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

The study, which focused on people who had undergone surgery to remove a melanoma, tested the drug's efficacy at reducing the risk of the cancer's return. — Megan Cerullo, CBS News, 19 Aug. 2026

The administration gutted the office that tests weapons systems for safety and efficacy. — Jesse Coburn, ProPublica, 19 Aug. 2026

Over the course of a month, our testers smoothed these lotions all over their body, taking notes on ingredients, texture, and efficacy in addressing their skin concerns. — Mary Honkus, Glamour, 18 Aug. 2026

Human safety and efficacy must be tested through staged clinical trials, followed by regulatory reviews. — Matthew Kayser, USA Today, 18 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