Which first-line TB drug requires routine visual testing due to optic toxicity?

• A. Ethambutol
• B. Isoniazid
• C. Rifampin
• D. Pyrazinamide

Answer: (A)

Ethambutol carries a unique risk among first-line TB drugs: optic toxicity. It can cause optic neuritis, leading to decreased visual acuity and impaired color vision, especially red-green discrimination. Because this toxicity is dose-related and can be reversible if caught early, patients on Ethambutol typically have baseline and periodic visual testing—checking sharpness of vision and color perception—to catch problems before lasting damage. If vision changes occur or testing worsens, the drug is stopped or its dose adjusted. The other first-line drugs don’t routinely require eye monitoring because their main risks lie in other areas (for example, hepatotoxicity for several of them), so routine visual testing isn’t standard with those agents.