Sure.
It's difficult to describe this system very well without getting somewhat advanced. Some of what I'll say here will be "take my word for it" type stuff--as its not possible to provide adequate justifications without significant prior knowledge of biological systems and biochemistry.
Ultimately there are 3 things I'll discuss here as it regards the action of opiates--there are far more to consider but this will consist of the basics:
1. Opiates mediate a release of dopamine into the brain and activate the brains inherant "reward" system--this is the facet which can lead to addiction.
2. Opiates act in the spinal cord to prevent pain messages from reaching the brain.
3. Opiates act in the brain to change the SUBJECTIVE experience of pain. I.E. a patient wills say they still feel pain but it "doesn't bother them" anymore.
A bit of further discussion on each point:
1. Opiate abuse can damage this reward or "pleasure" system in the brain. The sudden release of so much dopamine can damage both receptors for the dopamine and the pumps which release it. The net result is that an opiate addict who has been addicted for awhile has reduced their "total pleasure capacity" by damaging the system responsible for mediating pleasure. This is like saying a person can only feel, say, 80% as "good" as they used to feel. Subjectively this effect can take the form of anything from fidgetiness, to laziness, to depression.
2. This is the effect which we're commonly after when we use opiates medically speaking. This is an effect known as analgesia--whereby opiates are able to block transmission of pain messages before they even reach the brain.
3. This is a system which is mediated by endorphins which are released by activation of opiate receptors by an opiate.
For 2 and 3:
These two, together, are what cause a big problem for pain management. Each of these systems can be damaged by opiate over-use or abuse. The effect, on each system individually, is to create a "positive" baseline for pain.
Put it this way:
Lets say your body normally produces 200 units of endorphins which mediate pain (as a baseline). When you are injured at this level, you feel the pain--but if you have 200 units and no injury, there is no pain.
Because you are injured you take medicine to increase the amount of endorphins you have, let's say to 400 units.
If you do this for a short period of time, the effect is not appreciable. However, if you do this continuously for a long period of time--the end result is that your "new" baseline is higher than 200 units. While you may experience pain relief at the level of 400 units, over time the amount of relief you experience will diminish--this is an effect commonly known as tolerance.
If, at this point, you quit the medication and go back down to 200 units (if you have been at elevated levels for too long) your body will actually "prefer" a level of 300 units to be at a baseline "no pain" condition--rather than the previous level of 200 units. Of course I choose this 300 number arbitrarily, my only suggestion is to say that the preference will be higher than its original baseline value, but still less than the medicated value.
The result of this "preference" of your body is to experience dull pain throughout the entire body, and acute pain (more severe than its physical condition should cause) anywhere there is legitimate reason for pain. This is commonly known as a major symptom of withdrawal (although there are others). Ultimately the reason for withdrawal, in this sense, is that your body is not capable of producing the amount of "pain blocking" chemical that the medication is able to give you--and your body has become used to having elevated levels and has now decided that anything below that level consists of a pain response.
I.E. the drug hijacks your pain system, and confuses it into believing that it needs more of what you've been giving it in order to not feel pain--and this is known as physical dependence.
In addition to all of the above, the pumps for these "pain relief" chemicals and their respective receptors may ALSO be damaged through abuse of opiates, and the result is that your new ability to produce the chemical is at a value of say 150 units, and your new ability to receive it (at this production level) is even further reduced from this number, say 100 units.
So lets review.
Your body now prefers 300 units or it feels pain.
You can only produce 150. You can only accept 100 (when you are producing 150--without getting too advanced I can't explain why, but the effect here is exponential).
The net result is that, if you originally needed 400 units to fully remove the experience of a given pain response from a baseline of 200 ( i.e. 400-200 = 200 units for pain relief)
You now FEEL pain about as bad in your entire body (300-100 = 200 units of pain), and if the injury responsible for the initial pain hasn't healed you are now experiencing it at a much higher level (200 units needed for pain relief, 100 available--200-100=100 or a 50% increase).
In the end, the pain you can ultimately experience from opiate misuse is much greater and more permanent than that which comes from injuries which can heal--and for injuries which will NEVER heal, the end result can be increased pain in a non-medicated state from a given baseline value before medication is administered.
This is why opiates are generally bad for chronic pain relief unless an absolute cliff in quality of life has been reached. They are best suited to treating pain stemming from injuries which will heal, as a temporary mask.
At low levels almost no ill-effects are observed such as have been described here, but going even a tiny bit beyond the threshold is all that is required to begin the cycle of damage (and it IS a cycle which falls back in on itself once it is begun). That is why it is absolutely IMPERATIVE that opiates be administered under the care of a physician, and it is ALSO the reason why physicians are so touchy about prescribing opiates when other chemicals are present in the system for which the interactions with opiates are not well understood.
The system itself (opiate pain relief) is robust enough, and misunderstood enough, to warrant caution in new territory (scientifically speaking). Opiates act on the brain stem to affect respiration and many other basal functions of the body. Misuse of this stuff can actually kill you simply through this action alone, and its for that reason that if you can show me a doctor who will prescribe you opiates knowing that you'll use MJ--I can show you an unethical doctor.
Even if experience tells us that opiates and MJ do not react badly with one another--experience isn't always right, especially when it doesn't come with careful scientific consideration. Ultimately, a doctor doing this could kill somebody--and a good doctor will know that.
Any questions, just ask.
Also, please understand that this is a SIGNIFICANTLY simplified overlook of the system at hand--the end results are the same no matter how you choose to describe them, however.
