The established Vitamin B12 dose for correcting a deficiency is 1000 mcg (1 mg) — whether by injection or, for most people, by high-dose oral tablet. Decades of clinical data have settled the core numbers: a short loading phase to refill stores, then spaced-out maintenance dosing to keep them topped up. The interesting part is that the route matters far less than most assume.
This guide covers the standard injectable protocol (cyanocobalamin, hydroxocobalamin, and methylcobalamin), the loading-versus-maintenance schedule, the case for high-dose oral B12, and how the three common forms differ in how often they're dosed.
Research-context information only. Vitamin B12 (cobalamin) is an essential nutrient available as both over-the-counter supplements and prescription products. Research-grade injectable forms are sold and labeled for research use. The protocols and doses reported below come from published clinical trials and established medical literature. This article reports what has been documented, not what should be done. Consult a licensed physician for diagnosis and personal medical decisions.
Vitamin B12 is unusual among the compounds covered on this site: it has an enormous human evidence base, an FDA-defined nutrient role, and one of the cleanest safety records in clinical nutrition. That means the dosing discussion can be specific and confident rather than speculative.
Quick Reference: Protocol
| Parameter | Detail |
|---|---|
| Standard dose | 1000 mcg (1 mg) per injection |
| Loading phase | 1000 mcg daily or alternate-day for 1–2 weeks (or 6 doses over 1–2 weeks with neurologic involvement) |
| Maintenance (cyanocobalamin) | 1000 mcg IM every 1 month |
| Maintenance (hydroxocobalamin) | 1000 mcg IM every 1–3 months (retained longer) |
| High-dose oral alternative | 1000–2000 mcg daily |
| Route | Intramuscular, subcutaneous, or oral/sublingual |
| Upper limit | None established (water-soluble, excess excreted) |
The numbers above reflect the deficiency-correction protocols described across the clinical literature. A clinician matches the schedule to the cause of the deficiency: dietary insufficiency, malabsorption, or pernicious anemia each behave differently over time.
Routes of Administration
Vitamin B12 is one of the few compounds where multiple routes are genuinely interchangeable for most patients — a point that is well documented but still widely misunderstood.
Intramuscular and subcutaneous injection. The classic deficiency protocol uses 1000 mcg of cyanocobalamin or hydroxocobalamin given intramuscularly (subcutaneous delivery is also used, particularly with hydroxocobalamin). Injection bypasses the gut entirely, which is why it became the default for malabsorption and pernicious anemia. Carmel's clinical review (PMID 18606874) describes the standard loading-then-maintenance structure: frequent dosing up front to refill depleted tissue stores, then spaced maintenance.
High-dose oral and sublingual. This is where the evidence is most counterintuitive. Even when intrinsic factor is missing, roughly 1% of an oral dose crosses the gut by passive diffusion — a mechanism documented across multiple randomized trials — without requiring intrinsic factor. At 1000–2000 mcg daily, that 1% is enough to correct deficiency. Kuzminski's randomized trial (PMID 9694707) found 2000 mcg daily oral cyanocobalamin matched or exceeded monthly 1000 mcg injections at normalizing serum B12, and Butler's systematic review (PMID 16585128) and the Cochrane review by Wang (PMID 29543316) reached the same conclusion across multiple trials. Sublingual lozenges are absorbed comparably to swallowed tablets — published comparisons have not shown a consistent advantage of sublingual over oral.
The practical reading: injections are reserved for severe deficiency, neurologic involvement, or situations where daily adherence to oral dosing is impractical — not because oral fails to work.
Reconstitution Quick Reference
Research-grade B12 is most commonly sold as a pre-dissolved liquid rather than a lyophilized powder, so reconstitution math is usually unnecessary. The forms tracked from research vendors map to the standard dose as follows:
| Form | Vial | Concentration | Standard dose volume |
|---|---|---|---|
| Cyanocobalamin (injectable) | 10 mg | 1 mg/mL (typical) | 1.0 mL = 1000 mcg |
| Methylcobalamin (injectable) | 10 mg | 1 mg/mL (typical) | 1.0 mL = 1000 mcg |
| Dissolvable / oral form | — | per label | per label (1000–2000 mcg) |
A 10 mg vial at 1 mg/mL holds ten 1000-mcg doses. Actual concentration is set by the vendor's label and the diluent volume — the table assumes the common 1 mg/mL preparation. Where a vial ships as powder, established protocols describe reconstituting with bacteriostatic water to a known concentration before drawing the labeled dose.
