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.