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Background and Objectives: While several studies have indicated a link between illicit drug use and the development of psychosis, the confounding role of pre-existing psychiatric illness is unclear. This study controls for this factor to a greater extent than has hitherto been possible, using a retrospective observational matched-cohort design controlling for age, gender, socioeconomic status and prior psychiatric illness.
Methods: 592 cases (diagnosed with drug misuse/dependence) and 592 controls (no recorded history of drug misuse/dependence) were drawn from all patients aged 16-44 in 183 practices within the General Practice Research Database (UK). On study entry, cases and controls had never had a psychiatric diagnosis since registering with their practice.
The average look-back period was 17.7 years. The main outcome measure was diagnosis of psychosis (including schizophrenia) from study entry onwards.
Results: Patients with a drug misuse/dependence diagnosis are significantly more likely to be diagnosed with psychosis than those with no drug misuse/dependence history (RR = 2.10, 95% C.I. = 1.23-3.59) with the relative risk increasing as the definition of psychosis gets narrower.
Conclusions: This study has established that, when the confounding presence of previous psychiatric illness is removed, the onset of problematic substance misuse severe enough to warrant primary care consultation is a risk factor for future onset of first-ever psychotic illness. Thus, there is a distinct sub-group of psychotic patients among whom drug misuse/dependence, with no prior psychiatric illness, is a risk factor for the development of psychoses.
