Here's an article from a newspaper in the UK. I thought it was interesting.
11:59pm UK, Tuesday January 13, 2009
Coffee may cause hallucinations, research by the University of Durham suggests
Research suggests people who drink more than seven cups of instant coffee a day have an increased tendency to hallucinate. High caffeine users may even think they sense non-existent people, according to researchers from the University of Durham.
They studied 200 students who were asked about their typical intake of caffeine products.
Those who had a high caffeine intake were three times more likely to have the heard voice of someone non-existent than "low" users who consumed less than one cup of instant coffee or its equivalent.
Seeing things that were not there, hearing voices and sensing the presence of dead people were among the experiences reported.
Besides coffee, caffeine can be obtained from sources such as tea, chocolate, "pep" pills and energy drinks.
However, the hallucinations are not necessarily a sign of mental illness. Around 3% of people regularly hear voices, the research said.
The ability of caffeine to exacerbate the effects of stress may be behind the study's findings, scientists believe.
Dr Charles Fernyhough, the co-author of the study, pointed out that the research only showed an association between caffeine intake and hallucination proneness, not a causal link.
"One interpretation may be that those students who were more prone to hallucinations used caffeine to help cope with their experiences," he said.