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

Immunity from flu vaccination is generally thought to last for around four to six months before losing some efficacy, according to the CDC. — Stephen Underwood, Hartford Courant, 2 Oct. 2026

Those figures are based on efficacy analysis that assumes patients remained on treatment in the trial. — Annika Kim Constantino, CNBC, 2 Oct. 2026

In early September, there was a virtual call with 80-plus artists, executives and stakeholders from the Asian music community with trustees and governors from the Recording Academy to review the efficacy of the new award. — Hannah Dailey, Billboard, 2 Oct. 2026

Antibody-drug conjugates, gene therapies, cell therapies and personalized medicines are creating new questions around dose, exposure, efficacy, safety and patient variability. — Mirjam Trame, Forbes.com, 2 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