From the American Pain Society:
Addiction is a primary, chronic, neurobiologic disease, with genetic, psychosocial, and environmental factors influencing its development and manifestations. It is characterized by behaviors that include one or more of the following: impaired control over drug use, compulsive use, continued use despite harm, and craving.
II. Physical Dependence
Physical dependence is a state of adaptation that is manifested by a drug class specific withdrawal syndrome that can be produced by abrupt cessation, rapid dose reduction, decreasing blood level of the drug, and/or administration of an antagonist.
Tolerance is a state of adaptation in which exposure to a drug induces changes that result in a diminution of one or more of the drug’s effects over time.
Everyone who takes opiates on a regular basis, either for pain or to get high, becomes dependent and tolerant of them – this is simply a biological consequence of taking medication. However, addiction is a distinct disease in itself, and addicts will use any combination of mind-altering substances to withdraw from “real life”, whether it be pills, heroin, methamphetamines, cocaine, barbituates, or the old standby, alcohol.
Opiate pain medications are popular among addicts only because, when they are not being used in the presence of severe pain, they can result in euphoria (the “high”) .
“any of a group of peptides, resembling opiates, that are released in the body in response to stress or trauma and that react with the brain’s opiate receptors to reduce the sensation of pain.”
In pain patients, opiates supplement the endorphins used up by the body to resist pain. Without pain to “soak up” the extra, the blast of unused endorphins causes euphoria.