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 adjectival form of effect, I figured efficacy had an adjectival form. Perhaps efficate, however, to my disappointment, I've found that efficate is not a real word.

Does efficacy have an adjectival 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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    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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    @EdwinAshworth I did check the Merriam Webster online. It did not list an adjective form. – user39425 Dec 11 '14 at 19:01
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    @fredsbend: My closevote wasn't intended to imply "everyone knows that". The point is that anyone who wants to know can easily look it up. Plus it's part of basic English "derivational rules" that efficacy leads to efficacious, which in turn leads to efficaciously - which can easily be double-checked using a dictionary by anyone isn't sure whether that's a "real" word or not. But I'm old enough to remember Lily the Pink, so I do know the words. – FumbleFingers Dec 11 '14 at 19:10
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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'.