Isoniazid is generally well-tolerated at currently recommended doses. However, patients who are slow acetylators of isoniazid appear to have a higher incidence of some adverse effects. Also, patients whose nutrition is poor are at risk of peripheral neuritis which is one of the most common adverse effect of isoniazid. Other neurological adverse effects include psychotic reactions and convulsions. Pyridoxine may be given to prevent or treat these adverse effects. Optic neuritis has also been reported.
Hematological effects reported following use of isoniazid include various anemias, agranulocytosis, thrombocytopenia and eosinophilia.
Other adverse effects include nausea, vomiting, pellagra, purpura, hyperglycemia, lupus-like syndrome, urinary retention and gynecomastia.
Treatment of Adverse Effects: Pyridoxine HCl 10 mg daily has been recommended to prophylaxis of peripheral neuritis associated with isoniazid although some have suggested using up to 50 mg daily.