Substance use disorder in patients with ID and mental health disorders admitted to psychiatric hospitalisation are prevalent, which makes this issue an area of interest for future improvements in case identification, proper referring to specialised treatment resources and an increasing research focusing on specific therapeutic approaches.

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Background. Cannabis is the most widely used illegal drug in the world. Both an early cannabis use onset and the amount used contribute to the risk of suffering mental disorders in adulthood. Objective. Review longitudinal studies conducted on adolescents associating early cannabis use with the subsequent occurrence of mental disorders. Method. A search was conducted in PubMed until December 2016, focusing on longitudinal researches and prioritizing studies that clearly distinguished between an early and late onset of cannabis use. To achieve the objective, the word "cannabis" was used in combination with the main psychiatric diseases. A manual search of articles that appeared in the references was also conducted. Results. Fifteen studies met the inclusion criteria, but only five studies discriminated clearly between early and late onset of cannabis use. An association was found between early onset of cannabis use and mental disorders, particularly psychosis. The information regarding affective and anxiety disorders is more controversial. Discussion and conclusion. Early onset of cannabis use is associated with mental disorders, especially psychosis. Prevention plans for cannabis use in adolescents should be developed in order to decrease the risk of mental disorders.