Injectable cyanocobalamin is an approved prescription medicine with a defined label. This page explains how that label structures repletion and why — it is education about the medicine's design, not instructions. Whether B12 is needed, in what form and how often, depends on the cause of deficiency and on lab values, and belongs to a prescribing clinician.
Overview
B12 dosing confuses people because three unrelated numbers circulate as though they were versions of the same thing: the 2.4 mcg daily dietary requirement, the 100 mcg labelled injection for pernicious anaemia, and the 1,000 mcg wellness shot. They differ by more than two orders of magnitude, and each answers a different question. Only one of them appears on an FDA label.
The labelled repletion schedule
Cyanocobalamin injection, USP is supplied at 1,000 mcg/mL as a sterile solution for intramuscular or deep subcutaneous injection. For pernicious anaemia the label describes a front-loaded, tapering schedule (DailyMed):
- 100 mcg daily for 6 or 7 days by intramuscular or deep subcutaneous injection.
- Following clinical improvement and reticulocyte response, the same amount on alternate days for seven doses.
- Then every 3 to 4 days for a further 2 to 3 weeks.
- Maintenance: 100 mcg monthly for life.
These figures describe the label's structure, not a plan for any reader. The label also warns that hypokalaemia and sudden death may occur in severe megaloblastic anaemia treated intensively — which is why the intensive phase happens under supervision with monitoring.
The shape is worth reading. It is a repletion curve: refill the stores quickly, confirm the marrow is responding, then taper to a maintenance dose that offsets ongoing losses. "For life" appears because pernicious anaemia is a permanent absorption failure, not a temporary shortfall.
Three different numbers, three different questions
2.4 mcg per day is the adult Recommended Dietary Allowance — the amount needed to maintain healthy haematological status and serum B12 in a person absorbing normally (NIH Office of Dietary Supplements).
100 mcg by injection is a repletion dose that bypasses absorption entirely. It is roughly forty times the daily requirement because the goal is refilling depleted stores, not meeting a daily need.
1,000 mcg orally is a workaround for broken absorption. Intrinsic-factor-mediated uptake saturates at about 1–2 mcg per dose, so absorption of a large oral dose falls to about 2% at 500 mcg and 1.3% at 1,000 mcg. Roughly 13 mcg of a 1,000 mcg tablet is absorbed — via passive diffusion that does not require intrinsic factor, which is precisely why high-dose oral works even in malabsorption.
Seen this way, the numbers stop looking arbitrary. Each is scaled to the absorption pathway it is trying to use or avoid.
Oral versus injection — what the trials show
Injections are widely assumed to be more potent. The controlled evidence is thinner and less decisive than that assumption implies. A Cochrane review found three randomised trials comparing oral with intramuscular B12, totalling 153 participants (74 oral, 79 IM), with three-to-four-month follow-up. In two trials using 1,000 mcg/day orally there was no clinically relevant difference in serum B12 levels versus injection; a third, using 2,000 mcg/day, found a mean difference of 680 pg/mL favouring oral. Oral treatment cost less. The authors graded the evidence low quality due to imprecision and noted that no trial reported clinical signs and symptoms of deficiency as an outcome (Wang et al., Cochrane Database Syst Rev 2018).
Route selection in practice therefore turns on severity, adherence, and whether neurological involvement makes rapid, certain repletion the priority — not on an assumption that a needle delivers more. Diagnosis and management framing is set out in Stabler's clinical-practice review (N Engl J Med 2013).
Where the wellness shot sits
The 1,000–5,000 mcg weekly or monthly injection sold in wellness clinics and compounded blends does not correspond to any labelled schedule. It is a supraphysiologic dose given to people who have usually not been tested, for outcomes — energy, mood, metabolism — that have not been demonstrated in B12-replete individuals.
The toxicological objection is weak: no Tolerable Upper Intake Level has been set, and surplus is excreted. The stronger objection is diagnostic. Supplementing before testing normalises a serum B12 result and can mask the malabsorptive condition underneath it, and a neurological deficiency identified late may not fully reverse. Form is the least important variable in the whole discussion — cyanocobalamin and hydroxocobalamin are both labelled injectables, and the body interconverts forms — while whether a real deficiency exists is the most important one.