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I feel that there is a subtle difference in meaning between 'efficacy' and 'efficiency', but I couldn't find any authoritative sources that could help me confirm or refute this. Is there any difference between the two words, especially in scientific context?

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Did you check in a dictionary? – Hugo Dec 5 '11 at 14:51
Agree with Hugo, the difference is not really subtle at all. Check a dictionary. – ghoppe Dec 5 '11 at 15:10

closed as general reference by Hugo, aedia λ, ghoppe, jwpat7, Mitch Dec 5 '11 at 18:58

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Efficacy means effectiveness, i.e. the ability of something, e.g. a medicine, to produce the results you want.

Efficiency means ability to do something well without a waste of time or money, e.g. I was impressed by her efficiency in this project.

For a more technical context, I found the following additional definitions of efficiency:

a)The ratio of the effective or useful output to the total input in any system. b) The ratio of the energy delivered by a machine to the energy supplied for its operation.

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Agreed - Efficacy is about the results, whereas Efficiency is about waste when achieving the results. Something could be said to have good efficacy but poor efficiency, if it was effective but comparatively costly or wasteful. Likewise, something could be considered to show good efficiency but poor efficacy, if it only is moderately effective, but isn't particularly wasteful in the process. – CJM Dec 5 '11 at 15:03

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