Just found this on the net:
Metformin, a medication for individuals with type 2 (non-insulin dependent) diabetes, decreases vitamin B12 absorption by tying up free calcium required for absorption of the IF-B12 complex. This effect is correctable by drinking milk or taking calcium carbonate tablets along with food or supplements (5).
Here's the site:
http://lpi.oregonstate.edu/infocente...ug_interaction