Caffeine (C8H10N4O2) is the common name for trimethylxanthine (systematic name is 1,3,7-trimethylxanthine or 3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione). The chemical is also known as coffeine, theine, mateine, guaranine, or methyltheobromine. Caffeine is naturally produced by several plants, including coffee beans, guarana, yerba maté, cacao beans, and tea. For the plants, caffeine acts as a natural pesticide. It paralyzes and kills insects that attempt to feed on the plants. The molecule was first isolated by the German chemist Friedrich Ferdinand Runge in 1819.
When purified, caffeine is an intensely bitter white powder. It is added to colas and other soft drinks to impart a pleasing bitter note. However, caffeine is also an addictive stimulant. In humans, it stimulates the central nervous system, heart rate, and respiration, has psychotropic (mood altering) properties, and acts as a mild diuretic.

A normal dose of caffeine is generally considered to be 100 mg, which is roughly the amount found in a cup of coffee. However, more than half of all American adults consume more than 300 mg of caffeine every day, which makes it America's most popular drug. Caffeine is generally consumed in coffee, cola, chocolate, and tea, although it is also available over-the-counter as a stimulant.

Caffeine is believed to work by blocking adenosine receptors in the brain and other organs. This reduces the ability of adenosine to bind to the receptors, which would slow down cellular activity. The stimulated nerve cells release the hormone epinephrine (adrenaline), which increases heart rate, blood pressure, and blood flow to muscles, decreases blood flow to the skin and organs, and causes the liver to release glucose. Caffeine also increases levels of the neurotransmitter dopamine.

Caffeine is quickly and completely removed from the brain. Its effects are short-lived and it tends not to negatively affect concentration or higher brain functions. However, continued exposure to caffeine leads to developing a tolerance to it. Tolerance causes the body to become sensitized to to adenosine, so withdrawal causes blood pressure to drop, which can result in a headache and other symptoms. Too much caffeine can result in caffeine intoxication, which is characterized by nervousness, excitement, increased urination, insomnia, flushed face, cold hands/feet, intestinal complaints, and sometimes hallucinations. Some people experience the symptoms of caffeine intoxication after ingesting as little as 250 mg per day. The lethal ingested dose, for an adult person, is estimated to be 13-19 grams. While generally considered safe for people, caffeine can be very toxic to household pets, such as dogs, horses, or parrots. Caffeine intake has been demonstrated to reduce the risk of type II diabetes mellitus. In addition to use as a stimulant and flavoring agent, caffeine is included in many over-the-counter headache remedies.