Pharmacokinetics

Absorption and Distribution. Caffeine can be effectively administered orally, rectally, intramuscularly, or intravenously; however, it is usually administered orally. Orally consumed caffeine is rapidly and completely absorbed into the bloodstream through the gastrointestinal tract, producing effects in as little as fifteen minutes and reaching peak plasma levels within an hour. Food reduces the rate of absorption. Caffeine readily moves through all cells and tissue, largely by simple diffusion, and thus is distributed to all body organs, quickly reaching equilibrium between blood and all tissues, including brain. Caffeine crosses the placenta, and it passes into breast milk.

Metabolism and Excretion. The bloodstream delivers caffeine to the liver, where it is converted to a variety of metabolites. Most of an ingested dose of caffeine is converted to paraxanthine and then to several other metabolites. A smaller proportion of caffeine is converted to theophylline and theobro-mine; both of those compounds are also further metabolized. Some of these metabolites may contribute to caffeine's physiologic and behavioral effects.

The amount of time required for the body of an adult to remove half of an ingested dose of caffeine (i.e., the half-life) is 3 to 7 hours. On average, about 95 percent of a dose of caffeine is excreted within 15 to 35 hours. Cigarette smoking produces a twofold increase in the rate at which caffeine is eliminated from the body. There is a twofold decrease in the caffeine elimination rate in women using oral contraceptive steroids and during the later stages of pregnancy. Newborn infants elimi nate caffeine at markedly slower rates, requiring over 10 days to eliminate about 95 percent of a dose of caffeine. By 1 year of age, caffeine elimination rates increase substantially, exceeding those of adults; school-aged children eliminated caffeine twice as fast as adults.

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