efficacy vs potency

Efficacy:

Efficacy indicates the maximum intensity of response to a drug, eg. if the intensity of drug A is higher than drug B, then drug A is more efficacious than drug B
ED50 (effective dose-50%) is the dose or drug that gives rise to the designated response in 50% of the subjects or produced a 50% response. ED50 is easier to measure than maximum effect and is used to determine the efficacy
Similar to ED-50, IC-50 (inhibitory concentration-50%), LD-50 (Lethal dose-50%) are also determined by the dose-response curve.


Potency:

The term potency is generally used to a comparison of ED50 of two or more drugs that have parallel log dose-response curves
The drug that reaches the ED50 at the lower dose is the more potent
Potency is a term that is often misused (confused with efficacy). Potency is not important if you can increase the dose of the less potent drug without causing side effects