Rare synonym for 鎮痛剤. The glosses 'sedative' and 'tranquilizer' are less precise; this term primarily refers to pain relief.
See also: 鎮痛剤
止痛剤 is a rare variant spelling of 鎮痛剤 and means the same thing.
Compound of 止痛 (pain relief) + 剤 (medicine). The spelling 止痛 is a rare variant of 鎮痛.