To be effective or take effect (e.g., a medicine or measure having an effect).
この薬は頭痛によく効く。
This medicine works well for headaches.
冷房が効いていて部屋が涼しい。
The air conditioning is working and the room is cool.
対策が効いて、感染者数が減少した。
The measures were effective and the number of infections decreased.
To function well or have keen senses—often written 利く for body senses or abilities.
彼は目が利くので古着の価値が分かる。
He has a keen eye and can tell the value of vintage clothes.
耳が利く人は小さな異変にも気づく。
A person with keen hearing notices small abnormalities.
この時計は精度が高く、よく利く。
This clock is precise and works well.
To apply or take effect in a given condition (e.g., a discount being applicable).
このクーポンは今日だけ効く。
This coupon is valid only today.
学割が効く店でチケットを買った。
I bought a ticket at a place that honors student discounts.
その説明だと割引は効かない場合があるので確認してください。
According to that explanation, the discount may not apply, so please check.
To feel the potency/effect (especially of alcohol or stimulants) or to try/taste to check its effect; often written in kana in some contexts.
そのウイスキーはかなりきくので、少しずつ飲んでください。
That whiskey is quite strong, so please drink it little by little.
眠気覚ましの薬はすぐにきく感じがした。
I felt the stimulant take effect right away.