Tolerance and physical dependence are common consequences of drug self-administration. For those interested in understanding and modifying alcohol and drug abuse and the problems they cause, the greatest importance of tolerance and physical dependence is in the contribution they make as determinants of drug self-administration. Some alcoholics, for example, can appear normal at BLOOD ALCOHOL CONCENTRATIONS (BAC) that would prostrate most social drinkers. What role, if any, does tolerance play in paving the way to an escalation in drug use and in the medical and psychological problems caused by heavy drug use? In addition to being highly tolerant, alcoholics will also be physically dependent on alcohol. What evidence is there to support the common assumption that physical dependence is a critical factor in maintaining drug self-administration?

Such questions are best answered in the context of a general theory of howdrug consumption is...