Best Vitamin B12 Injection Sites: Where to Inject B12 PA Relief 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 Do You Need a Prescription for B12 Injections? OmegaQuant 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