A drug is a chemical substance that can change the way a person feels, thinks, or behaves. Some drugs are legal, like alcohol and tobacco, while others are illegal, like marijuana, cocaine, and heroin. Some drugs can be helpful, like medicine, while others can be harmful, like alcohol and tobacco. The effects of a drug depend on many factors, including how much of the drug is taken, how often it is taken, the person’s age, health, and other factors.