noun. A bitter white alkaloid, C₈H₁₀N₄O₂, found in certain plants such as cacao, coffee, kola, and tea, that stimulates the central nervous system and body metabolism and is used in medicine, usually in combination with other drugs, to relieve headaches and treat respiratory conditions in premature infants.

noun. An alkaloid, C8H10N4O2, found naturally in tea and coffee plants which acts as a mild stimulant of the central nervous system.

noun. a bitter alkaloid found in coffee and tea that is responsible for their stimulating effects

"While none of the participants were given a drink high in caffeine, the researchers concluded that caffeine is probably more helpful than sugar in staying alert."

alkaloid  

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adrenaline  alcohol  amphetamine  arsenic  aspirin