Caffeine
<p ><b>Caffeine</b> is a bitter, white <a href="http://en.wikipedia.org/wiki/Crystalline" title="Crystalline" class="mw-redirect" >crystalline</a> <a href="http://en.wikipedia.org/wiki/Xanthine" title="Xanthine" >xanthine</a> <a href="http://en.wikipedia.org/wiki/Alkaloid" title="Alkaloid" >alkaloid</a> that acts as a <a href="http://en.wikipedia.org/wiki/Stimulant" title="Stimulant" >stimulant</a> drug and a reversible <a href="http://en.wikipedia.org/wiki/Acetylcholinesterase_inhibitor" title="Acetylcholinesterase inhibitor" >acetylcholinesterase inhibitor</a>. </p>
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Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants.</p>
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It is most commonly consumed by humans in infusions extracted from the seed of the <a href="http://en.wikipedia.org/wiki/Coffea_arabica" title="Coffea arabica" >coffee plant</a> and the leaves of the <a href="http://en.wikipedia.org/wiki/Camellia_sinensis" title="Camellia sinensis" >tea bush</a>, as well as from various foods and drinks containing products derived from the <a href="http://en.wikipedia.org/wiki/Kola_nut" title="Kola nut" >kola nut</a>. Other sources include <a href="http://en.wikipedia.org/wiki/Yerba_mat%C3%A9" title="Yerba maté" class="mw-redirect" >yerba maté</a>, <a href="http://en.wikipedia.org/wiki/Guarana" title="Guarana" >guarana</a> berries, <a href="http://en.wikipedia.org/wiki/Ilex_guayusa" title="Ilex guayusa" >guayusa</a>, and the <a href="http://en.wikipedia.org/wiki/Ilex_vomitoria" title="Ilex vomitoria" >yaupon holly</a>.</p>
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In humans, caffeine acts as a <a href="http://en.wikipedia.org/wiki/Central_nervous_system" title="Central nervous system" >central nervous system</a> stimulant, temporarily warding off drowsiness and restoring alertness. </p>
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It is the world's most widely consumed <a href="http://en.wikipedia.org/wiki/Psychoactive_drug" title="Psychoactive drug" >psychoactive drug</a>, but, unlike many other psychoactive substances, it is both legal and unregulated in nearly all parts of the world. Beverages containing caffeine, such as <a href="http://en.wikipedia.org/wiki/Coffee" title="Coffee" >coffee</a>, <a href="http://en.wikipedia.org/wiki/Tea" title="Tea" >tea</a>, <a href="http://en.wikipedia.org/wiki/Soft_drink" title="Soft drink" >soft drinks</a>, and <a href="http://en.wikipedia.org/wiki/Energy_drink" title="Energy drink" >energy drinks</a>, enjoy great popularity; in North America, 90% of adults consume caffeine daily.<sup id="cite_ref-demon_drink_4-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Caffeine#cite_note-demon_drink-4" >[5]</a></sup></p>
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Caffeine is toxic at sufficiently high doses. </p>
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Ordinary consumption can have low health risks, even when carried on for years – there may be a modest protective effect against some diseases, including certain types of <a href="http://en.wikipedia.org/wiki/Cancer" title="Cancer" >cancer</a>. </p>
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Caffeine can have both positive and negative effects on anxiety disorders. Some people experience <a href="http://en.wikipedia.org/wiki/Insomnia" title="Insomnia" >sleep disruption</a> if they consume caffeine, especially during the evening hours, but others show little disturbance and the effect of caffeine on sleep is highly variable.</p>
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Evidence of a risk to <a href="http://en.wikipedia.org/wiki/Pregnancy" title="Pregnancy" >pregnancy</a> is equivocal, but some authorities have concluded that prudent advice is for pregnant women to limit consumption to the equivalent of two cups of coffee per day or less.<sup id="cite_ref-MayoPreg2_5-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Caffeine#cite_note-MayoPreg2-5" >[6]</a></sup> </p>
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Caffeine has <a href="http://en.wikipedia.org/wiki/Diuretic" title="Diuretic" >diuretic</a> properties when administered to people who are not used to it, but regular users develop a tolerance to this effect, and studies have generally failed to support the common notion that ordinary consumption contributes significantly to <a href="http://en.wikipedia.org/wiki/Dehydration" title="Dehydration" >dehydration</a>. </p>
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With heavy use, strong <a href="http://en.wikipedia.org/wiki/Tolerance" title="Tolerance" >tolerance</a> develops rapidly and caffeine can produce clinically significant <a href="http://en.wikipedia.org/wiki/Physical_dependence" title="Physical dependence" >physical</a> and <a href="http://en.wikipedia.org/wiki/Addiction" title="Addiction" >mental dependence</a>.</p>