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 health 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. Addiction is when people are used to taking a substance, and they feel bad when they are unable to get it. Very often, such people will also change their lives, so they can get the substance or drug they need more easily.