The findings of a recent study, based on nationwide Danish health records found that cannabis use disorder (CUD) is associated with an increased risk of ‘unipolar depression and bipolar depression’. This risk of depression was highest just after diagnosis of cannabis use disorder but was still significantly high five to ten years after CUD diagnosis.
Previous studies have found similar evidence of an association between depression and heavy marihuana use, but these have not been as convincing as this Danish study.
Strong evidence of an association between marihuana use, schizophrenia and psychosis has been available for a long time. This association is particularly strong in people with a genetic predisposition to schizophrenia and psychosis.
To explain some of the diagnostic terms used: ‘Bipolar depression’ refers to a depressive disorder where there have also been manic episode or episodes (including persistently elevated mood and activity). ‘Unipolar depression’ refers to major depression without associated manic episodes. ‘Cannabis Use Disorder’ (CUD) is defined by features including the heavy use of cannabis and evidence that this heavy use dominates the person’s life.
It must be acknowledged that there are other possible explanations for this association apart from a causative one, but the research methods used pointed strongly to a causative association.
In many countries including Australia, marihuana is being seen increasingly as a benign substance and as a treatment for anxiety and depression. The Danish study and other evidence is particularly important in arguing against its wide and careless use, especially in treatment of depression.
My simplistic hypothesis to explain any association between marihuana and depression is that marihuana interferes with adaptive function, making it difficult for heavy users to deal successfully with minor or major stress.
This possible connection between marihuana and depression is discussed in my book (page 277 and pages 463-4).