IDEALLY be prescribed by one major doctor - the one you see the most often is best. This way should you need any additional meds [say in the case of a broken foot] the immediate prescribing doc KNOWS who is in control of all your primary pain needs and can contact your main pain med doc if they have any concerns about those who are ubiquitous pain-med seekers. This way, one key doc can keep track [as must you!] of what works and doesn't and hopefully will keep up to date on new meds and methods to keep the pain at a lower HUMMM than it currently is.
Strangely, it is my neuro who prescribes the majority of my pain meds. I see that doc and one more often than my GP and other docs.
I would think in most cases that it would be either the neurologist or pain medicine doc who does the prescribing....but in other cases, if your GP is playing an agressive or active role in your care, and they are physically more accessable, there is absolutely no reason not to let the GP be the prescriber.
All that is important is a doc who knows you and your pain needs and one who is willing to work to help you as best they can.
hope this helps? - j