Background: Analgesics and sedatives are key elements to reduce physiological and psychological stress associated with treatment in paediatric intensive care. Prolonged drug use may induce tolerance and development of iatrogenic withdrawal syndrome (IWS) during the tapering phase. Our primary aim was to describe the prevalence of IWS among critically ill ventilated patients in two Norwegian paediatric intensive care units (PICUs), and secondary to investigate what motivated bedside nurses to administer additional drug doses. Methods: Mechanically ventilated patients (n = 40) from newborn to eighteen years of age, with continuous infusions of opioids and benzodiazepines for 5 days or more, were included consecutively from May 2016 to June 2018. By using Withdrawal Assessment Tool-1 (WAT-1) twice daily we recorded the prevalence of IWS. Additionally, we recorded signs and symptoms that led bedside nurses to administration extra bolus medication. Results: Peak WAT-1 score indicated an IWS prevalence of 95% in this selected group. The first days of the tapering phase were most critical for IWS. The most frequent symptoms triggering administration of additional bolus doses were agitation/restlessness, and thiopental and propofol were the bolus drugs used most frequently. Conclusions: IWS affected 95% of the children having received infusions of opioids and benzodiazepines for 5 days or more in PICUs without a tapering protocol for these drugs. This calls for implementation and testing of such weaning protocols.