This is because issues with regular function of the gastrointestinal tract can prevent the proper release and absorption of the vitamin which causes deficiency
While you can pre-mix a larger batch, its stability will diminish over time, even with bacteriostatic water
Scientific Classification A synthetic 16-amino-acid C-terminal peptide fragment derived from residues 177191 of a larger human polypeptide
Certain medications may also interfere with absorption and cause deficiency, in particular, medicines prescribed for heartburn, antibiotics, blood pressure medicine, cholesterol-lowering drugs, diabetes medications and antipsychotic drugs
Akinmoladun I, Afolabi O, Jolaoso A, et al
Vitamin B12: Fact Sheet for Health Professionals