A drug class is a set of medications and other compounds that have a similar chemical structures, the same mechanism of action (i.e. binding to the same biological target), a related mode of action, and/or are used to treat the same disease.
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In several dominant drug classification systems, these four types of classifications form a hierarchy. For example, the fibrates are a chemical class of drugs (amphipathic carboxylic acids) that share the same mechanism of action (PPAR agonist) and mode of action (reducing blood triglycerides), and that are used to prevent and treat the same disease (atherosclerosis). Conversely, not all PPAR agonists are fibrates, not all triglyceride lowering agents are PPAR agonists, and not all drugs used to treat atherosclerosis are triglyceride-lowering agents.
A drug class is typically defined by a prototype drug, the most important, and typically the first developed drug within the class, used as a reference for comparison.