The FDA lists caffeine as a food additive "generally recognized as safe," and most experts agree that moderate consumption of up to 300 milligrams a day (one to two 12-ounce coffees or six to eight 12-ounce cans of soda) is not dangerous.
(See National Geographic's caffeine photo gallery)
Jones and colleagues speculate in the journal Personality and Individual Differences that the drug may cause hallucinatory experiences by amplifying the physiological effects of stress—including driving increased production of a stress-related hormone known as cortisol.
Jones notes that other research has linked some hallucinations to traumatic events like abuse or bereavement, which may make people more sensitive to stress.
Jack Bergman, a behavioral pharmacologist at Harvard Medical School unaffiliated with the study, noted that heavy caffeine intake can produce jitteriness and other adverse effects, which typically prompt users to avoid higher doses.
"Overdose with much higher intake of caffeine produces myriad effects, one of which may be considered hallucination," he added.
"It's important to remember that hallucination can be most simply defined as a sensory disturbance. There certainly are reports that extremely high doses—overdoses—of caffeine can produce sensory disturbances and, in some cases, it may get more complex than just 'seeing stars' or 'hearing noises.'
"It's also important to realize that such very high doses of caffeine also produce, in some individuals, paranoid reactions that can intensify the experience of a sensory disturbance."
Bergman also noted that psychiatric conditions like psychotic depression can distort the effects of many drugs, including caffeine, so that it is difficult to predict how afflicted individuals will respond to high doses.
Study author Jones hopes to continue probing possible links between nutrition and sensory disturbances.
"We're planning to look at those who are disturbed by severe hallucinations and see whether dietary changes like caffeine, sugar, or fat intake can help people cope with their hallucinations better," he said.
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