- capable of having the desired result or effect; effective as a means, measure, remedy, etc.: The medicine is efficacious in stopping a cough.
- capable of or successful in producing an intended result; effective as a means, remedy, etc
adj.“sure to have the desired effect” (often of medicines), 1520s, from Latin efficaci-, stem of efficax (see efficacy) + -ous. Related: Efficaciously; efficaciousness.