What Size Needle for B12 Injection? Dr. prescribed B12 injections. Pharmacy only gave me vials. Where do you get your needles and syringes? What size?? Where do I inject? : r B12_Deficiency Does size matter? Historically, a bigger needle gauge was considered better to prevent damage to red blood cells. However, our recent guidelines recommend using smaller gauge needles (2325 G) for all 24 Hour B12 Injections ThinWell Clinic How to Give a B12 Injection: Step By Step Instructions