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': 'Tolerance develops to all of the following actions of opioids except? \nSelect and explain. \nA. Miosis, B. Analgesia, C. Euphoria, D. Nausea and vomiting', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Repeated administration of morphine results in the development of tolerance to some of its effects including respiratory depression,analgesia, sedation and euphoriant effects and other CNS depressant effects. Constipation and miosis shows no tolerance Ref: KD Tripathi 8th ed.', 'role': 'assistant'}]