Metformin is one of the most often prescribed and reliably effective medications for controlling blood sugar levels in type 2 diabetes. Increasing insulin sensitivity in the liver, it lowers blood glucose concentrations without increasing insulin production, helping its users control their diabetes without gaining weight, and without the risk of "falling out" from hypoglycemia.
Type 2 diabetics who have neuropathy, however, may need to ask their doctors for a different medication or be sure to take a combination of folic acid and vitamin B12. The two B vitamins help the body keep homocysteine levels low. When homocysteine levels are high, metformin aggravates neuropathy. It's better to take your vitamin B, but if for some reason you simply cannot take supplemental folic and homocysteine (if you live in a European country that already enforces the Codex Alimentarius, for example), then you may need to see your doctor about alternatives to metformin. Or, better yet, control your blood sugar levels with the right diet.
Wile DJ, Toth C. Association of metformin, elevated homocysteine, and methylmalonic acid levels and clinically worsened diabetic peripheral neuropathy. Diabetes Care. 2010 Jan;33(1):156-61. Epub 2009 Oct 21.