the difference between drug potency & efficacy

directory of Chem Help ASAP videos: https://www.chemhelpasap.com/youtube/ Potency is the concentration at which a compound exerts its effect or causes its response. Activity values like IC50 and EC50 both are a means of quantifying potency, and those activity values are read off the x-axis. A compound with a lower IC50 or EC50 value has a higher potency because the compound requires a lower concentration to give rise to its effect. Most activity curves have a logarithmic x-axis, and the curve has a sigmoidal shape. The x-axis value for the point of inflection of the sigmoid is the log IC50 or log EC50 for the curve. IC50 is normally used for enzyme inhibitors and antagonists. EC50 values are for agonists. Compounds with a smaller IC50 or EC50 value have a higher potency. Sometimes people say that curves correspond to a higher potency if they are "shifted left" along the x-axis. The magnitude of change in the curve on the y-axis corresponds to the efficacy of a compound. Compounds with a larger change on the y-axis have a higher efficacy. So, while potency is read off the x-axis, efficacy is read off the y-axis. Efficacy and potency do not share the same meaning. They are different words with different meanings.