Pharmacodynamics is the study of the relationship between the concentration of drug at the site of action and the biochemical and physiological effect. The most common mechanism is by the interaction of the drug with tissue receptors located either in cell membranes or in the intracellular fluid. The extent of receptor activation, and the subsequent biological response, is related to the concentration of the activating drug. Pharmacodynamics is the study of the biochemical, physiologic, and molecular effects of drugs on the body and involves receptor binding, postreceptor effects, and chemical interactions.The effects can include those manifested within animals including humans, microorganisms, or combinations oforganisms. Pharmacodynamics places particular emphasis on dose–response relationships, that is, the relationships between drug concentration and effect.