The co-occurrence of two conditions may also come about due to the presence of shared biological, psychological, social, or environmental risk factors. That is, the factors that increase the risk of one condition may also increase the risk for another [49–57]. For example, both AOD and mental health conditions have been associated with lower socioeconomic status, cognitive impairment, the presence of conduct disorder or behavioural disinhibition in childhood and antisocial personality disorder (ASPD). It is also possible that a genetic vulnerability to one disorder may increase the risk of developing another disorder [49, 58–60]. An increasing body of research has focused on epigenetics; that is, the way in which a person’s environment and experiences can influence gene expression. For example, childhood trauma and adversity has consistently been associated with alterations in the expression of genes that have been associated with the development of mental health and AOD use disorders [61]. Importantly, research has also shown that psychotherapy can also impact positively on genetic expression [62]. However, just as with other risk factors, underlying genetic vulnerability may increase the risk of developing a disorder – it does not mean a person is predestined to develop an AOD use disorder or depression, for example.
Figure 2: Explaining co-occurring conditions