efficacious
/ˌɛf.əˈkeɪ.ʃəs/adjective
efficacious is spelled E-F-F-I-C-A-C-I-O-U-S: 11 letters and 4 syllables. It appears about once in every 3 million words in written English, the 49,949th most frequent word form (wordfreq, 2021 data). As an adjective, efficacious means “Effective; possessing efficacy” (English Wiktionary, September 2026).
How many letters and syllables are in efficacious?
| Letters | 11: E-F-F-I-C-A-C-I-O-U-S |
|---|---|
| Syllables | 4 (from its American pronunciation) |
| In writing | about once in every 3 million words; the 49,949th most frequent word form |
| Wrong spellings searched | none measured in search |
| Rhymes with |
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); rhymes and same-letter words are computed by PlainSpell from its own word list.
How common is efficacious?
Efficacious appears about once in every 3 million words in written English, the 49,949th 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 efficacious?
- UK /ˌɛf.ɪˈkeɪ.ʃəs/
- US /ˌɛf.əˈkeɪ.ʃəs/
- Australia /ˌef.əˈkæɪ.ʃəs/
Efficacious 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-efficacious.wav, Wikimedia Commons (author and licence on each file's page).
What rhymes with efficacious?
The 12 most written words on PlainSpell end in the same sounds as efficacious from its last stressed vowel (/eɪʃəs/):
- tenacious
- audacious
- herbaceous
- ostentatious
- rapacious
- sebaceous
- pugnacious
- loquacious
- perspicacious
- argillaceous
- contumacious
- pertinacious
Computed by PlainSpell from the pronunciations in the English Wiktionary (dump of September 2, 2026): a rhyme shares every sound from the last stressed vowel to the end; ordered by written frequency (wordfreq 3.1).
Forms of efficacious
- more efficacious comparative
- most efficacious superlative
Source: English Wiktionary, dump of September 2, 2026 (CC BY-SA 4.0), through kaikki.org (Wiktextract).
What does efficacious mean?
The meanings the English Wiktionary records, quoted as written there (source below).
adjective
- 1
Effective; possessing efficacy.
This medicine is efficacious.
Source: English Wiktionary, dump of September 2, 2026 (CC BY-SA 4.0), through kaikki.org (Wiktextract).
Efficacious in Spanish, French, German and Portuguese
In the sense effective – see also effective:
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 efficacious come from?
From Old French efficacieux (whence -acious), from Latin efficāx (“efficacious”) + -ous, from efficere (“to effect, to accomplish”); see effect.
Source: English Wiktionary, dump of September 2, 2026 (CC BY-SA 4.0), through kaikki.org (Wiktextract).