Comorbidity between Substance use disorders (SUD) and psychotic disorders is common but the temporal relation of the first episodes of SUD and psychosis and how it affects the disorders has not been extensively investigated.
A nation-wide cohort (n = 2494) with a first hospitalization for psychosis at ages between 16 and 25 was identified. Psychiatric hospitalizations were followed from birth until up to 5 years after the first psychosis hospitalization. Risk factors for new SUD or psychosis hospitalization after the index hospitalization were analyzed by Cox regression.
30 % of the cases had SUD hospitalizations in the 5 years before or as a comorbid diagnosis at the first psychosis hospitalization. An additional 9% had a first SUD hospitalization in the five years after. The incidence of SUD hospitalizations increased year by year before and decrease year by year after the index hospitalization. The hazard ratio for a new SUD hospitalizations after the index hospitalization was significantly higher (hazard ratio 6.7, p-value<0.001) in cases with SUD before or at the index hospitalization compared to in cases without previous SUD. In cases with previous SUD, there was a strong association (p < 0.001) between a new psychosis hospitalization and a new SUD hospitalization the year after the index hospitalization, indicating that SUD may continue to aggravate the psychotic disorder in this group.
SUD is very common before a first hospital treated psychosis. The SUD likely aggravates early psychotic disorders in many cases.

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