There is an increased comorbidity, or co-occurrence, of substance dependence in individuals who have mental illness as compared to individuals without any mental disorder. This indicates either a shared neurobiological basis for both, or an interaction of effects at some level. Research on the origins of both mental illnesses and substance dependence will help to shed light on treatment and prevention strategies for both. There are several hypotheses as to why mental illness and substance dependence may co-occur:
- There may be a similar neurobiological basis to both;
- Substance use may help to alleviate some of the symptoms of the mental illness or the side effects of medication;
- Substance use may precipitate mental illnesses or lead to biological changes that have common elements with mental illnesses.
There is some evidence for all of these hypotheses. It is interesting that the effects of many psychoactive substances can produce psychiatric-like syndromes. For example, amphetamines and cocaine can induce psychotic-like symptoms. Hallucinogenic substances can produce hallucinations, which are an aspect of some psychoses. Furthermore, psychoactive substances regularly alter mood states, producing eithereuphoric and happy feelings, or inducing depressive symptoms, especially during substance withdrawal. Psychoactive substances can alter cognitive functioning, which is also a core feature of many mental illnesses. These factors all suggest common neurobiological substrates to both mental illnesses and substance dependence.
Some studies in the US have reported that more than 50% of the people with any mental disorder also suffer from substance dependence compared to 6% of the general population; and the odds of exhibiting substance dependence are 4.5 times higher for people with any mental disorder than for people without mental disorder (52). Clearly, there is a substantial overlap in these disorders.
The lifetime prevalence of alcohol dependence is 22% for individuals with any mental disorder compared to 14% for the general population, and the odds of having alcohol dependence if a person also has any mental disorder is 2.3 times higher than if there is no mental disorder (52). Studies in the United States over the last 20 years indicated that lifetime rates of major depressive disorder were 38-44% in people with alcohol dependence compared with only 7% in non-dependent individuals (35, 53-61). Further, approximately 80% of people with alcohol dependence have depressive symptoms (52, 62-64). An individual with alcohol dependence is 3.3 times more likely to also have schizophrenia, while a person with schizophrenia is 3.8 times more likely to exhibit alcohol dependence than the general population (52).
Higher percentages of people with mental illness, particularly people with schizophrenia, smoke tobacco compared to the general population. Depending on the particular mental illness, it has been reported that 26-88% of psychiatric patients smoke, compared to 20-30% of the general population (65-67). There are several close links between a major depressive disorder and tobacco smoking. In the US, up to 60% of heavy smokers have a history of mental illness (67, 68), and the incidence of major depressive disorder among smokers is twice that of non-smokers (65). Moreover, smokers who had a history of clinical depression were half as likely to succeed in quitting smoking than smokers without such history (14% versus 28%) (65). Epidemiological data indicate that the lifetime rates of major depressive disorder were 32% in cocaine users, and only 8 - 13% among non-cocaine users (52, 54, 56, 58, 69).
There is also a high degree of comorbidity of schizophrenia with psychostimu- lant use. Psychostimulant use is 2-5 times higher among patients with schizophrenia compared to people without schizophrenia, and more prevalent than in other psychiatric populations (70). Thus, it seems clear that substance dependence shares a considerable link with mental illness. Although most of the research on comorbidity has been carried out in only a few countries and the cultural validity of the data is unknown, neuroscience research into the treatment and prevention of one disorder will be beneficial to the other.