efficacy
/ˈɛf.ɪ.kə.si/noun
efficacy is spelled E-F-F-I-C-A-C-Y: 8 letters and 4 syllables. It appears 4.8 times per million words in written English, the 11,541st most frequent word form (wordfreq, 2021 data). As a noun, efficacy means “The ability to produce a desired effect under ideal testing conditions” (English Wiktionary, September 2026). The plural of efficacy is efficacies.
How many letters and syllables are in efficacy?
| Letters | 8: E-F-F-I-C-A-C-Y |
|---|---|
| Syllables | 4 (from its American pronunciation) |
| In writing | 4.8 times per million words; the 11,541st most frequent word form |
| Wrong spellings searched | none measured in search |
Sources: letters and syllables from the English Wiktionary's hyphenation or pronunciation and the CMU Pronouncing Dictionary; written frequency: wordfreq 3.1 (a 2021 snapshot of Wikipedia, subtitles, news, books, web text and social media).
How common is efficacy?
Efficacy appears 4.8 times per million words in written English, the 11,541st most frequent word form in the list.
Source: written frequency: wordfreq 3.1 (a 2021 snapshot of Wikipedia, subtitles, news, books, web text and social media).
How do you pronounce efficacy?
- US /ˈɛf.ɪ.kə.si/
- Australia /ˈef.ɪ.kə.si/
- Australia [ˈef.ɪ.kə.sɪi]
Efficacy has 4 syllables (counted from its American pronunciation in the CMU Pronouncing Dictionary).
Source: English Wiktionary, dump of September 2, 2026 (CC BY-SA 4.0), through kaikki.org (Wiktextract). Recordings: LL-Q1860_(eng)-Vealhurl-efficacy.wav, Wikimedia Commons (author and licence on each file's page).
Forms of efficacy
- efficacies plural
Source: English Wiktionary, dump of September 2, 2026 (CC BY-SA 4.0), through kaikki.org (Wiktextract).
What does efficacy mean?
The meanings the English Wiktionary records, quoted as written there (source below).
noun
- 1
The ability to produce a desired effect under ideal testing conditions.
- 2
A degree of ability to produce a desired effect; effectiveness.
Toothbrush with improved efficacy
Source: English Wiktionary, dump of September 2, 2026 (CC BY-SA 4.0), through kaikki.org (Wiktextract).
Efficacy in Spanish, French, German and Portuguese
| Language | Words |
|---|---|
| French | efficience, efficacité |
| German | Wirksamkeit, Wirkungsvermögen, Wirkvermögen, Wirkungsfähigkeit, Wirkfähigkeit , Wirkungsgrad |
| Portuguese | eficácia |
| Spanish | eficacia |
Source: English Wiktionary, dump of September 2, 2026 (CC BY-SA 4.0), through kaikki.org (Wiktextract). Translations are Wiktionary's, by sense; a linked word has its own PlainSpell page.
Where does efficacy come from?
From Old French efficace, from Late Latin efficācia (“efficacy”) (whence -acy), from efficāx (“efficacious”); see efficacious.
Source: English Wiktionary, dump of September 2, 2026 (CC BY-SA 4.0), through kaikki.org (Wiktextract).