In humans, caffeine is a central nervous system (CNS) stimulant, having the effect of temporarily warding off drowsiness and restoring alertness. Beverages containing caffeine, such as coffee, tea, soft drinks, and energy drinks enjoy great popularity.

Caffeine is the world's most widely consumed psychoactive substance, but unlike many other psychoactive substances it is legal and unregulated in nearly all jurisdictions. In North America, 90% of adults consume caffeine daily.

Caffeine is found in many plant species, where it acts as a natural pesticide, with high caffeine levels being reported in seedlings that are still developing foliages, but are lacking mechanical protection; caffeine paralyzes and kills certain insects feeding upon the plant.  High caffeine levels have also been found in the surrounding soil of coffee bean seedlings. It is therefore understood that caffeine has a natural function as both a natural pesticide and as an inhibitor of seed germination of other nearby coffee seedlings thus giving it a better chance of survival.

The most commonly used sources of caffeine are coffee, tea, and to a lesser extent cacao.

Caffeine content of select common food and drugs:

Product / Serving size / Caffeine per serving (mg) / Caffeine per litre (mg)

Caffeine tablet (regular strength) 1 tablet 100 —
Hershey's Special Dark (45% cacao content) 1 bar (43 g; 1.5 oz) 31 —
Coffee, espresso 44–60 mL (1.5-2 U.S. fl oz) 100 1691–2254
Coffee, Starbucks (Tall 12 U.S. fl oz) 240 650-700
Black tea 177 mL (6 U.S. fl oz) 50 282
Coca-Cola Classic 355 mL (12 U.S. fl oz) 34 96
Red Bull 250 mL (8.2 U.S. fl oz) 80 320

Humans have consumed caffeine since the Stone Age. Early peoples found that chewing the seeds, bark, or leaves of certain plants had the effects of easing fatigue, stimulating awareness, and elevating one's mood. Only much later was it found that the effect of caffeine was increased by steeping such plants in hot water. Many cultures have legends that attribute the discovery of such plants to people living many thousands of years ago.

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