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What Is Vitamin B12 (Methylcobalamin)?

Vitamin B12 is the common name for a nutrient known scientifically as methylcobalamin.

It's used as a dietary supplement and to treat certain anemias.

Vitamin B12 plays an important role in helping the body make red blood cells.

Mechanism of Action

 Vitamin B12 is used in the body in two forms, methylcobalamin and 5-deoxyadenosyl cobalamin.  The enzyme methionine synthase needs methylcobalamin as a cofactor.  This enzyme is involved in the conversion of the amino acid homocysteine into methionine which is, in turn, required for DNA methylation.  The other form, 5-deoxyadenosylcobalamin, is a cofactor needed by the enzyme that converts L-methylmalonyl-CoA to succinyl-CoA.  This conversion is an important step in the extraction of energy from proteins and fats. Furthermore, succinyl CoA is necessary for the production of hemoglobin, the substance that carries oxygen in red blood cells.

Vitamin B12 Dosage

How much vitamin B12 you should take will depend on your age and condition.

Talk to your doctor or pharmacist about the right dose for you, and read the label carefully.

For vitamin B12 deficiency, your doctor may prescribe a cyanocabalamin injection of 1000 mcg each day for one week, then once a week for one month, then once a month.

How is this medication best taken?

This medicine is injected into muscle or deeply under the skin. It is usually given by a health care professional in a clinic or doctor's office. However, your doctor may teach you how to inject yourself. Follow all instructions. Talk to your pediatrician regarding the use of this medicine in children. Special care may be needed.

Vitamin B12 Side Effects

  • Headache
  • Itching
  • Swelling
  • Nervousness and anxiousness
  • Involuntary or uncontrollable movements

Vitamin B12 Warnings

Talk to your doctor before taking vitamin B12 if you:

  • Are allergic to vitamin B12 or any other ingredients found in the drug
  • Have a genetic condition called optic atrophy where the nerve that connects the eye to the brain wastes away
  • Have blood in your urine
  • Have an ongoing infection
  • Have low iron or folate levels

Vitamin B12 and Alcohol

Alcohol actually decreases levels of B vitamins in the body -- especially if you drink a lot.

You should avoid or limit alcohol consumption while taking vitamin B12.

Vitamin B12 and Grapefruit Juice

You should avoid eating grapefruit and drinking grapefruit juice while taking vitamin B12.


 Keep this medicine in a refrigerator below 41°F (5°C) and away from moisture and light. Keep all medicine out of the reach of children. Throw away any unused medicine after the expiration date. Do not flush unused medications or pour down a sink or drain