Medications
In the United States, medical professionals may obtain drugs from drug companies or pharmacies (which in turn purchase drugs from the drug companies). Pharmacies may also supply a drug directly to patients, authorized by a prescription from a medical professional, if the drug can be safely self-administered. Most drugs are relatively high-cost for patients to purchase directly when first distributed, although health insurance may mitigate some of the cost. When the patent for a drug runs out, a generic drug (some known as simply a "generic") is usually synthesized and released by competing companies, causing the price to drop markedly. Drugs which don't require prescription by a medical professional are known as over-the-counter (OTC) drugs and can be sold in stores without pharmacy association.

Prohibition
Illegal drug use is often termed recreational, but recreational drug use can be quite legal. Recreational use of alcohol, for example, is quite legal in many states and countries. Also, illegal users may claim that their use is medicinal or therapeutic: medical necessity has been used successfully in England as a defence against charges of illegal possession of cannabis.

The quality of a drug supplied illegally can be very unreliable.

Drug addiction
Drug addiction, or substance dependence is the compulsive use of drugs, to the point where the user has no effective choice but to continue use. This phenomenon has occurred to some degree throughout recorded history (see "opium"), though modern agricultural practices, improvements in access to drugs, and advancements in biochemistry have exacerbated the problem significantly in the 20th century with the introduction of purified forms of active biological agents, and with the synthesis of hitherto unknown substances, such as methamphetamine and gamma-hydroxybutyrate (GHB). While "addiction" has been replaced by "dependency" as a clinical term, the terms are used interchangeably here.

Drug Classification

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