The brain adapts to all exogenous chemicals that alter its activity, but the degree of tolerance, dependence, and craving in opiate users is legend—stronger than for almost every other drug. Adaptations that underlie opiate addiction, including the production of anti-opiates, begin during the very first administration (this is true of all drugs) and rapidly gain strength with use. The strength of these opponent processes may be so robust because the sensation of pain is so critical for survival.
