Vitamin B12 Shot Side Effects

Vitamin B12, a water-soluble vitamin that can be found in animal proteins, is necessary for red blood cell production and neurological function. Individuals whose diets do not include animal proteins, or who are unable to absorb Vitamin B12 from the foods they eat, may require supplementation by mouth, nasal spray or injection. Vitamin B12 has few known side effects, according to the Mayo Clinic, and those side effects are very uncommon.

Allergic Reactions

Although rare, allergic reactions such as rashes, itching and wheezing have been reported by patients taking Vitamin B12, according to the Mayo Clinic. Vitamin B12 supplements should not be taken by patients who are sensitive to cobalt, cobalamin or other product ingredients. If an allergic reaction occurs, stop the medication and contact a health care provider immediately.


Diarrhea has been reported in some patients taking Vitamin B12 supplements, according to the Mayo Clinic. Medical attention usually is not required, and the symptoms may go away as the body adjusts to the medication. Contact your health care provider for information if the problem becomes bothersome.

Excess Intake of Vitamin B12

According to the Office of Dietary Supplements of the National Institutes of Health, no adverse effects of excess Vitamin B12 intake from food and supplements have been documented in healthy individuals.

Reporting Side Effects

Although the U.S. Food and Drug Administration categorizes Vitamin B12 as a supplement rather than as a drug, you may report side effects to the FDA at 1-800-FDA-1088.

About this Author

Marcy Brinkley’s articles about health care and legal issues have appeared in “Texas Health Law Reporter” and the “State Bar of Texas Health Law Section Report.” She holds a bachelor’s degree in nursing, a master’s degree in business administration and a Doctor of Jurisprudence.