Many years ago Memantine was trialed in Europe for PN.
That was almost a decade ago.

It did not pass FDA in US for this indication however. It was instead approved for Alzheimer's.

I don't know why it failed for PN. We had several people very interested in it for PN, and this was back in 2000-2001.

I will say I had some positive experience with dextromethorphan in Feb. I had a terrible bronchitis and was taking alot of cough syrup for it. My feet which should have flared with that illness did not hurt at all. So I added up my DM
intake and it was 80mg/24 hr period!
Now, my pain issues are not anywhere as intense as you all have here on RSD, but the NMDA blockade did work.
And on the other hand, I am very very sensitive to MSG content of foods. So perhaps for me the NMDA receptors are a big factor.

DM cannot be used by people taking SSRI or SSNI/SSRI drugs.
That would be Cymbalta, Effexor, Prozac, Lexapro etc etc.
It may cause serotonin syndrome in those cases. It should not be used with triptan migraine drugs either.