Why does comorbidity occur?

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There are a number of possible explanations as to why comorbidity may occur (see Figure 2):

  • The presence of a mental health condition may lead to an AOD use disorder, or vice versa (known as the direct causal hypothesis).
  • There may be an indirect causal relationship.
  • There may be factors that are common to both the AOD and mental health condition, increasing the likelihood that they will co-occur.