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

For Lilly, Zepbound’s higher efficacy than the original doses of Wegovy has also been key to securing its market share lead over Novo in the injectable space. — Annika Kim Constantino,elsa Ohlen, CNBC, 25 Sep. 2026

Despite growing evidence supporting safety and efficacy, and the lifting of the warning, a lingering stigma is keeping women from life-saving care. — Theara Coleman, TheWeek, 24 Sep. 2026

It’s proven to reduce the number and severity of hot flashes, day and night, based on clinical studies measuring efficacy over 24 hours at four and 12 weeks. — Partner Content, Variety, 23 Sep. 2026

Leaders in the field demanded better research, standardized methods, and clinical trials to confirm safety and efficacy. — Lorenzo Cohen, STAT, 22 Sep. 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