To determine the tolerance and safety of moricizine, the incidence and nature of its noncardiac adverse effects and organ toxicity reported during short- and long-term clinical studies were examined. From a pooled data base of 1,256 adult patients and healthy subjects, the most frequent non-cardiac adverse events were gastrointestinal (nausea) and neurologic (dizziness) complaints which occurred in 10 to 15% of patients during short-term (less than 3 months) studies and increased to 20 to 25% during long-term (greater than 12 months) studies. Adverse effects led to discontinuation of moricizine therapy in 116 patients (9%). Organ toxicity consisted predominantly of drug fever, possible thrombocytopenia and elevated liver function tests and was quite low (less than 0.7%) for both short- and long-term studies. Moricizine appears to be a well-tolerated antiarrhythmic drug with low occurrence of noncardiac adverse effects without significant serious organ toxicity.