The word efficacy is often used in the medical field. For example, a paper might be on the efficacy of a treatment or a particular drug. The word, of course, implies how effectual that treatment or drug is.
Because effectual is the adjective form of effect, I figured efficacy had an adjective form. Perhaps efficate, however, to my disappointment, I've found that efficate is not a real word.
Does efficacy have an adjective form or is the only real option effectual? Though efficate is not a real word, would it be well understood if it were used in this sense? What are some other options I might use that would be a suitable, real-word substitute for efficate?