Substance abuse is defined by Webster as : excessive use of a drug (such as alcohol, narcotics, or cocaine): use of a drug without medical justification. (Webster, 2017)
Substance abuse and mental illness go hand and hand usually when an individual is trying to self medicate ( for various reasons) or becomes dependant on it to feel good. Not realizing this ulcers your brain chemistry even more and can make your mental health worse. Sometimes addictions can happen by accident with naracotics used to treat anxiety. Which is why a person on Klopopin etc must be monitored as they can cause your brain to become dependent on them.
Lots of people when they are dealing with substance abuse find it hard to quit because the withdrawal symptoms are very difficult and can last a long time. The key is early detection, education and prevention.
Please see more info at https://www.canada.ca/en/health-canada/services/substance-abuse.html