I usually use the word "effectiveness" in conversation, but sometimes I use the word "efficacy" then self-correct with "effectiveness" . Is there a practical difference between them?


For all practical purposes, I don't think there's any difference in meaning between efficacy and effectiveness - or indeed efficaciousness, come to that.

My own feeling is that efficacy is somewhat stilted - pompous, even, despite the fact that it's a shorter word...maybe people like to use efficacy because it sounds more scientific. Possibly my perception is influenced by the fact that effectiveness has been the more common version for the past century...

Here a link that notes the difference: http://www.ahrq.gov/clinic/tp/efftrialtp.htm...Note "to distinguish effectiveness (pragmatic) from efficacy (explanatory) trials " Also, more at wikipedia.

However, based on past experience with hearing the word in scientific contexts, I googled efficacy and found that "In medicine, it is the ability of an intervention or drug to reproduce a desired effect in expert hands and under ideal circumstances." I guess that distinguishes it from effectiveness in practical real-world application. I know that one company I worked for conducted both "efficacy trials" and "field trials" for its products. enter image description here

  • It rings true to me that in the specialied jargon of the pharmaceutical industry, effficacy might well be even the preferred term. Why don't you copy the whole of my answer in to yours, which I can then delete? I think between us we have it nailed (and you could use the points more! :) – FumbleFingers Jan 5 '12 at 21:52
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    How about I just downvote yours and you upvote mine, and we'll call it even? :) I think you nailed it on the pomposity factor, though...people like to use efficacy because it sounds more scientific. – JeffSahol Jan 6 '12 at 0:20
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    Well, one good answer is better than two half-good ones. Unless you ask me not to I might just copy mine into yours anyway (wonder what the site etiquette is on that! :) – FumbleFingers Jan 6 '12 at 1:36
  • @FumbleFingers: having two almost identical answers from two different authors looks really...really suspect. At least there's a bit trail in the editing, but the first impression is that one of the authors is plagiarizing the other. I think I'd prefer each to stand alone. – Mitch Jan 6 '12 at 14:06
  • I added his answer to mine, per his request....@FumbleFingers, can you delete now? – JeffSahol Jan 6 '12 at 14:17

James Fernald, Funk & Wagnalls Dictionary of English Synonyms, Antonyms & Prepositions (1947) distinguishes between the adjectives effective and efficacious (and effectual) as follows:

That is effective which accomplishes an intended effect with emphasis, decision, and certainty; that is effectual which acts with such finality as to leave no more to be done. Effective measures will put an effectual stop to objectionable proceedings. That is efficacious which is thought of as having power, which may be active or may continue latent, to produce an effectual result; [examples omitted]. A person ... may be called effective in some special relation; as, an effective speaker; ... Efficacious and effectual are not used of persons.

Merriam-Webster's Dictionary of Synonyms (1984) lists efficacious (along with effectual and efficient) as synonyms of effective. Here are the relevant comments from the entry for effective:

effective, effectual, efficient, efficacious all mean producing or capable of producing a result or results, but they are not freely interchangeable in idiomatic use. Effective emphasizes the actual production of an effect or the power to produce a given effect [examples omitted] ... Efficacious implies the possession of the quality or virtue that gives a thing the potency or power that makes it effective [examples omitted]

Consistent with Merriam-Webster's distinction, we might say, for example,

Orange juice is efficacious as a cold remedy because it contains vitamin C, which is effective in combatting cold symptoms.

Switching to the noun forms that the poster asked about, we might put it this way:

The efficacy of orange juice as a cold remedy is owing to the effectiveness of vitamin C in combatting cold symptoms.

I must say, though, that these are extremely fine hairs that the dictionaries are splitting; I doubt that many English speakers maintain a clear distinction between effectiveness and efficacy.


In general usage, effectiveness and efficacy have the same meaning. In medical jargon there is a subtle, but important difference. From Wikipedia:

In medicine, effectiveness relates to how well a treatment works in practice, as opposed to efficacy, which measures how well it works in clinical trials or laboratory studies.

  • Wikipedia doesn't provide a source that effectiveness is used that way in medicine and efficacy is not. In my experience, efficacy is used for both circumstances. – user39425 Dec 11 '14 at 16:54
  • Wikipedia is neither an effective nor efficacious source of research – Kirby Aug 22 '16 at 19:06

Not synonyms. Possibly hair-splitting but trial-linked terminology often is (e.g. validation vs verification). Efficacy is the ability to produce a desired or intended result. Effectiveness is the degree to which something is successful in producing a desired result: hence, a measurement often preceded by a quantitative adjective. In credentialed trials, the extra nuance is added that "efficacy" often refers to predictions or results in ideal (i.e. "laboratory") conditions whereas "effectiveness" refers to field (i.e. "real world") results. For example, product efficacy was reduced to 83% effectiveness because some users applied it on rainy days.

  • I read this as "efficacy is the potential ability and effectiveness is the actual ability to produce a desired result". Suddenly all became clear! – m-smith Feb 24 '20 at 20:05

Based on what I have read, I would say that efficacy could be looked at as the "effective ability" of something whereas effectiveness could be looked at as the "effective outcome" of something. But I agree with another comment that it really seems to be splitting hairs.


To me, "efficacy" and "effectiveness" are essentially parallel to "utility" and "usefulness". The former connotes an idea of being used for the proper purpose while the latter simply indicates that it works. Using a screwdriver to put in a screw is both effective and efficacious. It has high utility and usefulness. Using the screwdriver to pry up a paint can would arguably do the same. But if you're using the same screwdriver as an improvised chisel, it is effective and it is useful, but that's not the intended purpose.

  • The distinction between use and utilise has been covered before, but I don't think that has any bearing on your (to me, spurious) distinction between efficacious and effective. – FumbleFingers Jan 5 '12 at 19:34
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    :) I honestly didn't know if anyone would share my impression. Connotations of words are always tricky because they've often picked up shades of meanings that have no real historical basis other than that of those involved in the conversation. – Sean Duggan Jan 5 '12 at 20:47
  • Yeah - apologies for that (to me, snarky) (to me spurious) there. In general I thoroughly approve of people sensing fine distinctions between two word. Collectively, it's how we enrich our language over time, and individually it's a real buzz discovering that you're not alone in detecting some subtlety that hasn't yet made it into "common knowledge" and dictionaries. It's all about having your finger on the pulse. Even so, in this case I do think "efficacy" is just a somewhat "dated" word, but that might be because "efficacious" is effectively a cliched archaism nowadays. – FumbleFingers Jan 5 '12 at 21:44

Some scientists do make a distinction.

Ernst & Pittler (2006, medical scientists):

Seemingly similar in meaning, 'efficacy' and 'effectiveness' express distinctly different concepts. A medical intervention is 'efficacious' if it works under strictly controlled (laboratory) conditions and it is considered 'effective' if it works under 'real life' conditions. Efficacy (or fastidious) trials test for efficacy and effectiveness (pragmatic) studies for effectiveness of a therapy.

Cartwright & Hardie (2012, philosopher of science and economist):

A great deal of this book is devoted to the significance of “it worked there” if you are trying to bet whether “it will work here” — in our and the literature’s language, the connection between evidence for efficacy and evidence for effectiveness.


Based on their definitions, and this thesaurus entry, I'd say they are synonyms, and the difference is purely stylistic word choice/personal preference.

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    -1 for differency and purley – James Waldby - jwpat7 Jan 5 '12 at 18:41
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    @jwpat7 For typos? Really? – yoozer8 Jan 5 '12 at 19:11
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    @jwpat7, you should have said "for typoes? rly?" – JeffSahol Jan 5 '12 at 20:56
  • You don't get a red line under misspelled words? What abut proof reading? – user39425 Dec 11 '14 at 16:55
  • I guess people coming to this page are hoping to gain more efficacy in all aspects of their English, including spelling. – Kirby Aug 22 '16 at 19:08

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