Tolerance and Dependence Behaviors of Drug Use

According to The Merck Manual of Diagnosis and Therapy (2005), a single definition of drug dependence is neither desirable nor possible because some illicit drug use does not involve dependence of any kind. Drug dependence of a specific type such as heroin or cocaine dependence, emphasizes the fact that different drugs have different effects as well as type and risk of dependence. Two primary concepts contribute to defining drug dependence: tolerance, which describes a need to progressively increase a dose to produce the effect achieved initially with smaller doses, and physical dependence defined as "a state of physiologic adaptation to a drug, manifested by a withdrawal (abstinence) syndrome (The Merck Manual of Diagnosis and Therapy, 2005).

Drugs producing strong physical dependence like heroin and alcohol are prone to abuse whether legal, in the case of alcohol in adults, or illegal in the case of heroin for individuals of any age. If a drug does not cause physical dependence, discontinuing the drug does not cause a major stereotypical withdrawal syndrome. However, t