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?

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    What about effective: producing a result that is wanted : having an intended effect. – user66974 Dec 11 '14 at 16:54
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    It's not efficient? – Digital Chris Dec 11 '14 at 16:55
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    I think the existence, meaning, and relevance of efficaciously are General Knowledge. – FumbleFingers Dec 11 '14 at 17:23
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    An efficacious approach would be to look up 'efficacy' and nearby entries in a dictionary. – Edwin Ashworth Dec 11 '14 at 18:43
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    One can't easily look up the adjectival form of efficacy because half a dozen common dictionaries I easily checked do not have an adjectival form listed. – pazzo Dec 11 '14 at 20:32

Effective is a common choice, or you may use the (slightly more highbrow) efficacious:

capable of or successful in producing an intended result; effective as a means, remedy, etc

I do not believe "efficate" would be well-understood.

  • Efficacious is a bit of a mouthful too. Why do you think efficate would not be well-understood? Remember, this is in a medical setting where the audience understands the meaning of efficacy. – user39425 Dec 11 '14 at 17:00
  • @fredsbend As a stand-alone word, the "-ate" makes me suspect the word is a verb. I know there are many adjectives with that ending (e.g., latinate) but there are probably many more (and much more common) verbs with the ending: "celebrate", "hyphenate", "pontificate". I expect it to mean "to perform an effect". Perhaps I'd think differently if I saw it in context, though. – apsillers Dec 11 '14 at 17:05
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    @fredsbend Also, The American Heritage Dictionary explicitly lists efficacious in its entry on efficacy. And for whatever my personal experience is worth, when I hear efficacious, I assume the context is medical by default. (I am not a medical professional.) – apsillers Dec 11 '14 at 17:08
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    If someone used the word "efficate" I'd point them to the room down the hallway. – Hot Licks Dec 11 '14 at 18:05
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    A pubmed search for "efficacious" turns up about 40 thousand results. A search for "efficate" yields exactly 1. This supports the idea that -- even in a medical context -- efficate is an unusual term. (Or at least one that biomedical researchers don't like to include in abstracts.) – Curtis H. Dec 11 '14 at 18:11

Conventionally, the adjectival form of efficacy is 'efficacious'. The adverb would be 'efficaciously'.