How Long Do Vitamin B12 Injections Stay In the Body? Vitamin B12 injections deliver high doses of this essential vitamin directly into your bloodstream for better absorption than oral supplements to boost energy, support red blood cell production and maintain nerve function. Vitamin B12 Monthly Injection Dose: Typical Dosages & Administration Methods Liv Hospital Vitamin B 12 Injectable for Horses, Dogs and Cats Jeffers B12 Injection Dose: 7 Key Guidelines for Adults and Common Prescriptions Liv Hospital