Understanding Vitamin B12 Injections for Health Benefits What are the possible side effects of a vitamin B12 (cobalamin) injection? If you've been feeling run down lately you could be deficient in Vitamin B12. Physical symptoms of B12 deficiency include mouth ulcers, fatigue, digestive issues and muscle aches. There are also neurological B12 Shots at Home: How, Where & How Often to Inject Yourself AZ IV Medics B12 Shot Guide: Benefits, Dosage & Side Effects BodySpec