効く
きく · kiku · JLPT N3 · verb-godan
to be effective
💊 kiku sounds like kick: the medicine kicks in, so it works
効 = 交 (exchange) + 力 (power): power that gets passed on and has an effect. Same sound as 聞く (to listen), but 効く is used for medicine or a remedy taking effect, as in 薬が効く (the medicine works).