It is a primary, complex brain disorder caused by compulsive drug seeking and use, despite harmful consequences. It is characterized by long-lasting chemical brain changes which interfere with a person's ability to think clearly, use good judgment, control behavior and feel normal without using drugs or alcohol.
When a person uses drugs and/or alcohol, the brain is flooded with dopamine. This is the neurotransmitter that resides in parts of the brain that control movement, motivation, emotion, cognition and pleasure. When the substance is abused, it overloads these systems and the user experiences euphoria. This, in turn, causes the user to continue to abuse the drug to get this same feeling. However, the brain adjusts by producing less dopamine or by reducing the number of receptors can can receive or transmit signals. As a result, many substance abusers feel depressed or flat when they are not taking drugs or using alcohol, thus need to take them to get their levels of dopamine back to normal.
Not everyone who uses substances is an addict or alcoholic. A person becomes an abuser when the substance becomes so crucial that he or she is willing to risk other important aspects of life in order to take the substance.