There are a number of theories that explain the relationship between mental illness and substance use.
Causality The causality theory suggests that certain types of substance use may causally lead to mental illness. There is strong evidence that using
cannabis can produce
psychotic and
affective experiences. When it comes to persisting effects, there is a clear increase in the incidence of psychotic outcomes in people who had used cannabis, even when they had used it only once. More frequent use of cannabis strongly augmented the risk for psychosis. The evidence for affective outcomes is less strong.
Attention-deficit hyperactivity disorder One in four people who have a substance use disorder also have
attention-deficit hyperactivity disorder, which makes the treatment of both conditions more difficult. ADHD is associated with an increased craving for drugs. Having ADHD makes it more likely that an individual will initiate substance misuse at a younger age than their peers. While generally stimulant medications do not seem to worsen substance use, they are known to be non-medically used in some cases. Psychosocial therapy and/or nonstimulant medications and extended release stimulants are ADHD treatment options that reduce these risks. However, certain forms of substance use disorders, especially alcohol use disorder, can cause or worsen certain neuropsychological symptoms which are common to autism spectrum disorder. This includes impaired
social skills due to the neurotoxic effects of alcohol on the brain, especially in the
prefrontal cortex area of the brain. The social skills that are impaired by
alcohol use disorder include impairments in perceiving facial emotions, prosody perception problems and
theory of mind deficits; the ability to understand humour is also impaired in people who consume excessive amounts of alcohol
Gambling The inclusion of behavioral addictions like pathological gambling must change our way of understanding and dealing with addictions. Pathological (disordered) gambling has commonalities in clinical expression, etiology, comorbidity, physiology and treatment with substance use disorders (DSM-5). A challenge is to understand the development of compulsivity at a neurochemical level not only for drugs.
Past exposure to psychiatric medications theory The past exposure theory suggests that
exposure to
psychiatric medication alters neural
synapses, introducing an
imbalance that was not previously present. Discontinuation of the drug is expected to result in symptoms of psychiatric illness which resolve once the drug is restarted. This theory suggests that while it may appear that the medication is working, it is only treating a disorder caused by the medication itself.
Self-medication theory The
self-medication theory suggests that people with severe mental illnesses misuse substances in order to relieve a specific set of symptoms and counter the negative side-effects of
antipsychotic medication. Khantizan proposes that substances are not randomly chosen, but are specifically selected for their effects. For example, using stimulants such as
nicotine or
amphetamines can be used to combat the
sedation that can be caused by higher doses of certain types of antipsychotic medication. (stiff muscles) and
dyskinesia (involuntary movement) being prevented.
Alleviation of dysphoria theory The alleviation of
dysphoria theory suggests that people with severe mental illness commonly have a negative
self-image, which makes them vulnerable to using
psychoactive substances to alleviate these feelings. Despite the existence of a wide range of dysphoric feelings (
anxiety, depression,
boredom, and
loneliness), the literature on self-reported reasons for use seems to lend support for the experience of these feelings being the primary motivator for alcohol use disorder and other drug misuse.
Multiple risk factor theory Another theory is that there may be shared risk factors that can lead to both substance use and mental illness. Mueser hypothesizes that these may include factors such as
social isolation, poverty, lack of structured daily activity, lack of adult role responsibility, living in areas with high drug availability, and association with people who already misuse drugs. Other evidence suggests that
traumatic life events, such as sexual
abuse, are associated with the development of psychiatric problems and substance use.
The supersensitivity theory The supersensitivity theory proposes that certain individuals who have severe mental illness also have biological and psychological vulnerabilities, caused by genetic and early
environmental life events. These interact with stressful life events and can result in either a psychiatric disorder or trigger a relapse into an existing illness. The theory states that although anti-psychotic medication can reduce the vulnerability, substance use may increase it, causing the individual to be more likely to experience negative consequences from using relatively small amounts of substances. These individuals, therefore, are "supersensitive" to the effects of certain substances, and individuals with psychotic illness such as
schizophrenia may be less capable of sustaining moderate substance use over time without experiencing negative symptoms. Although there are limitations in the research studies conducted in this area, namely that most have focused primarily on schizophrenia, this theory provides an explanation of why relatively low levels of substance misuse often result in negative consequences for individuals with severe mental illness. ==Avoiding categorical diagnosis==