Giving Your Cat a Vitamin B12 Boost. The Art of Doing Stuff 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 Subcutaneous Injection Sites and Instructions for Safe Self Administration Subcutaneous (SC) and Intramuscular (IM) Injections Must Knows for Subcutaneous administration: Pinch skin with non dominant hand and hold syringe with dominant hand Always inject: 2 inches away from umbilicus (for abdomen) 3 inches