The same patient has been taking isoniazid long-term. What is the side effect?

Correct answer: D. Hepatitis

• A. Cardiac arrhythmia is not a recognised principal adverse effect.
• B. OPTIC NEURITIS with red-green colour vision loss is the hallmark toxicity of ETHAMBUTOL, a different first-line anti-TB drug.
• C. Renal impairment is not the characteristic isoniazid toxicity.
• D. Correct. ISONIAZID is HEPATOTOXIC — drug-induced hepatitis is its major adverse effect, and risk increases with age and with concurrent alcohol use. It also causes PERIPHERAL NEUROPATHY by interfering with pyridoxine (vitamin B6) metabolism, which is why B6 is co-prescribed.

Note: First-line TB drugs and their signature toxicities: Isoniazid — hepatitis and peripheral neuropathy; Rifampin — hepatotoxicity and ORANGE-RED discoloration of body fluids; Pyrazinamide — hepatotoxicity and hyperuricemia; Ethambutol — optic neuritis. DENTAL RELEVANCE: hepatic impairment affects the metabolism of local anaesthetics and analgesics, so avoid additional hepatotoxins such as acetaminophen at high doses.