Myocarditis is a rare, but potentially serious side effect of clozapine. It can be fatal if not detected and managed. Myocarditis is inflammation of the heart muscle. Symptoms of myocarditis can include fever, fatigue, rapid heartbeat, or chest pain. Most cases of myocarditis develop within 2 to 3 weeks of starting clozapine. If you or a loved one are concerned about symptoms that could be myocarditis, you should quickly contact the patient’s clinician or present for emergency care.