Case reports have described individuals who consume large amounts of caffeine—exceeding one gram per day (1,000 milligrams). This excessive intake, observed particularly among psychiatric patients, drug and alcohol abusers, and anorectic patients, can produce a range of symptoms— muscle twitching, ANXIETY, restlessness, nervousness, insomnia, rambling speech, tachycardia (rapid heartbeat), cardiac arrhythmia (irregular heartbeat), psychomotor agitation, and sensory disturbances including ringing in the ears and flashes of light.

The disorder characterized by excessive caffeine intake has been referred to as caffeinism. There is some suggestion that excessive caffeine consumption can be linked to psychoses and anxiety disorders. Substantial amounts of caffeine are also used by a small percentage of competitive athletes, despite specific sanctions against such use.

Abused drugs are reliably self-administered under a range of environmental circumstances by humans and most are also self-administered by laboratory animals. Caffeine has been self-injected by laboratory nonhuman primates and self-administered orally and intravenously by rats, but there has been considerable variability across subjects and across studies.

Human self-administration of caffeine has been variable, as well; however it is clear that human subjects will self-administer caffeine, either in cap sules or in coffee, and even when they are not informed that caffeine is the drug under study. For example, heavy coffee drinkers given repeated choices between capsules containing 100 milligrams caffeine or placebo under double-blind conditions showed clear preference for the caffeine capsules and, on average, consumed between 500 and 1,300 milligrams of caffeine per day. Experimental studies with low to moderate caffeine consumers have found that between 30 and 60 percent of those subjects reliably choose caffeine over placebo in blind-choice tests. Subjects tend to show less caffeine preference as the caffeine dose increases from 100 to 600 milligrams, and some subjects reliably avoid caffeine doses of 400 to 600 milligrams.

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