However, this is preliminary research only, and as the authors state, the effect was only weak. Also, the questionnaire assessed the students ‘predisposition to hallucinations’, rather than their prior experiences of having actual hallucinations. The study’s limitations also mean that it cannot prove that caffeine causes increased susceptibility to hallucinations; therefore it should not be a cause for alarm in people who drink coffee or other beverages containing caffeine.
It should be noted the research paper contained no specific claims about the supernatural.
Where did the story come from? Simon Jones and Charles Fernyhough of the Department of Psychology, Durham University carried out this research. No sources of funding were reported. The study was published in the peer-reviewed journal Personality and Individual Differences.
What kind of scientific study was this? This was a cross-sectional study designed to investigate the theory that the release of cortisol in response to stress factors (or stressors) plays a role in the development of psychotic experiences. By extension, an individual’s propensity towards psychosis may be expected to be in relation to their cortisol response.
Caffeine is believed to heighten the cortisol response to any given stressor. This investigation aimed to see whether, at a controlled stress level, caffeine intake was related to hallucinations and ideas of persecution. Previous studies investigating caffeine and psychotic experiences have produced mixed findings.
A total of 214 students (70% female; average age 20 years) were recruited, and filled in questionnaires on caffeine use. All respondents remained anonymous and only age, sex and weight of the participants was known. Smokers were excluded.
The questionnaire on caffeine intake used a tool known as the Durham Caffeine Inventory, which presents caffeine-containing food and drinks and asks respondents to rate their typical intake over the past year on a 12-point scale from none to 8+ times per day. Set values of caffeine content were determined for each item, either from the FSA or sourced from the manufacturers.
The questionnaire also contained questions using the Launay-Slade Hallucination Scale, which is a 16-item tool designed to measure predisposition to hallucinations on a 5-point scale from ‘certainly doesn’t apply to me’ to ‘certainly does apply’.
Persecutory ideas were assessed using the Persecutory Ideation 10-item Questionnaire (responses from ‘very untrue’ to ‘very true’). Stress was assessed using the Perceived Stress 30-item Questionnaire, which looked at several aspects of stress, tension and worry over the past year (responses ‘almost never’ to ‘usually’).
Researchers then looked at the relationship between the level of hallucinations, feelings of persecution, stress reported, and caffeine consumption per kilogram of bodyweight.
What were the results of the study?Across participants, average daily caffeine intake was 141mg/day. This level was comparable to that in previous student studies, and represents about four cola drinks, three cups of strong tea or instant coffee, or one cup of brewed coffee a day.
Higher levels of caffeine intake were found to be associated with higher perceived stress levels and a higher hallucinatory score. But they were not linked to persecutory score (although hallucinatory and persecutory scores were positively correlated with each other). On further statistical analysis, they found stress to be predictive of proneness to hallucination and persecutory ideas.
After then controlling for age, sex, weight and stress, and then looking at the effect of caffeine, they found that caffeine still predicted proneness to hallucinations but not persecutory ideas.
What interpretations did the researchers draw from these results?The researchers conclude that their analyses found that caffeine intake was positively related to stress levels, and that caffeine intake was also related to hallucination proneness but not persecutory ideas. The researchers say the observed relationship between caffeine intake and proneness to hallucinations was weak.
They also state that the study is not causal, i.e. it cannot prove that an increased proneness to hallucinations comes from an increased caffeine intake, only that the two factors are associated.
What does the NHS Knowledge Service make of this study?As the authors say, their study lends support to their hypothesis that when stress levels are controlled for, caffeine intake is positively related to levels of psychosis-like experience.
This is a preliminary study only, and has several limitations:
- The authors report that “the effect was found to be weak and specific to hallucination-proneness and not persecutory ideation”.
- For every milligram increase in daily caffeine intake per kilogram of bodyweight (equivalent to an extra 1.5 cups of instant coffee for an 11-stone person), there was only an increase of 0.18 on the hallucination score (this score can range from 0 to 64, with a higher score indicating greater level of hallucinations). It is unclear how an increase this small would affect an individual’s experiences.
- It is important to note that the scale used measured “hallucination proneness” rather than strictly “hallucinations”, and it includes assessment of what most people might consider “normal” experiences. For example, one of the areas assessed includes having vivid daydreams, which might not be generally considered to be abnormal.
- In cross-sectional studies, it is not possible to determine cause and effect, i.e. in this case, whether increased caffeine caused increased hallucinations or stress, or whether increased caffeine consumption was a result of hallucinations or stress.
- This was a small, selected sample of university students, which cannot be assumed to be representative of the population as a whole. In addition, as the participants are likely to have been mostly healthy, the results cannot be assumed to apply to people who have been diagnosed with psychotic illnesses such as schizophrenia.
- All responses were self-reported, and participants were asked to give broad responses to caffeine, stress levels, and psychotic experiences over the past year. It is likely that this would lead to a considerable degree of recall and reporting bias, and very variable responses between participants. As the authors acknowledge, their self-report measure scale of caffeine was not validated.
- All participants were anonymous and, with smoking being the only criteria for exclusion, there are a number of unassessed factors that may have affected the results, e.g. medications being taken, diagnosis of depression, anxiety or psychosis, family history, etc.
- The reasons behind these findings are unclear as this study did not directly examine the theory that hallucinations and other psychosis-like experiences are related to cortisol release in response to stress.
- The study report that was analysed here did not express its results in terms of increased risk of hallucinations per cup of coffee. It is unclear where the figures quoted in the newspaper have come from.
The vast majority of the UK population drinks coffee and other caffeine-containing drinks without experiencing any hallucinations and should not be overly concerned by these findings.
Anyone who is experiencing psychotic experiences should always consult a doctor, rather than assuming it is is caused by caffeine.
Links to the headlines
Three cups of brewed coffee a day 'triples risk of hallucinations'. The Daily Telegraph, January 14 2009
Go easy on the coffee, you could start seeing things. Daily Mail, January 14 2009
Coffee link to 'visions'. Daily Mirror, January 14 2009
'Visions link' to coffee intake. BBC News, January 14 2009
Danger from just 7 cups of coffee a day. Daily Express, January 14 2009