Efficacious n. : Possessing the quality of being effective; productive of, or powerful to produce, the effect intended; as, an efficacious law.

Example:
Is it really Efficacious to use shampoos made for a particular color of hair?

Taxonomy
efficacious, Merriam-Webster Online Dictionary, Dictionary, ef, fi, cious, webster, Word Games, Webster Online Dictionary with audio pronunciations, glossary, english, language, ESL, e-f, adjective, Etymology, Latin, efficax.

Defination from the web:
efficacious from the Merriam-Webster Online Dictionary with audio pronunciations, thesaurus, Word of the Day, and word games. ... having the power to produce a desired effect <an efficacious remedy> synonyms see effective. — ef·fi·ca·cious·ly adverb. — ef·fi·ca·cious·ness noun ...