Although these problems often occur together, this does not mean that one caused the other, even if one appeared first. In fact, it can be hard to figure out which came first. Researchers think that there are three possibilities as to why they occur together: Common risk factors may contribute to both mental disorders and substance use disorders. These factors include genetics, stress, and trauma. Mental disorders can contribute to drug use and substance use disorders. For example, people with mental disorders may use drugs or alcohol to try to feel better temporarily. This is known as self-medication. Also, mental disorders may change the brain to make it more likely you will become addicted. Substance use and addiction can contribute to the development of a mental disorder. Substance use may change the brain in ways that make you more likely to develop a mental disorder.