question and answer
B12 deficiency: when are injections needed?
June 2007
PATRICIA MENARD, MD, of Antigonish, NS, wants to know: "Is it helpful to initiate vitamin B<sub>12</sub> therapy with intramuscular (IM) injection before going onto maintenance with an oral regimen? How long would it take for oral B<sub>12</sub> alone to normalize serum B<sub>12</sub> levels?"
You can treat patients with vitamin B12 deficiency with oral B12 alone, but in a symptomatic individual, it's not unreasonable to give one parenteral dose of B12 at the same time you start the oral regimen. The only patients who require longer and multiple doses of parenteral B12 are those with neurologic complications of B12 deficiency.

In an individual with pernicious anemia or some other form of malabsorption disorder, the amount of B12 retained from 1,000 µg of B12 IM once a month is similar to that retained following daily intake of 500 µg of oral B12.

It takes about 60 parenteral doses of 1,000 µg to replenish the body stores of an adult with B12-deficiency megaloblastic anemia. However, you don't have to replenish the body stores of B12. Once the patient is started on vitamin B12 1,000 µg IM once a month or 500 µg of B12 daily, the individual fully recovers from any symptoms at the same rate, except the neurologic ones, even if the serum level remains low. The serum B12 on maintenance oral dose is more related to the time of B12 ingestion compared to the drawing of the blood sample, than to the level of body stores.

About 1% of a large oral dose of B12 is absorbed by passive absorption without the need for intrinsic factor or an intact terminal ileum. So if a patient ingests 500 µg of B12 daily, about 5 µg is absorbed, and this is about twice the recommended daily requirement. There's no need to check serum levels once the patient is started on daily oral or monthly parenteral B12 therapy.
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