Every drug that enters the body gets removed over time. The drug is either excreted intact or metabolized first and then eliminated from the body. The rate of drug elimination is related to the drug's plasma concentration through a parameter called clearance. It estimates the volume of plasma being cleared of the drug in unit time. Usually, drug clearance follows first-order kinetics, where a constant fraction of the drug is eliminated per unit of time. As a result, the plot of drug concentration versus time is exponential, indicating that an increase in drug concentration increases its elimination rate. In contrast, some drug eliminations, such as for phenytoin and aspirin, follow zero-order kinetics. High therapeutic doses of these drugs saturate the metabolic capacity of enzymes, resulting in only a fixed amount of drugs being eliminated at a constant rate. The plot of drug concentration versus time becomes linear, implying that increasing the drug concentration does not affect the elimination rate.