Any pharmacologically active substance that is not part of the body's normal metabolism - that is, a substance alien to the body's natural biochemistry that causes demonstrable effects once introduced to it. The line between a drug and a poison is blurry at best and relies mostly on being able to define what is - and isn't - a desirable effect.
The effects that the user of a drug intends to occur are generally defined as the 'main effect', 'primary' or 'indication'. Others, besides those actually desired - and particularly deleterious ones - are known as side-effects.
In the modern era the trade in drugs is heavily regulated by most governments due to the immense potential for harm caused by rogue operators - licensing of who can make, store, handle and sell what is strict in most jurisdictions and enforced by powerful agencies with wide ranging powers. Where this is not the case the existence of contaminated, counterfeit or otherwise unsafe compounds can be a serious menace to the general population.
List of Drugs
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