Substance abuse, or drug abuse, happens when a person uses a drug over and over again, in ways that hurt their health. The person is using the drug to change their mood or to feel better, not for any healthy reason. Sometimes when the person uses the drug for a long time, they will start to act differently. Some of the drugs are illegal to have or use, or can have certain limits that the person does not follow. Someone who thinks that they need a drug is called an addict.