A prospective multicenter observational study will be carried out in two University-affiliated hospital intensive care units (Hôpital du Sacré-Coeur and Montreal General Hospital). All mechanically ventilated ICU patients receiving continuous infusions of benzodiazepines and/or opioids for more than 96 hours will be prospectively screened daily for withdrawal symptoms by an intensivist. Withdrawal symptoms for opioid and benzodiazepines will be identified using the DSM-V criteria. Patients with moderate to severe head trauma, a severe neurological disease, or prior to admission substance abuse (significant alcohol, illicit or prescription drugs) will be excluded. We will also exclude patients with a regularly prescribed chronic opioid or benzodiazepine use prior to ICU admission. Risk factors for withdrawal including type of agents exposed, dosing, duration of exposure, weaning of agents, organ dysfunction, age, gender, smoking, severity of disease and admission diagnosis will be prospectively collected. The Withdrawal Assessment Tool-1 (WAT-1), a screening tools validated in the paediatric population, will also be administered daily. The criterion validity of this screening tool will be compared to the DSM-V criteria.