It can be difficult to determine which condition started first. Scientists do know that if you have a mental illness like anxiety or PTSD, you are at higher risk for developing a substance use disorder. And if you have a substance use disorder, you are more at risk for developing mental health conditions. Research show that regular drug use can negatively affect areas of the brain involved in stress. In addition, drug use, like smoking marijuana regularly or drinking too much, can also make you worried that your parents or teachers will find out, and you could have some anxiety that you cannot stop using these substances.
Scientists also know that some of these conditions run in families, so genetics might play a role in both substance use and other mental illnesses. Genes (inherited traits) also play a role in how you respond to a drug. For example, some teens try marijuana and have a psychotic reaction---feeling and seeing things that aren’t really there. This could be related to certain genes.
Secondly, environmental influences can lead to co-occurring conditions. Children who are bullied, abused, or who have experienced trauma are more at risk for substance use disorders and other mental illnesses.