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A person vulnerable to one type of brain disease may also be vulnerable to another. Addiction and mental illness are both brain diseases. They involve the same pathways, molecules, and chemicals in the brain.
Increased dopamine activity is characteristic of both schizophrenia and many stimulants such as cocaine.
A serotonin transporter is associated with both mood disorders and alcoholism.
Both cocaine use and schizophrenia result in a dysfunctional reward pathway due to increased dopamine activity.