Efficacy and Potency

Potency denotes the amount of drug needed to produce a given effect.

Efficacy is the maximal effect that a drug produces irrespective of concentration (dose)

Potency: We generally refer to potency as the amount of drug dose that produces a quantal effect in 50% of the population- quantal dose response ED50. Potency can also be referred as the amount of drug required to produce 50% of it maximal drug effect-graded dose response EC50

A highly potent drug evokes a larger response at lower concentrations

Efficacy: Refers to the relative ability of a drug-receptor complex to produce a maximum functional response. A highly efficacious agonist can produce the maximal response of the receptor system while occupying a relatively low proportion of the receptors. This maximal effect produced is called maximal efficacy (Emax) and is irrespective of drug concentration.

Efficacy can only be plotted on a graded dose response curve and not on a quantal dose response curve because you are measuring the maximal efficacy REGARDLESS of the concentration (dose) required to produce the effect

In deciding which of two drugs to administer to a patient, the prescriber must usually consider their relative effectiveness rather than their relative potency. Potency largely determines the dose of the chosen drug.