Caffeine refers to an alkaloid found in coffee, cocoa beans, tea, kola nuts and guarana.
Also added to many fizzy drinks, energy drinks, pep pills and cold and flu remedies.
A stimulant of the central nervous system. Pure caffeine is a moderately powerful drug and is sometimes passed off as amphetamine. In small doses, such as the 150 milligrams in a typical cup of filter coffee, it increases alertness and promotes wakefulness. Caffeine also raises heart and respiration rate and promotes urine production. Higher doses induce jitteriness and anxiety. The fatal dose is about 10 grams. It raises heart and respiration rate and promotes urine production.
Other /More definition:
Caffeine refers to a drug present in coffee and other drinks that constricts blood vessels to the brain and prevents adenosine from inhibiting the release of dopamine and acetylcholine. It is a chemical compound with stimulant effects