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Pharmacokinetics of caffeine. Caffeine consumed orally is rapidly absorbed into the general circulation, metabolized by liver CYP1A2 enzyme, and reabsorbed by renal tubules with only 1-5% excreted unchanged in the urine within 48 hours. Caffeine can enter several organs including the brain, leading to increased adenosine receptor blockage (from Gonzalez de Mejia and Ramirez-Mares2; reprinted with permission from Elsevier)
