Drug addiction is a brain disease because the abuse of drugs leads to changes in the structure and functioning of the brain. The initial use of alcohol or other drugs, for most people, is voluntary. But over time, the changes in the brain caused by repeated use and abuse can affect a person’s self-control and their ability to make good decisions, and at the same time send intense impulses to take drugs. These effects are what make it so challenging for a person who is addicted to stop abusing drugs.
Similar to other chronic, relapsing diseases, such as diabetes, asthma, or heart disease, drug addiction can be managed successfully. And, as with other chronic diseases, it is not uncommon for a person to relapse and begin abusing drugs again. Relapse, however, does not signal failure—rather, it indicates that treatment should be reinstated, adjusted, or that alternate treatment is needed to help the individual regain control and recover.
(Adapted from: http://www.nida.nih.gov/scienceofaddiction/brain.html)