efficacious

Etymology

It is borrowed from Latin efficāx (“powerful, effectual, efficient”), stem efficāc-, with the suffix -ous,^[2][3][4] which Merriam-Webster and the Oxford Dictionary of English give as -ious.^[5][6] The Latin adjective is from efficiō (“to work out, accomplish”),^[3][7][8][4] which is formed from ex (“out”) and faciō (“to make, do”),^[9][3] from Proto-Indo-European *dhe- (“to set, put”).^[3] The verb is also the source of efficient and, through its past participle, of effect.^[9][10]

Earliest recorded meaning

Producing, or sure to produce, the intended effect: 1528, in W. Roy's satire against Cardinal Wolsey.^[2][5] Dictionary.com gives 1520–30,^[11] and Etymonline the 1520s.^[3]

Semantic evolution

The OED says that it is used of instruments, methods and actions and, in prose, not of personal agents.^[2] Etymonline notes that it is often said of medicines,^[3] and the Shorter Oxford illustrates it with P. Medawar's doubt that psychoanalytic treatment as such is efficacious.^[12] Johnson (1755) defined it as "productive of effects; powerful to produce the consequence intended", and quoted Philips's "by one efficacious breath".^[13] Earlier English had the noun efficace, from Old French, from about 1225 to 1712;^[2] efficacy is recorded from the 1520s,^[3] and an adjective efficace is recorded only once, about 1572.^[2]

Timeline

• c. 1225 – the earlier noun efficace, efficacy (obsolete by 1712).^[2]
• 1520s – the noun efficacy.^[3]
• 1528 – the adjective efficacious, producing the intended effect.^[2][5]
• c. 1572 – the adjective efficace, in the same sense.^[2]

Meanings

Adjective

1. Producing, or able to produce, the intended result; effective, especially of a remedy or method: an efficacious treatment for hay fever.^[8][5][2][3]

Word family

• efficaciously
• efficaciousness
• nonefficacious
• unefficacious

Related words

• effect
• effective
• effectual
• efficacity
• efficacy
• efficiency
• efficient
• feckless

References

1. ^ a b Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h i Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
3. ^ a b c d e f g h i Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
4. ^ a b Klein, Ernest. A Comprehensive Etymological Dictionary of the English Language. 2 vols. Amsterdam: Elsevier, 1966–1967.
5. ^ a b c d Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
6. ^ Stevenson, Angus, ed. (2010), Oxford Dictionary of English, 3rd ed., Oxford: Oxford University Press
7. ^ The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
8. ^ a b Collins English Dictionary (2023), Glasgow: HarperCollins
9. ^ a b Ayto, John (2005), Word Origins: The Secret Histories of English Words from A to Z, 2nd ed., London: A & C Black
10. ^ Weekley, Ernest (1921), An Etymological Dictionary of Modern English, London: John Murray
11. ^ Dictionary.com Unabridged (2016), based on the Random House Unabridged Dictionary, New York: Random House
12. ^ Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
13. ^ Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.