effects of b12 injections vs Pills: Richmond's Complete Guide How Long Does It Take

Brain Sci 13:272 Santos-Garca D, Macas M, Llaneza M, Grande M, De La Fuente-Fernndez R (2011) Serum vitamin B 12 and folate levels in Parkinsons disease patients treated with duodenal levodopa infusion

Weak Immune System : Increased susceptibility to illness or infections

Of note, however, rLCMV/LASV LII A immunization elicited also viral epitope-specific CD8 T cell responses (Figure S2D), and rLCMV/LASV LII A -immunized sIgM / x AID / mice that were not depleted of CD8 T cells completely suppressed rLCMV/LASV LIV C challenge viremia (Figure S2E)

You could be having an allergic reaction or could be taking too high of a dose