This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Central Nervous system, Subject name: Pharmacology
[{'content': 'Naltrexone is used to maintain abstinence following opioid withdrawal in addicts. It blocks all of the following features of opioid use, except: \nSelect and explain. \nA. Euphoriant effects of opioid, B. Craving for opioid, C. Miosis, D. Respiratory depression', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ref: Goodman and Gilman 12th/661 *Naltrexone has no action in the absence of agonists but promptly reverses the opioid effects when administered i.v. *It can reverse all effects of opioid like sedation, analgesia, constipation, respiratory depression and Miosis etc but it do not reduce craving.', 'role': 'assistant'}]