This article will focus on first-order half-life elimination as it is the most frequently encountered in clinical practice. In contrast, a few drugs follow zero-order elimination in which the drug amount decreases by a constant amount over time regardless of initial concentration (i.e., ethanol). Most clinically relevant drugs tend to follow first-order pharmacokinetics that is, their drug-elimination rates are proportional to plasma concentrations. The characteristic decreases of drugs over time have long been studied in a field known as pharmacokinetics and are depictable by basic differential equations. Different drugs have different half-lives however, they all follow this rule: after one half-life has passed, 50% of the initial drug amount is removed from the body. Understanding the concept of half-life is useful for determining excretion rates as well as steady-state concentrations for any specific drug. Half-life in the context of medical science typically refers to the elimination half-life. The definition of elimination half-life is the length of time required for the concentration of a particular substance (typically a drug) to decrease to half of its starting dose in the body.
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