how long are b12 injections good for Vitamin Injections: What You Need To Know Vitamin B12 Supplements and Injections

It's a very small needle nothing like a vaccine or blood draw

[25] Supplementation with vitamin B 12 during conditions of deficiency restores the intracellular level of cobalamin and maintains a sufficient level of the two active coenzymes: methylcobalamin and adenosylcobalamin

The MHRA product information for hydroxocobalamin preparations lists several recognised side effects that patients should be aware of before commencing treatment

The injection is given every six months if due to a dietary deficiency, or two to three months if the cause of your low B12 levels is pernicious anaemia (an autoimmune condition that can cause a deficiency)