Besides coffee, caffeine is found in tea, energy drinks and chocolate
People who drink too much coffee could start seeing ghosts or hearing strange voices, UK research has suggested.
People who drank more than seven cups of instant coffee a day were three times more likely to hallucinate than those who took just one, a study found.
A Durham University team questioned 200 students about their caffeine intake, the journal Personality and Individual Differences reported.
However, academics say the findings do not prove a "causal link".
They also stress that experiencing hallucinations is not a definite sign of mental illness and that about 3% of people regularly hear voices.
"This is the first step toward looking at the wider factors associated with hallucinations," said psychology PhD student Simon Jones, who led the study.
He said previous research had suggested factors such as childhood trauma could be linked to hallucinations.
When under stress, the body releases a hormone called cortisol which is produced in greater quantities after consuming caffeine.
The extra cortisol boost could be what causes a person to hallucinate.
Therefore, Mr James added, it made sense to examine the link between caffeine and mood.
Besides coffee, sources such as tea, chocolate, "pep" pills and energy drinks contain caffeine.
After asking the students about their typical intake, the research team assessed their susceptibility to hallucinatory experiences and stress levels.
Among the experiences reported were seeing things that were not there, hearing voices, and sensing the presence of dead people.
However, Mr James stressed more work was needed to pin down the link.
He said: "Stressed people may simply drink more caffeine."
And he added: "Even if caffeine were responsible for hallucinations in some way, the part it plays would be small compared to other factors in life."
The researchers now plan to investigate whether other aspects of diet, such as sugar and fat consumption, might be associated with hallucinations.
Dr Euan Paul, of the British Coffee Association, stressed the study only focused on people with a very high caffeine intake.
He also said no details were recorded of other substances consumed that might have had hallucinogenic effects.
"There are literally thousands of well conducted published studies looking at all aspects of the coffee, caffeine and health debate and the overall conclusion clearly demonstrates that moderate caffeine intake, 400-500mg per day, is safe for the general population and may even confer health benefits."
Recent research has linked high caffeine intake among pregnant women to miscarriage or low birth weight.
Other studies suggested it could help prevent skin cancer, reduce nerve damage associated with multiple sclerosis, or cause problems for diabetes sufferers.
The Durham study was funded by the Economic and Social Research Council and Medical Research Council.