also: こうげん
Refers to the beneficial result or power of something, such as a medicine, treatment, or method. Somewhat formal and often used in written contexts.
The efficacy of this medicine was proven in clinical trials.
The effectiveness of the new treatment is anticipated.
Standard kanji spelling for this word.
Obsolete kana usage; the modern reading is こうけん.
Compound of 効 (effect, efficacy) and 験 (test, effect). The exact historical derivation is uncertain, but the combination naturally conveys 'proven effectiveness'.