic neuritis**, which can cause: - Color blindness (red-green color discrimination loss) - Decreased visual acuity - Central scotoma **High-Yield:** Ethambutol requires: - **Baseline visual acuity and color vision testing** before starting - **Monthly monitoring** during treatment - **Immediate discontinuation** if visual symptoms develop (potentially irreversible) ### Monitoring Summary Table | Drug | Key Adverse Effect | Monitoring Required | Prevention/Management | |------|-------------------|-------------------|----------------------| | Isoniazid | Peripheral neuropathy | Clinical assessment | Pyridoxine 10 mg daily | | Rifampicin | Drug interactions, orange secretions | Drug interaction review | Counseling, adjust concurrent meds | | Pyrazinamide | Hyperuricemia, gout | Serum uric acid (baseline) | Allopurinol if history of gout | | Ethambutol | **Optic neuritis** | **Baseline + monthly vision tests** | **Discontinue if symptoms develop** | **Clinical Pearl:** Ethambutol-induced optic neuritis is dose-dependent and more common at doses >25 mg/kg/day. At standard doses (15 mg/kg/day), the incidence is <1%, but monitoring is still essential because the vision loss can be irreversible if not caught early. [cite:Harrison 21e Ch 158, Robbins 10e Ch 8]
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