Meaning
効 means effect, efficacy, or result — the sense that something is actively producing an outcome. A medicine taking hold, a strategy that delivers, a contract still in force: 効 covers them all.
It shows up most in 効果 (kouka, effect/result) and 有効 (yuukou, valid/effective), two words that come up constantly across medicine, law, and daily conversation.
The character pairs 交 (crossing, exchange) on the left with 力 (strength) on the right — force meeting a crossing point to produce something tangible. That radical 力 runs through every compound: 効 always implies energy being applied, not just a passive state.
8 strokes, Grade 5. Hán-Việt: HIỆU — the same root appears in Vietnamese as hiệu quả (effectiveness) and hiệu lực (force of law), giving Vietnamese learners an immediate grip on the meaning.
Readings
On'yomi (音読み) — Chinese-derived readings
One on'yomi: コウ (kou). It covers virtually every compound form. When 効 appears alongside another kanji in a noun or adjective, read it as コウ.
- 効果 (kouka) — effect, result, impact. Broad usage across science, medicine, and daily speech: この薬は効果がある (This medicine has an effect).
- 効率 (kouritsu) — efficiency, productivity. Standard in business and technology: 効率よく働く (to work efficiently).
- 効力 (kouryoku) — efficacy, legal force. Common in contracts and formal writing: この契約は効力を持つ (This contract has legal force).
- 効能 (kounou) — beneficial effect, especially of medicines. Often printed on product labels and pharmaceutical packaging.
Kun'yomi (訓読み) — Native Japanese readings
The kun'yomi き(く) gives us the verb 効く (kiku): to be effective, to work, to take effect. Reach for it when asking whether something does its job — a medicine, a remedy, an effort. In speech, kiku sounds identical to 聞く (to listen) and 利く (to be capable), so kanji context is the only distinguisher.
- 効く (kiku) — to be effective, to work. Example: この薬がよく効く (This medicine works well).
- 効き目 (kikime) — effectiveness, how well something works. 効き目がない (It has no effect).
- 効き過ぎる (kiki-sugiru) — to be too effective. Used when a remedy works more strongly than expected.
Common Words & Compounds
効 appears in compound words across medicine, law, business, and daily life. Here are the ones worth knowing at N2.
Effectiveness and Results:
- 効果 (kouka) — effect, result, impact
- 効果的 (koukateki) — effective, impactful (な-adjective)
- 効力 (kouryoku) — efficacy, legal validity
- 効能 (kounou) — beneficial effect, virtue (especially of medicines)
- 効用 (kouyou) — utility, usefulness, benefit
Presence or Absence of Effect:
- 有効 (yuukou) — valid, effective, in force
- 無効 (mukou) — invalid, null, void
- 有効期限 (yuukou kigen) — expiration date, period of validity
Speed and Degree of Effect:
- 即効 (sokkou) — immediate effect
- 速効 (sokkou) — quick-acting effect (especially for medicine)
- 薬効 (yakkou) — medicinal effect, pharmacological action
Efficiency:
- 効率 (kouritsu) — efficiency
- 効率的 (kouritsu-teki) — efficient (な-adjective)
- 非効率 (hikouritsu) — inefficiency, inefficient
Other Important Compounds:
- 時効 (jikou) — statute of limitations, prescription period
- 実効 (jikkou) — actual effect, practical result
Example Sentences
この薬はよく効く。
Kono kusuri wa yoku kiku.
This medicine works well.
新しい勉強法は効果的だった。
Atarashii benkyouhou wa koukateki datta.
The new study method worked really well.
このパスポートの有効期限はいつですか。
Kono pasupooto no yuukou kigen wa itsu desu ka.
When does this passport expire?
契約は来月から無効になります。
Keiyaku wa raigetsu kara mukou ni narimasu.
The contract will become void starting next month.
彼は仕事を効率よく進めるのが得意だ。
Kare wa shigoto wo kouritsu yoku susumeru no ga tokui da.
He's good at getting work done efficiently.
この湿布は肩こりに効き目がある。
Kono shippu wa katakori ni kikime ga aru.
This patch works well for stiff shoulders.
罪の時効が成立してしまった。
Tsumi no jikou ga seiritsu shite shimatta.
The statute of limitations for the crime has expired.
非効率な作業方法を見直す必要がある。
Hikouritsu na sagyou houhou wo minaosu hitsuyou ga aru.
The inefficient work process needs another look.
運動はストレス解消に実効のある手段だ。
Undou wa sutoresu kaishou ni jikkou no aru shudan da.
Exercise is a genuinely effective way to relieve stress.
Memory Tip
Split 効 into 交 (crossing) + 力 (power). Power applied at a crossing point produces a result — that's the whole kanji. When you see 効, something is working, taking effect, delivering an outcome.
Vietnamese learners already know this as HIỆU through hiệu quả and hiệu lực. The meaning arrives without effort.