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

Marlow’s testimony also challenged the efficacy of Flock cameras and argued that the only situations that they have been studied and found to deter crime is in relation to car thefts and property crimes. — Graham Hurley, CNN Money, 1 Nov. 2026

The committee noted conflicting evidence on the drug’s efficacy and clear safety risks, including brain bleeds, in large numbers of patients. — C. Joseph Ross Daval, STAT, 30 Oct. 2026

For years, Apple questioned the necessity and efficacy of laptop touch screens but worked on implementing the technology—both behind the scenes and in the open—through its iPhone and iPad product lines. — Joe Osborne, PC Magazine, 29 Oct. 2026

But the peptides covered in the latest HHS policy haven’t undergone the safety and efficacy testing required by the FDA to obtain approval for human use. — Emily Kay Votruba, EverydayHealth.com, 29 Oct. 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