What Are The Side Effects of Vitamin B12 Injections?

Vitamin B12 plays a vital role in cell growth and reproduction, protein and tissue synthesis and blood formation. Generally, a healthy diet rich in meat, fish and dairy products provide a natural source of vitamin B12 required for the body, but it may be deficient in vegetarians and vegans. Vitamin B12 deficiency can also be found in people with poor nutrition, infection, cancer, anemia, stomach/intestinal problems and/or nerve damage. Vitamin B12 deficiency is mostly treated by injecting cyanocobalamin, which is a synthetic form of vitamin B12. It is administered either intramuscularly or subcutaneously. Dosage should be taken as directed by the physician.

What are the Side Effects of Vitamin B12 Injections?

What Are The Side Effects of Vitamin B12 Injections?

Generally, the injections of vitamin B12 are considered safe when they are administered in a proper manner. However, every drug carries some potential side effects and the same is true for cyanocobalamin also. The most common side effects include sensation of warmth, redness and pain at the injection site, mild diarrhea, nausea, headaches, pain in joints and a feeling of swelling in the whole body. Although, rare, but it can also cause adverse effects on many organs including heart, lungs, muscles, skin and nerves too. The musculoskeletal side effects include muscle pain, muscle weakness, cramps and swelling in extremities including hands, arms, ankles, feet and/or lower legs. The cardiovascular and respiratory side effects include chest pain, tachycardia (increased heart rate), heart palpitations, low blood pressure, and chest tightness, cough with or without production of pink frothy sputum, wheezing and difficulty breathing. Dermatological side effects include rash on the skin, itching, hives, skin redness, blisters, pale skin, peeling and swelling with or without fever.

Swelling might be present in and around the eyelids, eyes, lips, tongue and/or throat. The neurologic side effects include confusion, dizziness, drowsiness, slurred speech, balancing issue, loss of consciousness, muscular paralysis, fatigue, numbness, tingling, and/or burning. Other metabolic side effects could be hypokalemia (low potassium levels in the body), extreme thirst, rapid weight gain, frequent urination and/or unexplained bleeding or bruising. It might also present with vision problems, ringing in ears, dilated neck veins, eye pain, and difficulty swallowing and/or bleeding from gums or nose.

The side effects might be either due to high dose of vitamin B12 or allergic reaction to its composition. Some discomfort at the injection site is common, but if the pain, swelling, redness persist for long or aggravate, then a doctor should be consulted. Gastrointestinal side effects such as nausea, abdominal discomfort and/or diarrhea should be monitored closely; diarrhea could lead to dehydration if it is severe. Additionally, taking food with the supplement could lessen the chances of gastrointestinal side effects. Few side effects are mild and go away on their own as body adjusts to the new high dose of vitamin B12 from a deficient state. Some patients might develop allergic reaction and present with symptoms of rashes, hives, swelling of lips, tongue and throat along with difficulty breathing, chest tightness due to inflamed airways. In such cases, the emergency help is called upon and a doctor should be contacted as soon as possible.


Vitamin B12 injection is contraindicated in pregnant and lactating women, liver disease, kidney disease, infection, iron deficiency, folic acid deficiency, the patient undergoing treatment that has a direct effect on bone marrow, an allergy to medication, vitamin, preservative or cobalt. A patient with a history of Leber’s disease is not a suitable candidate for vitamin B12 injection. Leber’s disease is a hereditary optic neuropathy that causes slow painless vision loss. Vitamin B12 injection has a tendency to worsen the condition. Vitamin B12 injection should be administered only under the recommendation and guidance of a medical professional and they should be aware of complete medical history including any conditions such as hypokalemia, polycythemia vera, gout, Leber’s disease, iron and/or folic acid deficiency. An individual should inform their doctor about the medicines they are taking as cyanocobalamin also interacts with certain prescription and OTC medications such as amoxicillin, erythromycin, methotrexate, and pyrimethamine.

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Pramod Kerkar, M.D., FFARCSI, DA
Pramod Kerkar, M.D., FFARCSI, DA
Written, Edited or Reviewed By: Pramod Kerkar, M.D., FFARCSI, DA Pain Assist Inc. This article does not provide medical advice. See disclaimer
Last Modified On:June 21, 2018

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