Most critically ill patients encounter pain and distress from acute illness, medical procedures and devices as well as routine care in the intensive care units (ICU). Opioids are principal analgesics that alleviate moderate to severe pain and facilitate patients to co-operate the course of treatment. However, prolong administration of opioids especially in mechanically ventilated patients can cause withdrawal symptoms if analgesics are rapidly weaning or acutely disruption. The opioid withdrawal symptoms (OWS) are well reported in critically ill children that cause discomfort and prolong weaning from mechanical ventilation. Weaning opioids and treatment of withdrawal symptoms are needed in order to decrease ventilator days, ICU and hospital length of stay. Conversely, there is lack of knowledge about incidence, clinical presentation, time course and appropriated assessment tool for withdrawal detection. Therefore, we conduct the study to explore an incidence of OWS, to identify factors associated OWS, to establish the assessment tool for OWS, and to report efficacy of the pharmacological treatment for OWS, in adult critically ill patients.