Alpha-lipoic acid reduces the pain level of my PN (hereditary, idiopathic). My pain level increases within a few days if I stop taking it. I take 600 mg/day of Jarrow "alpha-lipoic sustain," have been for a few years. It has a very short half-life in the blood so the sustained release may help. But since mrsD mentioned the r-lipoic, I should give that a try. From what I have read, ALA is commonly used by those with diabetic PN. The studies tend to be done with either 600mg or 1200mg and as I recall, there was not much benefit going to 1200mg. I have experienced no side effects from it.
Electron