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

Missing even a few doses every week can dramatically reduce the efficacy of PrEP. — Michal Ruprecht, NPR, 13 Aug. 2026

Towards the end there’s a dreamlike montage of uncertain efficacy, though stunning images of Toni and young Adela on a bed covered in vines and grasses are so mesmerizing that the eye wants to linger further. — Jay Weissberg, Variety, 13 Aug. 2026

However, effective delivery remains a critical challenge; Junho Jung stresses that topical applications often lack the efficacy of clinical injections, as ingredient stability and form are crucial for skin absorption. — Laia Farran Graves, Forbes.com, 12 Aug. 2026

Even before Bridge, the introduction of the current generation of GLP-1 medications for weight loss has brought prescribing challenges as profound as the medications’ remarkable efficacy. — Jeffrey Millstein, STAT, 11 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