Efficiency:
Efficiency is doing things in the most economical way whether in terms of time, energy, or money. It is the ratio of the output to the inputs of any system (good input to output ratio). Once you have found a solution, you can then try to improve it by making it more efficient. In clinical trials, two drugs could be equally effective: they both manage to improve patients’ symptoms in the real world. However, if one of them is much more costly than the other one, that drug won’t be considered as efficient.
Efficacy:
Efficacy is getting things done. It is the ability to produce a desired amount of the desired effect, or success in achieving a given goal. The word efficacy is used in pharmacology and medicine to refer to the maximum response achievable from a pharmaceutical drug in research settings. It simply focuses on the output without considering the input.
To summarize: efficacy means getting things done (is it working?), effectiveness means doing the right things (is it actually working well?), and efficiency means doing things right (is it working in the most economical way?)