The injections were likely cyanocobalamin, which stays in the bloodstream only a short time - less than a week (though B-12 is stored in the liver for several months).

http://www.nature.com/nature/journal.../198200a0.html

After you feel recovered, you might consider trying sublingual methylcobalamin.

http://neurotalk.psychcentral.com/thread85103.html

HTH,

Doc