What's the difference between "efficacy" and "effectiveness"?

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.

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.