Classes are subcomponents of drug categories and are based either on the chemical structure of the drug or on its 'mechanism of action,' i.e., how it works to achieve its results. For example, the analgesic category, or drugs which treat pain, is broken down into two classes - opioids (such as codeine or morphine) and non-opioids (such as ibuprofen or aspirin). Certain classes are subdivided into an additional level of specificity. For example, the beta-adrenergic blocking agent class of the cardiovascular category, or drugs used to treat hypertension, is subdivided into alpha-beta-adrenergic blocking agents (such as Normodyne), cardioselective beta-adrenergic blocking agents (such as Brevibloc), and nonselective beta-adrenergic blocking agents (such as Inderal). In 2004, the United States Pharmacopeia (USP), a non-profit non-governmental organization, received directive from the Medicare Modernization Act to publish guidelines on drug categories and classes. These guidelines are to be used by prescription drug plans (PDPs) in developing their formularies for the Medicare population. The USP defined 41 therapeutic categories, 32 of which are further divided into pharmacologic classes. Overall, the draft model includes 137 classes and 9 categories that have no classes, for a total of 146 unique therapeutic categories and pharmacologic classes. PDPs that adopt the guidelines are required to include at least two drugs from each class in a category. If a category is not broken into classes, the PDP must include at least two drugs from the category. For classes that have additional subdivisions, PDPs are required to cover 1 drug from each such subdivision. See also Medicare Part D.