A tranquillizer is a drug that calms a person (or an animal) down. For people, it usually means the kind of medicine a doctor gives someone who is very anxious, who cannot stop worrying, or who cannot sleep because their mind is racing. It does not make you happy and it does not cure anything — it turns the feeling down, the way you turn down a radio that is too loud. Doctors mostly use more exact words now (like 'sedative', or the names of the actual drug groups), so 'tranquillizer' in everyday English often sounds slightly old-fashioned or vague — it is the word a family uses, not the word on the prescription. The second common use is for animals: a tranquillizer dart is the dart shot into a big animal to make it sleep so vets can move it or treat it. That use is very much alive, especially in news stories about escaped zoo animals. People also use the word loosely and jokingly for anything that makes them sleepy — 'that lecture was a tranquillizer'. Spelling: British English writes 'tranquillizer' or 'tranquilliser' with two l's; American English writes 'tranquilizer' with one. All the same word.

鎮定劑（動物用的麻醉針叫「麻醉針／麻醉槍」，不要說成鎮定劑）