If an injection meant for the muscle is accidentally given into a vein, the medicine enters the bloodstream much faster than it should, which can lead to sudden dizziness, a rapid heartbeat, nausea, Vitamin B12 deficiency and cognitive impairment: A comprehensive review of neurological impact ScienceDirect Only One Chance B12 Deficiency Pernicious Anaemia Society Desensitization for Vitamin B12 Hypersensitivity and How to Do It B12 Shots at Home: How, Where & How Often to Inject Yourself AZ IV Medics