When Mary was feeling down, she drank way too much.
Jim drank too much too, but his dependence on alcohol was so severe that it caused uncontrollable cravings and he made very unhealthy decisions.
What’s the difference between Jim and Mary?
Mary probably has a substance abuse problem – a serious issue, but maybe different from Jim’s issue. Jim’s issue is might be somehting more serious though – substance use disorder, which is more commonly known as substance dependence.