There is no blood test or brain scan to diagnose addiction. We base our diagnosis on patterns of behavior.
Clinically we figure out the degree by which a substance is causing problems by asking questions in four categories. Risk taking such as driving intoxicated on weed or alcohol impaired control, like ongoing compulsive thoughts to use physical dependence such as needing more to get the same effect and social problems like neglecting responsibilities or impacting relationships. There can be problems in just one or two of these categories when there are significant problems in many of these areas that point to clinical addiction.
