Pharmacodynamics is the study of how drugs have effects on the body. The most common mechanism is by 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 (the agonist). This relationship is described mathematically by the dose–response curve, which plots the drug dose (or concentration) against its effect. This important pharmacodynamic relationship can be influenced by patient factors (e.g.