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Cigarettes, containing nicotine, are one of the world’s most widely consumed drugs.<ref>According to the statistic of the Food and Agriculture Organization the production quantity in 2006 of coffee was 7.8 million tonnes and of tobacco was 6.7 million tonnes.</ref>

A drug is, in the broadest of terms, a chemical substance that has known biological effects on humans or other animals.<ref name="Merriam Webster: Concise Encyclopedia">"Drug." Merriam Webster: Concise Encyclopedia</ref> Foods are generally excluded from this definition, in spite of their physiological effects on animal species.<ref name="diccom">"Drug." Unabridged (v 1.1), Random House, Inc., via Retrieved on 20 September 2007.</ref><ref name=" Drug Definition">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Merriam Webster: Drug - Definition">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

In pharmacology, a drug is "a chemical substance used in the treatment, cure, prevention, or diagnosis of disease or used to otherwise enhance physical or mental well-being."<ref name="diccom" /> Pharmaceutical drugs may be used for a limited duration, or on a regular basis for chronic disorders.<ref name="ahsci">"Drug." The American Heritage Science Dictionary, Houghton Mifflin Company, via Retrieved on 20 September 2007.</ref>

Psychoactive drugs are chemical substances that affect the function of the nervous system, altering perception, mood or consciousness.<ref name="bushbook">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> Alcohol, nicotine, and caffeine are the most widely consumed psychoactive drugs worldwide.<ref></ref>

Recreational drugs are drugs that are not used for medicinal purposes, but are instead used for pleasure.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Common recreational drugs include alcohol, nicotine and caffeine, as well as other substances such as opiates and amphetamines.

Some drugs can cause addiction and habituation<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> and all drugs can cause side effects.<ref name="MHRA Side Effects of Medicines">"MHRA Side Effects of Medicines." MHRA Side Effects of Medicines,</ref> Many drugs are illegal for recreational purposes and international treaties such as the Single Convention on Narcotic Drugs exist for the purpose of legally prohibiting certain substances.

Drug sections
Intro   Etymology    Medication    Spiritual and religious use    Self-improvement    Recreational drug use   Administering drugs   See also    References    Further reading    External links   

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