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. Researchers also found that people drinking as few as three cups of brewed coffee each day may experience mild hallucinations, such as hearing voices that aren't there.
Researchers believe the hallucinations stem from the caffeine, rather than the coffee. Besides coffee, caffeine can be obtained from sources such as tea, chocolate, "pep" pills and energy drinks.
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.