I was kinda hoping that a Mexican doc as a professional courtesy to his American doc counterpart would at least give some weight to a professional American doc giving me a professional prescription for a narcotic pain reliever

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Without having to go through an exam all over again.
I understand that such may not be realistic though.
I guess we can conclude this much from what has been said so far...
If you want an OTC (over the counter) medication that is available in both countries as such...no problem. Get it and bring it across the border as you please (though I would still declare it to be on the safe side).
If you want a medication that is available by prescription in one country but OTC in the other...get a prescription for the country which needs a prescription and bring it into the country that doesn't. No problem.
Do not bring a medication into a country that needs a prescription from a country that doesn't without a prescription to have it in the country that needs a prescription!
So I think if we honor the prescription requirements for either the U.S. or Mexico within their respective borders we can go back and forth just fine with whatever we are prescribed.
As long as we declare it at the border to our respective government border agents.
Does that sound about right?
Personally I won't go into Mexico to get a medication that I do not have a U.S. prescription for (assuming that a U.S. prescription is needed to get that medicine in the U.S.).
Carlos