B12 Injection: Uses, Administration & Side Effects

Clinical review: Flávia Costa
August 2022
  1. Uses
  2. Administration
  3. Side effects
  4. Contraindications

A B12 injection is made-up of artificial B12 vitamin and is used to treat low B12 levels in the body. B12 levels can become low as a result of inadequate nutrition, decreased absorption of this vitamin in the intestines, stomach problems, infections or cancer. 

Persistently low B12 levels can cause pernicious anemia, stomach problems or nerve damage. This vitamin is important for the growth and proliferation of cells, blood cell formation, protein formation, and nervous system maintenance. 

This medication can be obtained in pharmacies as the generic cyanocobalamin infection with a prescription. It is injected intramuscularly by a trained health care professional. 

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What it is for 

B12 injections are prescribed for the treatment of many illnesses like: 

  • Penicious anemia
  • Vitamin B12 deficiency
  • Intense nerve pain or inflammation 
  • Intense muscular pain
  • Sciatica 
  • Lumbar osteoarthritis
  • Arthritis
  • Diabetic neuropathy

Vitamin B12 injections can also be prescribed in some cases of a herniated disc or narrowing of the spinal cord near the neck, especially when the pain starts in the neck and radiates to an arm. 

Learn more about vitamin b12 deficiency and the symptoms it can cause.

How to take it

A vitamin B12 injection can be injected into the gluteal muscle by a healthcare professional. Dosing depends on the condition being treated, and use should be monitored by the doctor. 

Generally, for treatment of mild nerve pain, 5000 iu is injected as the initial dose, with subsequent doses administered until pain resolves. For intense nerve pain, dosing can increase to 15000 iu per day until pain is better managed. 

Anemia pernicious is treated with 100 to 1000 iu per day, injected into the muscle. Treatment duration varies from person to person, and should be monitored with frequent blood tests. 

Possible side effects

Some side effects can occur following the first B12 injection, or they can emerge after several doses. The most common ones are itchiness and diarrhea. 

With ongoing treatment, other, more serious side effects can emerge. These should be communicated to the prescriber. Other side effects that can occur include: 

  • Excessive sweat production
  • Rapid weight gain
  • Pain when breathing 
  • Shortness of breath when lying down
  • Anxiety
  • Increased or irregular heart rate
  • Numbness, tingling or muscular weakness
  • Increased thirst 
  • Urinary frequency 

B12 injections can also cause allergy symptoms immediately after administration, like difficulty breathing, a swollen throat, mouth, or swelling of the mouth, tongue or face. Therefore, vitamin B12 should only be injected by a health care professional who is prepared to provide first aid immediately. 

Contraindications for use

B12 injections should not be administered to children or pregnant or breastfeeding women. They are also contraindicated or people with vision problems, Leber’s disease, kidney failure, liver failure, folic acid deficiency or any type of systemic infection. 

This medication is also not advised for people with an allergy to any of the components in the solution. 

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Written by Daisy Oliveira - Registered Nurse. Updated by Tua Saude editing team on August of 2022. Clinical review by Flávia Costa - Pharmacist, on August of 2022.


  • VIDAL-ALABALL, J.; et al. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database Syst Rev. 3. CD004655, 2005
  • STABLER, Sally P. Clinical practice. Vitamin B12 deficiency. N Engl J Med. 368. 2; 149-60, 2013
Show more references
  • DRUGS.COM. Vitamin B12 Injection. Available on: <https://www.drugs.com/vitamin-b12.html>. Access in 28 Apr 2021
  • LANGAN, Robert C.; GOODBRED, Andrew J. Vitamin B12 Deficiency: Recognition and Management. Am Fam Physician. 96. 6; 384-389, 2017
  • THAKKAR, K.; BILLA, G. Treatment of vitamin B12 deficiency-methylcobalamine? Cyancobalamine? Hydroxocobalamin?-clearing the confusion. Eur J Clin Nutr. 69. 1; 1-2, 2015
Clinical review:
Flávia Costa
Graduated in Pharmacy at Centro Universitário Newton Paiva, Brazil, in 2003. Master in Biomedical Sciences at UBI, Portugal.