Efficacy vs Potency

Potency is the concentration (EC50) or dose (ED50) of a drug required to produce 50% of that drug’s maximal effect. Efficacy (Emax) is the maximum effect which can be expected from this drug (i.e. when this magnitude of effect is reached, increasing the dose will not produce a greater magnitude of effect) To further explained the difference between efficacy and potency, please have a look at below figure. There are 2 drugs in the figure i.e. DrugA and DrugB:

• Both Drug A and Drug B achieve the same maximum effect, i.e. they have equal efficacy.
• However, drug A achieves this effect at a lower dose. 
• Thus, Drug A has higher potency than Drug B.