WordWeb Online

1. Marked by qualities giving the power to produce an intended effect
   "written propaganda is less efficacious than the habits and prejudices...of the readers"; "the medicine is efficacious in stopping a cough"
    
2. Producing or capable of producing an intended result or having a striking effect
   "an efficacious law";
   - effective, effectual

See also: effective, effectiveness, effectivity, effectuality, effectualness, efficaciousness, efficacy, efficient, hard-hitting, impelling, impressive, potent, powerful, rough-and-ready, stiff, strong, telling, trenchant, useful, utile

Antonym: inefficacious

Encyclopedia: Efficacious