Young peoples drug and alcohol problems are rarely freestanding but linked to a wide range of other mental health and behavioral difficulties. Some of these relate to poor impulse control disorders, but a another sub-population of heavy drug and alcohol using youth is linked to mental health problems such as depression. Establishing a link between the two has been difficult. This is because mental health problems can be protective against drug and alcohol involvement for some youth but a risk for others. However, in a rare long term follow-up study by University of Washington Health Sciences/UW Medicine found very strong evidence that early adolescent depression was highly predictive of heavy cannabis use at aged 18, click here. Further research has established that cannbis use does worsen depressiv symptoms in young people (click here).
This is supported by a host of wider studies.