Background: Clonidine is used in critically ill patients for sedative and analgesic purposes. Objectives: 1) to review, systematically, the literature on prolonged intravenous (iv) clonidine for sedation in critically ill patients; 2) to evaluate the evidence from related fields of medicine on the use of clonidine for sedative or analgesic purposes, and; 3) to investigate prescribing practices of clonidine. Methods: We searched the literature for: 1) the use of prolonged iv clonidine for sedation in adult critically ill patients; 2) evidence of iv clonidine in the perioperative setting and in patients suffering from alcohol withdrawal; 3) information on haemodynamic side effects of clonidine. We performed a telephone and email enquiry to investigate dosing schemes. Results: Three published prospective trials were found. Clonidine shortened the duration of ventilation. Clonidine also had an analgesic and opioid-sparing effect. The cardiovascular events reported included bradycardia and first-degree atrioventricular block. Clonidine doses in the literature varied from 50 to 3360 μg/70 kg/day. All of the 14 responding intensive care units (ICUs) confirmed the prolonged use of iv clonidine. Doses varied from 240 to 2400 μg/70 kg/day and loading doses varied from 0 to 150 μg. Conclusion: Prolonged use of iv clonidine is common in critically ill patients, but the evidence is limited. Clonidine can decrease postoperative opioid consumption and pain intensity and can produce dose-dependent sedation. Dosing schemes varied considerably between the surveyed ICUs as well as in the literature. Cardiovascular side effects were reported. Clonidine should be used cautiously in high-risk cardiovascular patients. © 2015, Netherlands Society of Intensive Care. All Rights Reserved.