Drug dependency, also known as substance dependence, is a condition where an individual develops a strong craving for a drug, leading to compulsive use despite harmful consequences. This can occur with both legal substances, like alcohol and prescription medications, and illegal drugs, such as heroin or cocaine. Over time, the body may adapt to the presence of the drug, resulting in tolerance, where higher doses are needed to achieve the same effect. Withdrawal symptoms can occur when the drug is reduced or stopped, making it difficult for individuals to quit without support or treatment.