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If you have a mental health problem, such as depression or anxiety, you are much more likely to have a substance use disorder (SUD). Similarly, if you have a SUD, you are more likely to experience mental health problems. This is clear from the statistics, which show a massive correlation between addiction and mental illness.
In 2018-19, 53% of people starting treatment for substance use in the UK said they had a co-occurring mental health problem.  The same correlation between poor mental health and addiction can be found in America, where 49.5 percent of adults surveyed with a serious mental illness admitted to having used illicit drugs in 2018. 
The answer to that question is a complex one. It has to do with shared risk factors, genetics, upbringing, environmental influences and the way in which mental disorders and addiction exacerbate each other.
There are several risk factors for mental illness, many of which overlap with the risk factors for addiction. We’ve listed some of the main overlapping risk factors below.
One study, by J-C Wang et al., found that between 40 and 60 percent of someone’s vulnerability to addiction comes from their genes.  That is a huge proportion, and one which science needs to get to grips with.
Research is still ongoing to find out how genes function in the development of comorbid disorders. Comorbid disorders are mental illnesses like depression combined with a SUD.
Scientists believe that these comorbid disorders may be caused by a mixture of genetic and environmental influences. One example which is often given is that of cannabis and psychosis. Those with a genetic vulnerability to psychosis are even more likely to develop psychosis if they use cannabis regularly.  In this instance, then, substance use and genetics may be the key risk factors in determining whether someone develops a mental illness.
Genetic risk factors can act both directly and indirectly. Examples of genes acting directly in the development of substance use disorders include:
Examples of genes acting indirectly in the development of substance use disorders/mental illness include:
This section gets a little technical, but bear with us!
What is epigenetics? ‘Epigenetics’ deals with the way in which gene expression is altered by outside influences. This is not to say that the gene sequence itself is being changed; rather, environmental factors such as stress, trauma and drug use have an influence on the way in which information from genes is interpreted by the body.
An epigenetic risk factor, therefore, is similar to a genetic risk factor but it also stems from someone’s environment, and the impact of that environment on someone’s genetic expression.
Epigenetics can lead to serious, long-lasting changes in genetic expression which can even be passed on to someone’s offspring. 
The most important stage for epigenetic alteration is the developmental stage. In other words, someone’s environment during their upbringing, and even while they are in the womb, can have a big impact on their genetic expression. Maternal diet is one factor which has been found to have an effect on neurotransmission, with high-fat diets leading to changes in levels of proteins in the brain.
Studies have also shown that environmental factors, such as drug use and stress, can lead to a greater risk of issues with mental illness and addiction through epigenetic processes. 
There is significant overlap between mental disorders and SUDs in terms of the parts of the brain they affect.
The brain circuits which control reward, impulse control, emotions and decision making are all disrupted by both SUDs and mental disorders.
What’s more, several neurotransmitters (as mentioned previously) are involved in both mental disorders and SUDs. They include serotonin, dopamine, GABA, and glutamate.
High stress levels have been linked with mental illness. Some scientists believe that stress also provides the missing link between mental disorders and SUDs.
Why might stress be the key to linking mental disorders with SUDs? Before we take a look at the science, it is worth noting that many people with SUDs and mental illnesses (especially anxiety) exhibit high stress levels, and stressors are often one of the leading causes of relapse. So, from a non-neurological perspective, there is some justification for looking at stress as a factor in the development of co-occurring mental illness and SUD.
How about from a neurological perspective? The hypothalamic-pituitary-adrenal gland (HPA) deals with stress responses. It also has an effect on the brain circuits which handle motivation. Due to this link between stress and the brain, when someone experiences greater amounts of stress they are also more likely to exhibit impulsivity and diminished control over behaviour. In the longer term, stress can cause changes in the HPA gland, which in turn affects learning and motivation. Lowered motivation and learning ability have consistently been associated with both mental disorders and SUDs. 
From an addiction recovery perspective, these findings are interesting. They suggest that treatments which attempt to reduce stress – such as mindfulness and yoga – may be especially effective in people with SUDs, people with mental illness, and (perhaps most of all) people with co-occurring mental illness and SUD.
There are a few high-risk populations when it comes to trauma and the development of mental health problems/SUDs. One is adolescents. One study found that up to 59% of young people with PTSD go on to develop substance use problems. 
Trauma and substance misuse is also a major problem among members of the armed forces, where a high proportion of veterans suffer from SUDs and PTSD.
We’ve looked at a range of risk factors for the development of SUDs and mental illness, including genetic, epigenetic, neurological and environmental risk factors such as stress and trauma. The evidence points towards the idea that these risk factors can give you a greater chance of developing both a SUD and a mental disorder. This goes some way towards explaining the correlation between the two things.
In the following section we take a brief look at another explanation for the correlation between mental illness and SUDs: the fact that these conditions seem to lead to one another.
In other words, something about having a mental disorder makes you more prone to misusing substances, and something about substance use can put you more at risk of developing a mental disorder.
The evidence suggests that mental disorders are a risk factor for developing a SUD. . This is especially true with certain mental disorders, such as bipolar disorder. Anxiety has also been linked with substance misuse, and those with anxiety who self-medicate with substances are more at risk of developing a social anxiety disorder.
The notion of self-medicating is the most popular explanation for why those with mental disorders are more likely to develop a SUD. Substances such as alcohol, marijuana or opioids have short-term effects which make them attractive options for those with mental disorders, even if their long-term effects are highly disruptive. Alcohol, for instance, has a de-inhibiting effect which might appeal to those with anxiety. Marijuana provides some pain relief, and opioids provide a lot of pain relief.
Despite these appealing qualities, these substances cause a host of adverse effects, especially with long-term use. Opioids, for instance, are highly addictive, and can lead to side effects such as hyperalgesia when taken for long periods. Marijuana can exacerbate conditions like schizophrenia, and alcohol can worsen pre-existing anxiety and depression.
From a neurological point of view, there is evidence that mental illness may cause changes in the brain which make a person more susceptible to misusing substances. Mental illness can increase the reward effect of substance use and decrease understanding of its negative consequences. When you combine this with the apparent ability of substances to dispel the symptoms of a mental disorder (even for a short time), it becomes clear why those with mental illnesses are more at risk of substance use and developing a SUD.
As mentioned already, substance use causes changes in the brain which are also found with mental disorders like anxiety and schizophrenia.
What’s more, it is thought that substance use might trigger the onset of mental illness in those who are already predisposed towards mental illness.
This may be true in the case of cannabis and psychosis. One study argues that, although cannabis is not sufficient to cause psychosis on its own, it may be a ‘component cause’.  In other words, cannabis may interact with other risk factors to elevate the chances of someone developing psychosis. It is worth saying that the precise relationship between cannabis and psychosis is still not fully understood, and research is ongoing to determine how exactly this drug might be involved in psychosis.
The idea of cannabis as a ‘component cause’ of psychosis raises interesting questions. Are there other examples of specific drugs that relate to specific mental illnesses? At this stage it is hard to know, but perhaps in twenty or thirty years we may have a better idea.
Due to the high correlation between mental illness and SUD, there are a lot of people who suffer from comorbid conditions, where they have a mental illness as well as a SUD. As mentioned at the start of this article, 53% of people starting treatment for substance use in the UK in 2018-19 said they had a co-occurring mental health problem.
For those with comorbid mental illness and SUD, it is very important that both conditions are diagnosed. However, it can also be very difficult when there are overlapping symptoms. For example, symptoms of SUDs, such as drug withdrawal, can be not dissimilar to some of the symptoms of mental illness.
There are a few ways to try to ensure that comorbid disorders are correctly diagnosed. Firstly, anyone who enters treatment for a mental illness should be screened for a SUD. Likewise, anyone who enters treatment for a SUD should be screened for a mental illness. Second, when someone enters drug or alcohol treatment, they should be observed for a period after withdrawal in order to ascertain whether they have a comorbid mental health condition. It should be easier to isolate the symptoms of a serious mental illness once withdrawal symptoms have subsided.
The hope is that people with comorbid conditions get better treatment as a result of more accurate diagnosis. Comorbid conditions are typically harder to treat because they are more complex. An accurate diagnosis is the first step in good treatment.
As well as the challenge posed by diagnosing comorbid drug use and mental illness disorders, there are several difficulties involved with treatment.
One such difficulty is the complexity of comorbid disorders. Service users often have complex needs, and a range of symptoms. A high level of understanding is necessary on the part of treatment staff in order to treat comorbid disorders correctly.
Another difficulty is the high rates of treatment dropout and low treatment adherence that persist among those with comorbid mental illness and SUD.  This makes effective treatment more difficult.
As scientists and medical professionals become more knowledgeable about comorbid conditions, the treatment options will become more varied, and treatment will get better overall.
Therapy for comorbid SUD and mental illness will often involve some form of behavioural therapy, such as cognitive behavioural therapy, as well as a more functional/motivation-based therapy.
We’ve listed some therapy options below for those with comorbid disorders, as well as a brief explanation of what each therapy is.
This is a link to the National Institute on Drug Abuse’s Research Report entitled ‘Common Comorbidities with Substance Use Disorders’. It contains lots of useful information about comorbid conditions.
Here are some statistics related to mental health and substance use in the UK.
And here is a document from Public Health England entitled ‘Better Care for people with co-occurring mental health and alcohol/drug use conditions: a guide for commissioners and service providers.’ It contains useful info for those involved with treating co-occurring conditions.
 Gov.uk, ‘Substance misuse treatment for adults statistics 2018-19’. https://www.gov.uk/government/statistics/substance-misuse-treatment-for-adults-statistics-2018-to-2019/adult-substance-misuse-treatment-statistics-2018-to-2019-report#mental-health
 SAMHSA.gov, ‘Key Substance Use and Mental Health Indicators in the United States: Results from the 2018 National Survey on Drug Use and Health’.
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