@Paralee - Just to be clear I don't believe this is going to be as simple as popping some pain killers from time to time, although I could be wrong. No doubt this will need to be approached with caution even for those without a proclivity to addiction I'm sure it could be a slippery slope. Luckily like you I've also never had any issues, so I am more confident about experimentation than many others, I think.
The intent for this type of "therapy" would be two fold. First we've covered to some degree, namely, to "clear the gunk" out of the receptors. As Hip mentions we would expect to have documented somewhere that a type of endotoxin does exist that would potentially bind to this receptor, perhaps there is not one that exists that has preferential binding but rather some affinity. In order to have any legitimate scientific methodology we would need to know what we're working with, so I suppose this this post is more a form of mental masturbation- albeit still worth effort
OK, all jokes aside. For any semi/permanent resolution some physiologic changes would be needing to take place. Unfortunately using a potent opioid you would typically run into the tug-a-war with homeostatic driven reductions in post synaptic receptors respective to the drug ingested. However in a few select circumstances I think the opposite may be possible
1) The opioid drug induces an overall sensitization effect. Drug sensitization has always been an interesting phenomena as it somehow over rides the massive numbers of feedback/feedforward loops and redundancies that exist in the neural circuitry. Keep in mind that CFS is a maladaptive physiologic change in response to some form of stress that is maintained by virtue of the exact same feedback mechanisms that are intended to keep homeostasis...Unfortunately the mechanisms of sensitization are far from clear. My overall understanding is it is more likely to occur if:
a) a stronger dose is used without a ramp-up otherwise habituation may occur. Keep in mind this is a bit dangerous and I'm speaking more theoretically.
b) a varying schedule of administration is utilized, meaning not every day at 10AM you take the dose. Some days you would take it others not, any pattern in the timing would need to be minimized and co-varying dosage would be vital.
c) ideally the environment should be different for each dose, look up conditioned place preference. The perception of external environmental cues essentially primes the system and offsets the induced effect. This is the same mechanism that underlies tolerance and habituation
d) a compound is chosen that hits multiple receptors rather than one discrete sub/receptor. In particular I think we would need something that hits the delta receptor and not just mu.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3832215/
2) The hope is that the increase in tonic opioid levels would lead to a self sustained system. I could see this possible if the induced perturbation caused an ability to optimize external environmental rewards; e.g. beginning a workout regime, getting on top of finances, change in personal/relationship interaction/dynamics.
If you look up effort discounting this is what I'm speaking of; a reduction in behavior repertoire due to a loss of willingness to engage any any behavior that requires any form of significant effort because of the reduced ability to experience any form of reward in doing so. Each and every decision we've ever made has been a result of our internal evaluation of effort to payoff, whether it be intrinsic or extrinsic. It's essentially an algorithm that is calculated by neural constructs, weighted nodes, all determined by network communications. Unfortunately with CFS pain and fatigue tends to mask all else, which is essentially like being punished at all times, and in particular in those where effort has been put forth. As classical conditioning dictates soon you will no longer wish to perform that particular or even similar actions, not because of laziness, but because our bodies are hardwired physiologically to conserve resources in the absence of reward. This in my mind is precisely a state of learned helplessness.
In this case the hope would be a neural kindling type of effect would be elicited and reinforced by engaging in particularly rewarding interactions. Unfortunately you can't just sit on the couch and take a pain-killer otherwise you're rolling the dice which direction this goes..