Instructions for B12 Injection Alan Health Vitamin B12 deficiency, sometimes called cobalamin deficiency, happens when your body is either not getting enough or not absorbing enough vitamin B12 from the foods you eat. Vitamin B12 is an important B12 Injections: Who Should Get Them and How Long Do They Last? Who Should Take Vitamin B12 Injections And Why? B12 Shots at Home: How, Where & How Often to Inject Yourself AZ IV Medics