There are several reasons why people use drugs [recreation, fun, peer pressure] but there is only one reason why people continue to use drugs and become addicted [mental illness].
For example, two people start using drugs, one of them stops and the other continues to use and becomes addicted to the drug. Why does one become addicted? Because the addicted person had a mental illness, anxiety, or a lack of coping skills to begin with the other did not.
If an addict stops using the drug the brain will eventually heal itself from the [addiction] but the mental illness will still need to be treated...
Psychiatrists normally treat the addiction first, then the mental illness. I wonder what would happen if you treated the mental illness first and then the addiction?