A case of ventricular tachycardia in a 37-year-old man after ingestion of heroin and cocaine, treated with naloxone is described. The arrhythmia is attributed to unopposed sympathomemetic effects of cocaine following administration of naloxone. The pro-rhythmogenic effects of naloxone may have contributed to the arrhythmia. He was initially treated sequentially with intravenous magnesium sulphate, calcium gluconate, sodium bicarbonate and lignocaine infusion, with no resolution of the ventricular tachycardia. He was admitted to the coronary care unit and was given an amiodarone infusion which converted the rhythm to sinus rhythm. Forensic physicians administering naloxone to poly-drug abuser in a custodial setting should be aware of potentially fatal consequences which they will be unable to detect. It is recommended that all such patients should be transferred to hospital for observation and investigation.