Substance use disorder (SUD) is a complex condition in which there is uncontrolled use of a substance despite harmful consequences. People with SUD have an intense focus--sometimes called an addiction--on using a certain substance(s) such as alcohol, tobacco, or other psychoactive substances, to the point where their 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. Repeated substance use can cause changes in how the brain functions. These changes can last long after the immediate intoxication wears off. Intoxication is the intense pleasure, euphoria, and calm that is caused by the substance; these symptoms are different for each substance. With continued use of a substance, tolerance can develop, where someone may require larger amounts in order to fell these effects. Additionally, discontinuing use can lead to symptoms of withdrawal and intense cravings to return to use, often experienced as anxiety. 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.According to the National Institute on Drug Abuse, people begin taking drugs for a variety of reasons, including: To feel good - feeling of pleasure, "high" or "intoxication." To feel better -relieve stress, forget problems, or feel numb. To do better - improve performance or thinking. Curiosity and peer pressure or experimenting. People with substance use and behavioral addictions may be aware of their problem but not be able to stop even if they want and try to. The addiction may cause physical and psychological problems as well as interpersonal problems such as with family members and friends or at work.