効|こうkou[1]atamadaka (head-high)

1. nounefficacy; benefit; efficiency; effect; result; success

• Synonym効能こうのうWrittenIn written/formal prose, this means the efficacy of medicines, hot springs and the like, as in 「効能書き」 or 「温泉の効能」.
• Synonym効果こうかNeutralThe neutral, most general word for effect, as in 「効果がある」; 「効」 on its own is rare.
• Synonym効力こうりょくWrittenIn written/formal prose; the effectiveness or validity of laws and medicines, as in 「効力を失う」 — leans toward validity.
• Synonym効用こうようWrittenWritten/formal prose word for usefulness or utility; in economics it also denotes the satisfaction gained from consuming goods.
• Synonym効き目ききめCasualColloquial; the visible effect or curative power, as in 「効き目がない」 — the usual spoken word for effect.
• Synonym効験こうけんWrittenWritten/formal prose and rare; miraculous efficacy, typically of gods and Buddhas or of medicine.
• Related利きききCasualColloquial; how well something works or its sensitivity, as in 「薬の効き」 — stresses the act of taking effect.
• Related利りWrittenWritten/formal prose; advantage, profit or benefit, mostly used as a morpheme, as in 「地の利」.
• Related実効じっこうWrittenWritten/formal prose; actual, practical effect, often in policy contexts, as in 「実効性」.
• Related益えきWrittenWritten/formal prose; benefit or gain, mostly used as a morpheme, as in 「有益」.
• Antonym無効むこうWrittenWritten/formal prose; invalid or void, the opposite of the efficacy of 「効」.

For effect, use 「効果」; in casual speech 「効き目」 is the usual choice. 「効」 alone is rare — it mostly appears as a morpheme or in set phrases like 「効を奏する」 (to take effect) and 「薬石効なく」 (medicine proves useless). For the efficacy of medicines or hot springs, say 「効能」; for invalid, 「無効」.

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

1. (コウ) ききめ。しるし。効能こうのう。