How to say "efficacy" in Japanese

• 効果こうかJLPT N3approx.Common
  no-adjectiveThe general-purpose word for effect and effectiveness: the result something produces. When English 'efficacy' simply means how well something works, this is the usual choice.
  彼かれらがいくら弁解べんかいしても彼女かのじょには何なんの効果こうかもなかった。↗

  Their excuses cut no ice with her.
• 効能こうのうCommon
  nounThe beneficial effect of a medicine or remedy — what it is good for. This is the word used on medicine packaging; choose it when 'efficacy' refers to a drug's curative powers.
  絵画かいがが有ゆうする第だい一いちの効能こうのうは見みる人ひとの目めのごちそうとなることである。↗

  The first virtue of a painting is to be a feast for the eyes.
• 効力こうりょくJLPT N2approx.Common
  nounEmphasizes force or potency: the power to produce an effect. It is also the word for legal validity — whether a law, contract, or judgment has force.
  その法律ほうりつはもう効力こうりょくはなくなっている。↗

  The law is not in effect any longer.
• 利きききCommon
  nounReally the noun form of the verb 利く 'to work, to take effect.' For 'efficacy' standing alone it is a marginal choice; 効果 or 効能 is what you want.
• 実効じっこうCommon
  nounEmphasizes effect in actual practice rather than in theory: whether something really works. At home in formal, technical, and administrative writing about policies, rules, and measures.

Choose 効果 for effectiveness in general, 効能 for the beneficial effects of medicines, 効力 for force or potency including legal validity, 実効 for practical real-world effect, and treat 「利き」 as marginal.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 効果こうかeffect
• 効能こうのうeffect
• 効力こうりょくeffect
• 利きききeffectiveness
• 実効じっこうpractical effect
• 効き目ききめeffect
• 効こうefficacy
• 奏効そうこうefficacy
• 効験こうけんefficacy
• 力ちからefficacy
• 験げんeffect (e.g. of prayers; as the result of intense training)
• 回りまわりeffect