Drug abuse has the dual effect of causing a short-term "spike" in the brain's dopamine levels and then reduces the capacity of the brain to produce dopamine. It is this latter phenomenon that scientists believe can result in the often uncontrollable compulsive craving, seeking, and use of drugs that persists with relapse possibilities for up to 2 years and sometimes longer. This behavior persists to the extent that the addict ignores life-sustaining biological functions; this is addiction. After describing how the brain works, this paper details the effects of drugs of abuse on the brain. Attention is given to the specific effects on the brain of marijuana, opiates, inhalants, hallucinogens, steroids, stimulants, and nicotine. Considerable progress has been made in developing effective treatment, including medications and behavioral therapies, and research continues to identify better ways to match treatment strategies to specific patient needs. Continued research is needed into new behavioral treatment modalities, recognizing that medication is unlikely to ever be the sole treatment for addiction, which has social and behavioral as well as biological components. An ultimate task for treatment, far beyond treating the immediate effects of withdrawal, is to reverse, or at least to compensate for, these brain changes. 13 references