What is the major hepatotoxicity risk with isoniazid and how is monitoring performed?

• A. Nephrotoxicity; monitor creatinine.
• B. Neurotoxicity; monitor ammonia.
• C. Cardiovascular toxicity; monitor ECG.
• D. Hepatotoxicity; monitor liver enzymes (ALT/AST) and symptoms.

Answer: (D)

Isoniazid’s major risk is hepatotoxicity. Isoniazid can cause drug-induced hepatitis, and the chance of it occurring rises with age, preexisting liver disease, heavy alcohol use, and use of other liver-toxic drugs. Because of this, monitoring focuses on liver function tests and watching for liver-related symptoms. Start with baseline liver tests (ALT and AST, and bilirubin if available) before therapy. Then check liver enzymes periodically during treatment—typically in the first couple of months—and reassess promptly if symptoms develop. Patients should be told to report fatigue, loss of appetite, nausea, vomiting, right upper quadrant pain, dark urine, or yellowing of the skin or eyes, as these suggest liver injury. If enzyme elevations are significant or liver injury is suspected, discontinue isoniazid and evaluate further. (Note: while neuropathy is another known risk with isoniazid, prevented with pyridoxine, the question focuses on hepatotoxicity.)