Potency is the amount of agonist needed to elicit a desired response. The potency of an agonist is inversely related to its EC value. The EC can be measured for a given agonist by determining the concentration of agonist needed to elicit half of the maximum biological response of the agonist. The EC value is useful for comparing the potency of drugs with similar efficacies producing physiologically similar effects.  The smaller the EC value, the greater the potency of the agonist, the lower the concentration of drug that is required to elicit the maximum biological response.