Causes
People show that most opioid dependent patients suffer from at least one severe psychiatric comorbidity. Since opioids used in pain therapy rarely cause any of these conditions, they are assumed to have existed prior to the development of dependence. Opioids are known to have strong antidepressive, anxiolytic and antipsychotic effects and thus opioid dependence often develops as a result of self medication.
Furthermore some studies suggest a permanent dysregulation of the endogenous opioid receptor system after chronic exposure to opiates. A recent study has shown that an increase in BDNF, brain-derived neurotrophic factor, in the ventral tegmental area (VTA) in rats can cause opiate-naive rats to begin displaying opiate-dependent behavior, including withdrawal and drug-seeking behavior. It has been shown that when an opiate-naive person begins using opiates at levels inducing euphoria, this same increase in BDNF occurs.
Another recent study concluded to have shown "a direct link between morphine abstinence and depressive-like symptoms" and postulates "that serotonin dysfunction represents a main mechanism contributing to mood disorders in opiate abstinence".
Read more about this topic: Opioid Dependence