Isoniazid 

Serum aminotransferase activity increases in 10% to 20% of patients taking isoniazid, especially in the early weeks of treatment, but often returns to normal even when the drug is continued. Severe liver damage due to isoniazid is less common than previously thought. It is more likely to occur in patients more than 35 years old, but can also occur in younger patients. Routine monitoring is not necessary except for patients with pre-existing liver disease. Medical Letter consultants recommend stopping isoniazid when serum aspartate amino transferase activity reaches five times the upper limit of normal or if the patient has symptoms of hepatitis, but it can sometimes be re-started later.

Peripheral neuropathy occurs rarely and can usually be prevented by supplementation with pyridoxine (Vitamin B6, 10-25 mg/day), which is recommended for patients with chronic alcohol use, diabetes, chronic renal failure or HIV infection, and for those who are pregnant, breast feeding or malnourished.