efficacy

noun

ef·​fi·​ca·​cy ˈe-fi-kə-sē 

plural efficacies

: the power to produce a result or effect

increased teacher efficacy in classroom management

The efficacy of this treatment has not yet been proved.

In the upcoming … clinical trial, researchers will further investigate the efficacy and safety of the vaccine.

—Julia Landwehr

Synonyms of efficacy

• effectiveness
• efficaciousness
• efficiency

Examples of efficacy in a Sentence

In the planner's view, Rumsfeld had two goals: to demonstrate the efficacy of precision bombing and to "do the war on the cheap." —Seymour M. Hersh, New Yorker, 7 Apr. 2003

Vaccines exist, but their efficacy against aerosolized plague is unknown. —Sharon Begley et al., Newsweek, 8 Oct. 2001

… efficacy does not have to be demonstrated before homeopathic products are marketed. —Alison Abbott et al., Nature, 26 Sept. 1996

questioned the efficacy of the alarms in actually preventing auto theft

Recent Examples on the Web

And with that practice come new, more advanced discussions about the safety, efficacy and depth of this technology in our workplaces. — Larry Bradley, Forbes.com, 6 Aug. 2026

Prasad defended his decision by arguing that the company didn’t compare its mRNA vaccine’s efficacy to the correct existing vaccine. — Beth Mole, ArsTechnica, 6 Aug. 2026

The drug has also shown higher efficacy than a rival obesity pill from Lilly, which launched a few months later than Novo's treatment. — Annika Kim Constantino, CNBC, 6 Aug. 2026

Green Science Alliance plans to expand production using various biomass waste feeds and evaluate efficacy against a broader array of agricultural pests. — Mrigakshi Dixit, Interesting Engineering, 4 Aug. 2026

Word History

Etymology

borrowed from Latin efficācia, from efficāc-, efficāx "capable of fulfilling a function, efficacious" + -ia -y entry 2

First Known Use

1527, in the meaning defined above

Rhymes for efficacy

• malagasy
• profligacy
• assay
• bassi
• brassey
• brassy
• cassie
• chassis
• classy
• gassy
• glassy
• grassy

Kids Definition

efficacy

noun

ef·​fi·​ca·​cy ˈef-i-kə-sē 

plural efficacies

: the power to produce a desired result