efficacy

Etymology

It is borrowed from Latin efficācia (“efficacy, efficiency”),^{[2][3][4][5][6]} from efficāx (“powerful, effectual”),^[3][4][5] from efficiō (“to work out, accomplish”),^[3][4][7] the source of effect.^[3][4] It replaced efficace, probably first recorded before 1200 in the “Ancrene Riwle”, from Old French efficace,^[4][3] and efficacite, from the early 15th century, from Latin efficācitās.^[3][4] The Anglo-Norman noun is recorded from 1160–74, for efficacy, virtue and power to produce an effect.^[8]

Earliest recorded meaning

The power to produce an effect, or the intended effect: 1527, in L. Andrewe's translation of Brunschwig's Vertuose Boke.^[2][4][3] Merriam-Webster Unabridged dates the word to the 13th century.^[9]

Semantic evolution

The main sense has not changed since the 16th century.^[2][6] For a time the word also meant an effect (from 1549, in Latimer, until 1614),^[2][6] and Locke used it in 1690 for a way of bringing about a result.^[2] Johnson (1755) defined it as the power to produce effects.^[10]

Timeline

• before 1200 – the earlier efficace.^[4][3]
• early 15th century – the earlier efficacite.^[3]
• 1527 – power to produce effects.^[2][4]
• 1549 – an effect (obsolete by 1614).^[2]
• 1690 – a way of bringing about a result, in Locke.^[2]

Meanings

Noun

1. The power or ability to produce the intended result; effectiveness: They doubted the efficacy of the new treatment.^{[9][11][2][6]}
2. (obsolete, 1549–1614) An effect.^[2][6]

Word family

• inefficacy
• nonefficacy

Related words

• effect
• effective
• effector
• effectual
• effectuate
• efficacious
• efficacity
• efficiency
• efficient

Usage notes

The OED notes that efficacy is not used of people as agents, unlike efficacious.^[2] Collins gives efficaciousness as a synonym.^[11]

References

1. ^ Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h i j k 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 c d e f g h i Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
5. ^ a b The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
6. ^ a b c d e Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
7. ^ Skeat, Walter W. (1911), A Concise Etymological Dictionary of the English Language, new ed., Oxford: Clarendon Press
8. ^ Anglo-Norman Dictionary (AND2), Aberystwyth University, anglo-norman.net (2nd ed., 2005–)
9. ^ a b Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
10. ^ Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.
11. ^ a b Collins English Dictionary (2023), Glasgow: HarperCollins