• Basic forms
• 利く — to be effective; to work well / base form
• 利きます — works; is effective / masu form
• 利いた — worked; was effective / ta form
• 利いて — working; effective and … / te-form
• 利かない — doesn’t work; not effective / negative
• 利けば — if it works / conditional
• 利け — work!; be effective! (rare) / imperative
• Other common forms
• よく利く薬 — a medicine that works well
• 頭が利く — to be sharp; quick-witted
• 利かせる — to make effective; to apply (causative)
• More
• 利こう — let it work; let’s apply it / 〜よう form
• 利かなかった — didn’t work
• 利いている — is working; is effective

利く（きく） is a godan verb ending in く. It is commonly used to describe effectiveness — especially of medicine, tools, skills, or mental sharpness. Unlike 効く, which focuses on results, 利く often emphasizes usefulness or functional impact.