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A Canadian family doctor reports that an interaction between tricyclic antidepressants and marijuana can produce a life-threatening arrhythmia. His patient, a 17-year-old boy, had been prescribed 25 mg of amitriptyline to help him sleep and alleviate some of his depression. A week after filling the prescription the boy showed up in the emergency ward with a supraventricular tachycardia rate of 300 beats per minute. He had been smoking marijuana the evening and night before. The doctor speculates that the amitriptyline and marijuana interacted to produce the arrhythmia and cautions against prescribing tricyclic antidepressants to adolescents who have a potential for drug abuse.