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B12 Injection Dosage

author image Leah DiPlacido, Ph.D.
Leah DiPlacido, a medical writer with more than nine years of biomedical writing experience, received her doctorate in immunology from Yale University. Her work is published in "Journal of Immunology," "Arthritis and Rheumatism" and "Journal of Experimental Medicine." She writes about disease for doctors, scientists and the general public.
B12 Injection Dosage
Woman holding a syringe Photo Credit: Photodisc/Photodisc/Getty Images

Vitamin B-12 is required for the transformation of protein and fat into energy. The metal cobalt is incorporated into vitamin B-12, which is the basis of the alternative name of this vitamin, cobalamin. A vitamin B-12 deficiency is more common in the elderly, affecting between 10 and 15 percent of those over 60 years of age. Some diseases are the cause of the deficiency, including pernicious anemia, and conditions that reduce the amount of vitamin B-12 absorbed by the gastrointestinal tract.

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Who Is Prescribed Vitamin B-12 Injections

Vitamin B-12 in the form of cyanocobalamin is prescribed to treat severe vitamin B-12 deficiencies. One manufacturer of injectable vitamin B-12 is American Regent Inc., and they have published prescribing information for this medication. According to this manufacturer, vitamin B-12 injections are generally prescribed for people in whom the deficiency is caused by insufficient absorption of vitamin B-12 through the gastrointestinal tract. Cyanocobalamin is used to treat pernicious anemia; illness, dysfunction and/or surgery of the gastrointestinal tract that prevents normal vitamin B-12 absorption; pancreas or bowel cancer; deficiency in folic acid; and/or a tapeworm infection. In addition, some people who eat a strict vegetarian diet do not get enough vitamin B-12, and may need injections or high dose oral vitamin B-12 to correct a deficiency.


Vitamin B-12 injections are given intramuscularly or subcutaneously, meaning that vitamin B-12 is injected directly into the muscle or underneath the skin, respectively. The injectable vitamin B-12 is in the form of cyanocobalamin, which is the principle form of this vitamin used in supplements.

Common Vitamin B-12 Doses

For those with pernicious anemia, monthly injected doses of vitamin B-12 should be taken for life. At the start of treatment, 100 mcg are injected each day for six or seven days. Blood tests reveal if the doses of vitamin B-12 are raising the level of this vitamin in the blood; if improvement is seen, then the doctor may change the dose to every other day for an additional seven days, followed by every three or four days for between two and three weeks. When levels of vitamin B-12 become normal in the blood, monthly injections at 100 mcg should be repeated for life. Make sure to consult a medical professional before beginning a regimen of vitamin B-12 injections.

Side Effects and Safety

Doses up to 1,000 mcg per month of injected vitamin B-12 have no toxic effects in those who have pernicious anemia but are otherwise healthy. The most common side effects of vitamin B-12 injection are diarrhea and swelling. Other symptoms that are less common include muscle pain, weakness and cramps; intense thirst and frequent urination; coughing, wheezing and shortness of breath; and fast heart rate and dizziness. If you are allergic to any of the components of the injectable vitamin, you may experience swelling, redness, hives, rash, itching and/or difficulty breathing or swallowing.

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