what's the difference between efficacy and efficiency
Quoting this article Efficacy, effectiveness, efficiency, 1  efficacy is the extent to which a drug has the ability to bring about its intended effect under ideal circumstances, such as in a randomised clinical trial. 2  effectiveness is the extent to which a drug achieves its intended effect in the usual clinical setting.
These terms cannot be interchanged and have a specific meaning. Efficiency is the ability of an intervention to have effects on a specific marker in relation to a condition without it necessarily been relevant to patients. The study is done under highly controlled conditions with highly compliant participants.