Efficacious rhymes with:
perspicacious, perspicacious, audacious, capacious, ceraceous, crustaceous, fallacious, flirtatious, gracious, hellacious, herbaceous, horatius, ignatius, loquacious, mendacious, ostentatious, perspicacious, predaceous, proteinaceous, salacious, sebaceous, spacious, tenacious, vexatious, vivacious, voracious
Efficacious sounds like:
eaves, ebasco, ebaugh, ebbs, efface, efficacy, effigy, eohippus, epes, epic, epic's, epics, epoch, epochs, epoxy, eppes, eppich, epps, epsco, eva's, evacuee, evacuees, eves, evoke, evokes, eyepiece