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

She's found that parents who are more open about these topics give their children more confidence and self-efficacy. — Madeline Mitchell, USA Today, 8 July 2026

The therapy remains controversial among mainstream oncologists, in part because there are few rigorous clinical studies evaluating its efficacy. — Shelby Hartman, Los Angeles Times, 8 July 2026

The clinical trials will test two separate drugs, MBP-134 and remdesivir, for efficacy in treating the Bundibugyo Ebola virus. — Omer Awan, Forbes.com, 7 July 2026

Neuropeptides Neuropeptides, short for neurotransmitter-inhibiting peptides, are the neurocosmetics with the most scientific backing to anchor their efficacy. — Deanna Pai, Vogue, 6 July 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