It is the standard multi-dose vial rule: under the pharmacy standard USP Chapter 797, once a multi-dose vial has been opened or accessed by a needle, it should be dated and discarded within 28 days unless the manufacturer specifies a different date
Vitamin B12 , Folate, and the Methionine Remethylation Cycle-Biochemistry, Pathways, and Regulation. Journal of Inherited Metabolic Disease, vol
Always label your BAC water vial with the date you first opened it
Pharmacological Reviews, 1998
But the question is how often can you get a B12 shot
Additionally, lifestyle factors like diet and exercise can influence the longevity of the shots effects