Drug addiction refers to a chronic, relapsing brain disorder characterized by an individual's compulsive need for drugs despite negative consequences. It is caused by changes in the brain's chemistry and function due to repeated exposure to certain substances, leading to cravings, loss of control over drug use, and continued use despite harm to oneself or others. Drug addiction can involve a wide range of substances, including illicit drugs like heroin, prescription medications, and even legal substances such as alcohol and nicotine. It requires professional treatment and support to overcome, but with the right interventions and resources, individuals can recover from drug addiction and lead healthy, productive lives.