The primary review question is: what is the global incidence rate of emergency medical services (EMS)-attended and EMS-treated out-of-hospital cardiac arrest (OHCA) precipitated by drug overdose in adults?The secondary review questions are: what proportion of adults with EMS-treated overdose OHCA: i) survived to hospital discharge or 30 days, ii) survived with intact neurological function at hospital discharge (i.e. Cerebral Performance Category or modified Rankin Scale ≤ 2), iii) achieved prehospital return of spontaneous circulation, iv) survived to hospital, and v) survived to 12 months?