Are you B-12 deficient? Youll probably have to use an online turkish or indian pharmacy.
B-12 is a water soluble vitamin which makes it extremely easy for your kidneys to piss out the excess. Within 48 hours you'd have pissed out all of the B-12, except for what you actually need. B-12 that isnt excess is stored in the liver and bone marrow and is excreted through the bile. I wont really go into deep discussion about it, but a percentage of things that are excreted through bile are reabsorbed including B-12, recycling it.
B-12 supplementation if you're not deficent is a waste of money and resources. There are even some observational studies linking higher cancer rates with higher levels of B-12.