Addiction: A chronic relapsing condition characterized by compulsive drug-seeking and abuse and by long-lasting chemical changes in the brain. Addiction is the same irrespective of whether the drug is alcohol, amphetamines, cocaine, heroin, marijuana, or nicotine. Every addictive substance induces pleasant states or relieves distress. Continued use of the addictive substance induces adaptive changes in the brain that lead to tolerance, physical dependence, uncontrollable craving and, all too often, relapse. Dependence is at such a point that stopping is very difficult and causes severe physical and mental reactions from withdrawal. The risk of addiction is in part inherited. Genetic factors, for example, account for about 40% of the risk of alcoholism. The genetic factors predisposing to addiction are not yet fully understood.