I take 100mcg of B-12 /day as a single pill in AM usually w/ my first round of PD meds. Also take 1000 iu of E and 500mg of C. My Gen Pract MD suggested it about 8 or 9 years ago when one of my "folate, folic acid" whatever levels was a little low. He also said it was a good vit for brain functions. I do notice that on days when i forget my Vits, that the PD meds don't seem to work quite as well, but since I take Vits together, I cant really say what B-12 alone does, but it does not appear to have any negatives that i have noticed. Steve 52/16