Abstract It is widely recognized that Vitamin B12, also known as cyanocobalamin, is a nutrient that keeps the body’s nerves and blood cells healthy and contributes in deoxyribonucleic acid synthesis. Megaloblastic anemia, a form of anemia that causes fatigue and weakness, can be prevented with Vitamin B12. The main objective of this case report was to draw attention to the rare hypersensitive reactions which can occur after injecting Vitamin B12 supplement therapy. The abovementioned reaction could be a result to an immunoglobulin E-mediated reaction or due to sensitization to the Vitamin B12 molecule itself. Here, we have reported the case of a 25-year-old female patient, a known case of pulmonary tuberculosis, who was hospitalized and on medication of fixed dose of directly observed treatment, short-course, nebulizer Combimist, tablet beclomethasone, and cyanocobalamin injection. We deduce that the anaphylactoid reaction to Vitamin B12 is a rare but serious side effect and that it should be vigilantly monitored while administering the drug to the patient, especially if the parenteral route is preferred.