Question of the Day:, How long does it take for a B12 injection to rev up the engines of energy? , For many people, the boost can be felt within 2448 hours. Others may notice gradual changes 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 Vitamin B12 Inejctions at Skin2Soul MedSpa & Regenerative Medicine Choosing the Right Needle and Syringe for B12 Injections: A Complete Guide