Substance use disorder (SUD) is complex a condition in which there is uncontrolled use of a substance despite harmful consequence. People with SUD have an intense focus on using a certain substance(s) such as alcohol, tobacco, or illicit drugs, to the point where the person's ability to function in day to day life becomes impaired. People keep using the substance even when they know it is causing or will cause problems. The most severe SUDs are sometimes called addictions.
People with a substance use disorder may have distorted thinking and behaviors. Changes in the brain's structure and function are what cause people to have intense cravings, changes in personality, abnormal movements, and other behaviors. Brain imaging studies show changes in the areas of the brain that relate to judgment, decision making, learning, memory, and behavioral control.
Repeated substance use can cause changes in how the brain functions. These changes can last long after the immediate effects of the substance wears off, or in other words, after the period of intoxication. Intoxication is the intense pleasure, euphoria, calm, increased perception and sense, and other feelings that are caused by the substance. Intoxication symptoms are different for each substance.