Substance use disorder is an illness that can affect anyone: rich or poor, male or female, employed or unemployed, young or old, and any race or ethnicity. Nobody knows for sure exactly what causes it, but the chance of developing a substance use disorder depends partly on genetics - biological traits passed down through families. A person's environment, psychological traits, and stress levels also play major roles by contributing to the use of alcohol or drugs. Researchers have found that using drugs for a long time changes the brain in important, long-lasting ways. It is as if a switch in the brain turned on at some point. This point is different for every person.

One of the most important signs of substance addiction or dependence is continued use of drugs or alcohol despite experiencing the serious negative consequences of heavy drug or alcohol use. Often, a person will blame other people or circumstances for his or her problems instead of realizing that the difficulties result from use of drugs or alcohol. For example, your partner may believe he was fired from jobs because his bosses didn't know how to run a business. Or your daughter may believe she got a ticket for driving under the influence of alcohol because the police was targeting here. Perhaps your loved one has even blamed you. People with this illness really may believe that they drink normally or that "everyone" takes drugs. These false beliefs are called denial, and denial is part of the illness.