Opiate addiction is recognized as a central nervous system disorder, caused by continuous opiate intake.
In a nutshell, after prolonged opiate use, the nerve cells in the brain, which would otherwise produce endogenous opiates (natural painkillers, or endorphins), cease to function normally. The body stops producing endorphins because it is receiving opiates instead. The degeneration of these nerve cells causes a physical dependency to an external supply of opiates. Abrupt or sudden abstinence from opiates induces opiate withdrawal syndrome. The key idea here is that the brain is trying to maintain a state of homeostasis, or equilibrium, that does not deviate from the norm. You start with a base level of endogenous opiates (endorphins) before you ever become dependent. These endorphins interact with opiate receptors in your brain. Opiate drugs mimic these natural endorphins, and can bind to the same receptors, sometimes even better than your own! If you continue to take opiates for an extended period of time, your brain will try to rebalance itself. It can do this by either stopping production of your own endorphins (thus lowering the total level of opiate like substances in your brain) or it can downregulate, or dispose of, some of your opiate receptors (thus making less binding sites available for opiates, again attempting to recreate the natural balance in the brain). The trouble is, once this adjustment occurs, the brain is then reliant on an external source of opiates to maintain this balance. If you stop supplying this external source (e.g. you stop taking your pills), the brain is once again out of balance. This is what produces withdrawal symptoms.
For a more in depth guide to the science behind addiction, see this page: %PAGENAME%
-Opiate Addiction Vs. Dependence
-Withdrawal Symptoms
-PAWS (Post Acute Withdrawal Syndrome)