A 45-year-old man on anti-TB therapy (2HRZE/4HR) presents at week 6 with complaints of inability to distinguish red from green. Visual acuity is normal. Which drug is responsible, and at what daily dose does this toxicity typically occur?

• A Isoniazid; dose >10 mg/kg/day
• B Ethambutol; dose >15-20 mg/kg/day ✓
• C Rifampicin; dose >15 mg/kg/day
• D Pyrazinamide; dose >30 mg/kg/day

Explanation

Ethambutol causes dose-dependent optic neuritis (red-green colour blindness) via chelation of copper in the optic nerve, impairing mitochondrial function. At standard doses (15 mg/kg/day) risk is low; it rises significantly above 25 mg/kg/day. Baseline and monthly visual acuity and colour vision testing are recommended. The toxicity is usually reversible if detected early and the drug is stopped.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.