"sure to have the desired effect" (often of medicines), 1520s, from Latin efficaci-, stem of efficax "powerful, effectual, efficient," from stem of efficere "work out, accomplish" (see effect (n.)) + -ous. Related: Efficaciously; efficaciousness.
efferent
effervesce
effervescence
effervescent
effete
efficacious
efficacy
efficiency
efficient
effigy
effleurage