Caffeine is an incredibly common drug that is used by more than 85% of adults and children in the United States, according to the Diagnostic and StatDSM-5. Caffeine, when ingested orally, is brought into the body via the digestive tract. Most of the caffeine digestion is done in the stomach and, depending on stomach contents and the amount, it can be digested in the intestines. Caffeine will take about a half hour to an hour to take its effect on the body. When the caffeine is in your body, the liver metabolizes it then the waste is excreted by the kidneys.

The effects of caffeine are longer lasting than most think, with a half-life of three hours. Caffeine is in a group of chemical compounds called xanthines; the main action of this compound is to inhibit the neurotransmitter called adenosine. When adenosine binds to its receptors, it causes sedation. Caffeine binds to these receptors and blocks the adenosine. Adenosine is a by-product of cellular metabolism, which means the more active neurons, the more adenosine becomes available; however, adenosine is unable to have its effect on the body due to the caffeine on the receptor site. Caffeine also causes vasoconstriction in the brain, but vasodilation in the body.

Despite caffeine's stimulant effects, there is a decrease in blood flow to the brain. This is how caffeine keeps you awake and alert through your day.