Comorbidity is defined as the co-occurrence of one or more disorders in the same child or adolescent either at the same time or in some causal sequence. In relation to internalising disorders, data from clinical samples points to a high overlap between these disorders and substance use disorders, independent of whether the referred problem is a substance use.

Reference: https://www.health.gov.au/internet/publications/publishing.nsf/Content/drugtreat-pubs-comorbid-toc~drugtreat-pubs-comorbid-4~drugtreat-pubs-comorbid-4-3 , consulted on july 1st,2019