expression, godan verb (-ku)
medicine is effective; medicine works
Used when a medicine or treatment produces the desired effect. Often appears in the negative form 薬が効かない (the medicine doesn't work).
この薬が効いて、熱が下がった。
This medicine worked and my fever went down.
痛み止めを飲んだけど、まだ薬が効かない
I took a painkiller, but the medicine still hasn't taken effect.