According to this little Science Blog post, caffeine has been linked to hallucinations. This was corroborated by (badly spelled) anecdotal evicence at the Lycaeum (only when dealing with “drug culture” does poor spelling adds to authenticity and credibility). The original article (subscription required):
In diathesis–stress models of psychosis, cortisol released in response to stressors is proposed to play a role in the development of psychotic experiences. Individual differences in cortisol response to stressors are therefore likely to play a role in proneness to psychotic experiences. As caffeine has been found to increase cortisol response to a given stressor, we proposed that, when levels of stress were controlled for, caffeine intake would be related to hallucination-proneness and persecutory ideation. Caffeine intake, stress, hallucination-proneness and persecutory ideation were assessed by self-report questionnaires in a non-clinical sample (N = 219). Caffeine intake was positively related to stress levels and hallucination-proneness, but not persecutory ideation. When stress levels were controlled for, caffeine intake predicted levels of hallucination-proneness but not persecutory ideation. Implications of these findings are discussed and avenues for future research suggested.
Translation: There are chemicals that seem to be related to craziness and also to caffeine. It turns out that, among 200 people, the crazy ones drink a lot more coffee… we don’t think it’s a coincidence.
I took their questionnaire. At the end it referred me to intervoiceonline.org, The International Community for Hearing Voices.