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Caffeine (C8H10N4O2, or 1,3,7-trimethylxanthine or 3,7-Dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) is an alkaloid of the methylxanthine family, which also includes the similar compunds theophylline and theobromine. These occur naturally in coffee beans, cacao beans, cola nuts, tea, yerba mate, and guarana. In its pure state it is an intensely bitter white powder.

Caffeine acts as a stimulant of the central nervous system, heart, and respiration. It is also a diuretic.

A typical cup of drip-brewed coffee contains 100 to 200 mg of caffeine. A single shot (1 - 1.5 oz.) of espresso contains about 100mg. Stimulant pills may contain as much as 200 mg each. The lethal dose for humans is estimated to be about 10 grams, the equivalent of about 80 cups of coffee.

Continued consumption of caffeine can lead to tolerance. Upon withdrawal, the body becomes oversensitive to adenosine, causing blood pressure to drop dramatically, leading to headache and other symptoms.