I have a confession: I’m a compulsive looker-upper. So today, even though I was pretty sure it was OK to use the word “efficacy” in the sentence I was writing, I checked anyway. Sure enough, in most contexts it’s an appropriate synonym for “effectiveness.” (I liked efficacy in this case, because I was talking about something that had been statistically proven, and I think it sounds more “science-y.” How’s that for a peek inside a writer’s brain?)
But I also like looking things up because I almost always learn something. Today’s lesson was that, in medical parlance, “efficacy” and “effectiveness” mean different things, and it’s a nuance that’s quite significant. Efficacy is a narrower definition that means how well something works in an ideal or controlled setting, such as a clinical trial. Effectiveness describes how well it works under “real-world” conditions. Effectiveness, for example, takes into consideration how easy a drug is to use, and potential side effects, whereas efficacy measures only how well it produces the desired result.
The moral: In academia, particularly in science or medicine-based fields, you probably want to be careful about interchanging these words. But for the layman, they are essentially synonymous.
Get edited. 
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Thank you for clearly explaining the difference between these two words. I learned something today!
I laughed when I read, “How’s that for a peek inside a writer’s brain?”
Excellent discussion. A suggested supplement:
You failed to note or discuss a still-valid synonym for “efficacy” – “efficaciousness” Of these two, I imagine efficacy is the newer word and I prefer it. I would guess that in common usage (the ultimate arbiter of such things) the less pretentious “efficacy” has thankfully won out.
Pardon my quasi (or complete?) digression but this seems somewhat analogous to the synonyms “relevance” (good word) and “relevancy.” (absurd!) (I recognize that I am, however, in this instance, championing the older – or at least not ending with a “y” – word, but “relevance” works just fine – it’s perfectly efficacious – while “relevancy” sounds like someone wanted to add a syllable to sound. . . fancy, which to me comes off as pretentious, and thus the word for a preening loser. Whatever.)
Where such synonyms battle, eventually one wins out and the other becomes marked as an “not preferred term of usage” which eventually fades to “obsolete” or “ancient usage” then “improper usage”, and ultimately on to what I would call an “extinct” word where it’s no longer even found in dictionaries. For examples,just read some Chaucer or Shakespeare original manuscripts. (Not being the type to “look up” as much as you – although I do “look up” to folks like you who do so * – I’ve not done so here. As such, in this paragraph, I”m fairly certain I’m (1) not using the proper dictionary terms, and/or (2) not being completely accurate in how dictionaries go through such words, over time, between proper and current preferred usage to where such words are no longer even listed.)
Again, you wrote a nice comment.
Thanks,
Mitchell
P.s., I find it interesting that another Mitchell has commented, albeit under the common contraction of the name. I happen to be a lawyer – though I try to avoid hanging out with such generally scurrilous folks – and I’ve been surprised that of all the “Mitchell’s” I’ve ever met, I’d guess 97% or more have likewise been or become attorneys, including the first one I’d ever met (in First Grade) – I’ve always found that interesting. (Now THAT’S a digression.)
M
*Seriously. I always mean what I say and say what I mean.
This is helpful. Thanks!!
Hows this for an example; “…flu vaccines showed an efficacy of 79% and an effectiveness of 33%”. No, seriously.
I frequently run into the need for a word that means effectiveness and efficiency together. Let me give an example that illustrates why. Suppose you have a sales force and you take action to improve their sales capabilities. This means you can either sell more with the same size of sales force (effectiveness) or you can achieve same sales with a smaller sales force (efficiency). When I first heard the word efficacy, I thought it was what I was looking for š
I enjoyed your comments. I am also a “look-it-upper”. š