What is the Latin word for efficacy?

The usual Latin words for “efficacy” are volō, virtūs, efficācitās; also efficiō, pondus, potestās. The right choice depends on the sense; see the distinctions below.

English → Latin

“efficacy” in Latin

volōto wish, to please; to want; to mean, to intend virtūsmanhood, manliness, virility; courage, valor, resoluteness, gallantry; virtue, goodness efficācitāsefficiency, efficacy, power efficiōto make out or work out; effect, execute, complete, accomplish, make, form, compose; to cause to occur, to bring about, pondusweight; weight of a pound; heaviness, weight of a body potestāspower, ability; mastery, control; authority, jurisdiction

The English word efficacy itself goes back to Latin efficiō, through efficācia.

Riddle & Arnold A Copious and Critical English–Latin Lexicon, 1864

efficacy

vis (power; or circumlocution with valere): virtus (medicinal property of herbs, etc.): efficacitas: efficientia (☞ efficacia, post-classical, in the elder Pliny). In poetry, sometimes pondera (interdum lacrimae pondera vocis habent, Ov.). Natural efficacy, naturalis efficientia or potestas.

Quick answer

What is the Latin word for efficacy?: The usual Latin words for “efficacy” are volō, virtūs, efficācitās; also efficiō, pondus, potestās. The right choice depends on the sense; see the distinctions below.

Entry from J. E. Riddle & T. K. Arnold, A Copious and Critical English–Latin Lexicon (rev. C. Anthon, New York: Harper, 1864), digitised by Fergus J. P. Walsh, CC BY-SA 4.0; Latin markup adapted by latindictionary.co.