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': 'Which of the following drug is not an opioid agonist? \nSelect and explain. \nA. Heroin, B. Ketamine, C. Methadone, D. Codiene', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Heroin is a semi synthetic opiod agonists . Codiene is a natural opium alkaloid . Methadone is a synthetic alkaloid Ref": KD Tripathi 8 th ed.', 'role': 'assistant'}]