Procainamide is metabolized by N-acetyltransferase (a phase II drug metabolism reaction) to form N-acetyl-procainamide (NAPA), which itself has antiarrhythmic activity. Patients who are classified as slow acetylators may develop SLE-like symptoms when treated with procainamide. Other drugs metabolized via N-acetyltransferase, including isoniazid and hydralazine, have also been associated with lupus-like symptoms in slow acetylators.