The potency of an agonist is usually defined by its EC50-value. This can be calculated for any agonist by determining the concentration of agonist needed to elicit half of the maximum response of the agonist. Knowing an EC50-value is useful for comparing the potency of drugs with similar efficacies producing physiologically similar effects. The lower the EC50, the greater the potency of the agonist the lower the concentration of drug that is required to elicit the maximum biological response.
Potency of a ligand depends on its affinity and its efficacy. Below a certain concentration of agonist, the effect 'E' is too low to measure but at higher concentrations it becomes appreciable and rises with increasing agonist concentration until at sufficiently high concentrations it can no longer be increased by raising the concentration and asymptotes to a maximum Emax.
The term potency refers to the concentration of agonist at the EC50 value. The lower this concentration, the less drug is required to produce 50% of maximum effect and the higher the potency.