What Factors Affect B12 Injection Dosage?

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  • Written By: Emma Miller
  • Edited By: Michelle Arevalo
  • Last Modified Date: 23 November 2019
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Low levels of vitamin B12 in the body are often treated with injections. There are many causes of low levels of B12 and these can include malabsorption issues in the small intestine, adherence to a strict vegan diet, and intake of anti-diabetic medications. The most common cause of vitamin B12 deficiency is the autoimmune disorder known as pernicious anemia. The main factor that usually determines B12 injection dosage is the level of this vitamin in the body, as determined by a blood test. Pregnancy may increase the need for supplemental B12 and dosage adjustments may be needed during this time.

Low levels of B12 are commonly seen in people over age 65 and in those suffering from pernicious anemia. Adequate levels of this vitamin are necessary for normal neurological function and for the conversion of nutrients into energy in the body. Deficiency may lead to a range of neurological symptoms and hematologic abnormalities.

Diagnosis of B12 deficiency is generally simple and done by measuring the levels of B12 in the blood. Serum levels of the vitamin are typically the main factor affecting B12 injection dosage. The lower the blood levels, the higher the supplemental B12 dose. In some cases, B12 supplementation may be discontinued after blood levels normalize. For most people, however, supplementation is ongoing.


Supplementation therapy is generally given via intramuscular injection, at a dose of 1,000 to 2,000 micrograms (mcg) daily for up to two weeks. This generally leads to a rapid increase of blood B12 levels. The common, long-term maintenance dosage ranges from 100 to 1,000 micrograms, administered once a month. Blood tests are normally done every few months to monitor levels during treatment. This usually helps ensure the correct B12 injection dosage is given.

Pregnancy can affect the levels of B12 in the body. People who require long-term B12 therapy should undergo regular testing during this time, as their B12 injection dosage may need to be adjusted. The body’s nutrient requirements typically go back to normal in the months following pregnancy, so B12 injection dosage may need to be changed accordingly.

Intake of certain medications may affect a person’s B12 injection dosage. Regular use of proton pump inhibitors, antacids, or some anti-diabetic drugs, such as metformin, may lead to decreased blood levels of B12 as they alter the way the vitamin is absorbed. Normal absorption resumes when the medications are discontinued, but people on long-term therapy with these drugs may need a higher B12 injection dosage for the duration of treatment.


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