This study further explores the association between schizophrenia and caffeine use by combining two prior
published Spanish samples (250 schizophrenia outpatients and 290 controls from the general population)
with two Spanish long-term inpatient samples from the same hospital (145 with schizophrenia and 64 with other
severe mental illnesses). The specific aims were to establish whether or not, after controlling for confounders
including tobacco smoking, the association between schizophrenia and caffeine is consistent across schizophrenia
samples and across different definitions of caffeine use. The frequency of caffeine use in schizophrenia inpatients
was not significantly higher than that in non-schizophrenia inpatients (77%, 111/145 vs. 75%, 48/64) or controls
butwas significantly higher than in schizophrenia outpatients. The frequency of high caffeine users among caffeine
users in schizophrenia inpatients was not significantly higher than in non-schizophrenia inpatients (45%, 50/111 vs.
52%, 25/48) or controls, but was significantly lower than in schizophrenia outpatients. Smoking was significantly
associated with caffeine use across all samples and definitions. Between 2 and 3% of schizophrenia inpatients,
schizophrenia outpatients and non-schizophrenia inpatients showed caffeinism (N700mg/day in smokers). Several
of these smoking patients with caffeinism were also taking other inducers, particularly omeprazole. The lack of
consistent association between schizophrenia and caffeine use is surprising when compared with the very consistent
association between tobacco smoking and caffeine use across all of our analyses (use and high use in users) and
all our samples. The confounding effects of tobacco smoking may explain in large part the apparent association
between schizophrenia and caffeine use.