efficacious

Efficacious is an adjective that describes something capable of producing the desired or intended result. It is commonly used of medicines, therapies, interventions, or policies, where a claim of efficacy is supported by evidence. In everyday language, “effective” is more common, but “efficacious” appears in scientific, medical, and formal writing to emphasize reliable outcomes.

Etymology: From Latin efficax “powerful, productive,” from efficire “to bring about, to accomplish.” The form entered English in the early modern period and has retained a formal tone.

Usage notes: In medicine and psychology, “efficacy” refers to how well a treatment works under ideal conditions, such as controlled clinical trials, whereas “effectiveness” refers to real-world performance. “Efficacious” describes the outcome demonstrated in those controlled settings; “effective” describes actual results in practice. The term is more common in technical writing and may sound archaic or overly formal in casual discourse.

Examples: “The vaccine was efficacious in phase III trials.” “The program proved efficacious in reducing relapse rates in the pilot study.” The word is also encountered in philosophy and theology to denote causes or mechanisms that reliably bring about a desired effect; in secular usage, it retains a formal, technical tone.

See also: efficacy (noun), effective, efficiency, efficacy vs. effectiveness. Efficacious is related but distinct from efficiency, which concerns the input-output ratio, not the success of producing the intended result.