We already produce natural opioid chemicals (endorphins/enkephalins) in the precise amounts our bodies were designed to handle. The problem arises when an individual has been using an opioid drug for a period of time.
After prolonged use of opioid drugs, the production of endogenous opioids is inhibited, which accounts in part for the withdrawal syndrome that results from the immediate cessation of the drug.
The continuous use of opioid overrides our natural ability to produce endorphins and enkephalins. The brain comes to rely upon the drugs to create these neurotransmitters.
When a person stops using the opioid drug, the brain doesn’t start creating these endogenous opioids right away. It short-circuits, leading to withdrawal symptoms, and deteriorating psychological function. Whether an individual is abusing opioids or even taking them as prescribed by a physician, the continued use quickly leads to tolerance. Tolerance is a state of adaptation in which exposure to a drug induces changes that result in a decrease of the drug’s effects over time.
If an individual continues using opioids after a tolerance has been established, they will eventually develop a physiological dependence. Dependence develops when the neurons adapt to the repeated drug exposure and only function normally in the presence of the drug.