Hi Jenny,
I am also diabetic (type II) and am just starting to take supplements to help control blood sugar. I just wanted to let you know I have taken megadoses of b-vitamins for years (since high school, and I'm well past 30). Like 6foot3 says it will just pass out of your body.
I can testify that taking high doses of Methylcobalamin has eased my neuropathy pains a great deal, and fairly quickly. The FDA recommends something like 2.4 Micrograms, I take well over 10000 milligrams daily with no problems. Of course I take a multi and other supplements, not just B12.
I have a friend with some sort of reaction to B vitamins, but I think that is extremely rare.
Besides, I read someplace, maybe here, that the FDA recommendations are the minimum requirements.
Anyway good luck to you,