Substance abuse is the medical term used to describe a pattern of using a substance (drug) that causes significant problems or distress. This may be missing work or school, using the substance in dangerous situations, such as driving a car. It may lead to substance-related legal problems, or continued substance use that interferes with friendships, family relationships, or both. Substance abuse, as a recognized medical brain disorder, refers to the abuse of illegal substances, such as marijuana, heroin, cocaine, or methamphetamine. Or it may be the abuse of legal substances, such as alcohol, nicotine, or prescription medicines. Alcohol is the most common legal drug of abuse.