A patient is taking isoniazid. Which adverse effect is of greatest concern?

Correct answer: A. Hepatotoxicity (drug-induced hepatitis)

• A. CORRECT. HEPATOTOXICITY is the principal and potentially fatal toxicity of isoniazid. Asymptomatic transaminase elevation is common; overt hepatitis is less common but serious, and risk rises with AGE, with concurrent ALCOHOL use, and with rifampin co-therapy. Patients on isoniazid have liver function monitored, and the dental implication is direct: avoid or minimise ACETAMINOPHEN, since it adds a second hepatotoxic burden. Choose an NSAID for analgesia if not otherwise contraindicated.
• B. Renal toxicity is not characteristic of isoniazid. Among antituberculous agents, nephrotoxicity and ototoxicity belong to the aminoglycosides such as streptomycin and amikacin.
• C. Cardiomyopathy is not an isoniazid effect.
• D. OPTIC NEURITIS with red–green colour blindness is the hallmark toxicity of ETHAMBUTOL, not isoniazid — a classic distractor pairing.

Note: Isoniazid's second well-known toxicity is PERIPHERAL NEUROPATHY from pyridoxine (vitamin B6) depletion, which is why B6 is co-prescribed. Toxicity map for the first-line regimen: Isoniazid → liver and neuropathy; Rifampin → liver and orange body fluids plus potent CYP450 induction; Pyrazinamide → liver and hyperuricaemia; Ethambutol → optic neuritis.