Caffeine
Caffeine is the most widely used psychoactive substance. Besides its well-known presence in coffee, tea, colas, and chocolate, it is available in many over-the-counter and prescription oral medications and as injectable caffeine sodium benzoate (occasionally used for neonatal apnea). Caffeine is widely used as an anorectant, a co-analgesic, a diuretic, and a sleep suppressant. Botanical forms of caffeine, including yerba mate, guarana (Paullinia cupana), kola nut (Cola nitida), and green tea extract, are common constituents of “thermogenic” dietary supplements that are widely touted for weight loss and athletic enhancement (see also Herbal and Alternative Products). Caffeine is occasionally combined in tablets with other stimulants, such as MDMA (methylenedioxymethamphetamine). Although caffeine has a wide therapeutic index and rarely causes serious toxicity, there are many documented cases of accidental, suicidal, and iatrogenic intoxication, some resulting in death.
Mechanism of toxicity
Caffeine is a trimethylxanthine that is closely related to theophylline. It acts primarily through nonselective inhibition of adenosine receptors. In addition, with overdose there is considerable beta1– and beta2-adrenergic stimulation secondary to release of endogenous catecholamines.
Pharmacokinetics. Caffeine is rapidly and completely absorbed orally, with a volume of distribution of 0.7–0.8 L/kg. Its elimination half-life is approximately 4–6 hours but can range from 3 hours in healthy smokers to 10 hours in nonsmokers; after overdose, the half-life may be as long as 15 hours. In infants younger than 2–3 months old, metabolism is extremely slow, and the half-life may exceed 24 hours (see also Table II–61
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