IM: intramuscular; SL: sublingual; CBC: complete blood count; MMA: methylmalonic acid (increased in vitamin B12 deficiency); MCV: mean corpuscular volume.
* Malabsorption is classically due to pernicious anemia (PA; vitamin B12 deficiency caused by autoantibodies to intrinsic factor or gastric parietal cells). Other causes may include bariatric, gastric, or small intestinal surgery. Some experts will use oral vitamin B12 as initial therapy for individuals with malabsorption if they do not have severe anemia or neurologic complications and if adherence was assured. Refer to UpToDate for diagnostic testing for PA and other evaluations.
ΒΆ Dose and frequency depend on the level of concern and the costs and burdens of therapy, with shared decision-making. For severe deficiency, daily dosing for the first week can be considered. If a dose increase is needed due to insufficient response, it is reasonable to increase the dosing frequency (eg, 1000 mcg IM every 2 weeks) and/or increase the dose (eg, 2000 mcg orally instead of 1000 mcg). Lower doses are used for children (refer to UpToDate for details).