Even though there is a high rate of comorbidity between addiction and mental illness, it does not mean that one necessarily caused the other—even if one condition appeared first. Instead, there are still a number of factors that need to be considered, according to the National Institute on Drug Abuse.

For instance -

➡️Drug abuse can cause people to experience one or more symptoms of another mental illness. For example, there is an increased risk of psychosis in some marijuana users.

➡️Mental disorders can lead to drug or alcohol abuse because some people use substances to self-medicate. For instance, the nicotine in tobacco products sometimes lessens certain symptoms of schizophrenia and may improve cognition.

There also is some evidence that indicates that addictions and mental illnesses are caused by underlying brain deficits, genetic influences, and/or exposure to trauma early in life. For instance, it is estimated that 40 to 60 percent of a person's vulnerability to addiction can be attributed to genetics. There also are several regions of the human genome that have been linked to an increased risk both for substance abuse and mental illness.

Another common factor between mental health issues and addiction is the age at which the symptoms appear. During the teen years, people are still developing, maturing, and growing. As a result, significant changes in the brain occur during adolescence. For instance, teenagers are more prone to take risks and act impulsively. These behaviors, while common among teens, can influence the risk of addiction and other mental disorders.

Finally, people who are physically or emotionally traumatized are at a much higher risk of substance use disorders. This connection is particularly concerning for veterans returning to the country. In fact, one in five military servicemen and women coming back from Iraq and Afghanistan have reported symptoms of post-traumatic stress disorder (PTSD) or major depression.

Sources - https://www.verywellmind.com/co-occurring-disorders-mental-health-and-addiction-4158280

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