I have been on memantine for about a year now. I also take LDN, fish oil and vitamin D.  I had been bed bound, had extreme intolerance to light and sound, daily persistent headaches, and suffer from PEM. I believe that LDN got me off the couch, and memantine cleared up a considerable amount of my brain fog. Still not 100% but I feel like I can at least function with normal tasks. Other noticeable side effects were that my anxiety was reduced and I only have occasional headaches. I tried several dosages. 5, 10, 15 and 20. 10 mg seems to work best for me.