§.01Why 5,000 mcg, when the RDA is 2.4 mcg.
Vitamin B12 (cobalamin) is one of the strangest nutrients to dose. The Recommended Daily Allowance is 2.4 mcg for adults — a tiny number. Yet B12 supplements commonly come in 1,000 mcg, 2,500 mcg, or 5,000 mcg sizes. There's a reason: oral B12 has appallingly bad absorption.
Normal B12 absorption requires intrinsic factor, a protein secreted by stomach parietal cells, which binds B12 and shepherds it to specific receptors in the terminal ileum. That mechanism is saturable. Even at peak efficiency, only about 1.5-2 mcg of B12 can be absorbed via intrinsic factor per meal. Anything more bypasses that pathway and gets absorbed by passive diffusion — at roughly 1-2% efficiency.
That is why a 5,000 mcg sublingual or oral dose makes sense even when the body only needs ~5 mcg per day: passive diffusion of 1% of 5,000 mcg = 50 mcg, which is 10x the daily target. It is over-dosing on purpose, because the math forces it.
§.02Who actually needs B12 supplementation.
- Vegans and most vegetarians. B12 is exclusively produced by bacteria. No plant food contains meaningful B12 (fortified foods and supplements are the exceptions). A strict vegan diet inevitably leads to B12 depletion over 2-5 years without supplementation.
- Adults 50+. Stomach acid declines with age, which reduces the release of food-bound B12 and reduces intrinsic factor availability. Roughly 15-30% of adults over 60 have suboptimal B12 status.
- Long-term proton pump inhibitor (PPI) users. Omeprazole, esomeprazole, pantoprazole reduce stomach acid → reduced B12 absorption. Over 2+ years, deficiency is common.
- Metformin users. Long-term metformin (5+ years) reduces B12 absorption in the terminal ileum. Diabetes guidelines now recommend B12 testing every 1-2 years on metformin.
- People with pernicious anemia. Autoimmune destruction of intrinsic-factor-producing cells. Requires sublingual high-dose B12 or injections.
- Gastric bypass or ileum resection patients. Surgical impact on absorption.
- Celiac, Crohn's, ulcerative colitis with terminal ileum involvement. Reduced absorption.
§.03The forms: methylcobalamin vs cyanocobalamin.
The cobalamin molecule needs a "cap" to be stable. Common caps:
| Form | Active | Best for | Cost |
|---|---|---|---|
| Methylcobalamin | Directly active | MTHFR variants, neurological recovery | $$ |
| Cyanocobalamin | Converted in liver | General use, most stable | $ |
| Adenosylcobalamin | Mitochondrial form | Energy / metabolic support | $$$ |
| Hydroxocobalamin | Injectable, long half-life | Deficiency reversal, cyanide poisoning | Rx |
For most people, cyanocobalamin works fine — the body converts it to active methyl- and adenosyl- forms in the liver. If you have an MTHFR polymorphism (~30% of the population has at least one variant), methylcobalamin bypasses the conversion step and may produce faster correction. The difference for healthy adults is small.
§.04How to take it.
Standard: 1,000-5,000 mcg orally or sublingually, 2-3 times per week for maintenance.
Deficiency correction: 5,000 mcg sublingual daily for 4 weeks, then test serum B12 (target > 400 pg/mL) and methylmalonic acid (MMA, target < 250 nmol/L). If MMA remains elevated, consider intramuscular hydroxocobalamin injections via your prescriber.
Sublingual or swallowed? Studies suggest similar efficacy at high doses. Sublingual feels more "active" but is not chemically necessary at 5,000 mcg — passive diffusion in the gut handles absorption either way at this dose.
Time of day: any time. B12 is water-soluble, no food interaction.
§.05Symptoms of deficiency.
Early B12 deficiency is sneaky because it looks like generic fatigue or aging. As it progresses:
- Macrocytic anemia. Larger-than-normal red blood cells. Detected on a basic CBC (look for MCV > 100 fL).
- Peripheral neuropathy. Tingling, numbness, especially in feet and hands. Reversible if caught early; permanent if left for years.
- Cognitive symptoms. Brain fog, poor concentration, memory issues. Can be misdiagnosed as early dementia in older adults.
- Glossitis. Smooth, beefy-red tongue from epithelial atrophy.
- Mood changes. Depression, irritability, sometimes psychiatric symptoms (rare but documented).
- Subacute combined degeneration of the spinal cord. Late-stage. Affects motor function. Often irreversible.
If you have any neurological symptoms and could plausibly be B12-deficient (vegan, PPI user, post-bypass), get serum B12, MMA, and homocysteine tested before starting high-dose supplementation. High-dose B12 can mask the hematological signs of deficiency while neurological damage progresses.
§.06What to look for on a label.
- Dose: 1,000-5,000 mcg. 5,000 mcg is overkill for maintenance but is the dose used in deficiency correction studies.
- Form named explicitly: methyl-, cyano-, adenosyl-, or hydroxocobalamin. "Vitamin B12" alone is ambiguous.
- Delivery: sublingual tablet, dissolve-under-tongue lozenge, liquid drops, or swallowable capsule. All work at this dose.
- No sugar / artificial sweeteners in gummy or lozenge forms (some have 4-6g added sugar per dose).
- USP / NSF verification if available.
§.07Frequently asked.
Can I overdose on B12?
Practically no. B12 is water-soluble; excess is excreted in urine. No upper limit has been established. Doses up to 1,000,000 mcg (1 g) have been given without serious adverse effects.
Why does my urine turn bright yellow?
That's riboflavin (B2) from a B-complex, not B12. B12 itself doesn't pigment urine.
Do I need injections?
Only if you have severe deficiency that doesn't correct with oral, or pernicious anemia with confirmed lack of intrinsic factor. Most B12 deficiency corrects with sublingual or oral 5,000 mcg daily for 4 weeks.
Will B12 give me energy?
If you are deficient, yes — the energy improvement is real and dramatic. If you are not deficient, B12 won't make you feel more energetic. It's not a stimulant; it's a coenzyme.
Methyl- vs cyano- for someone with MTHFR?
Methylcobalamin is the form most physicians recommend for MTHFR variants because it bypasses the conversion step. The clinical difference is modest for most people but real for some.
§.08The bottom line.
Vitamin B12 deficiency is one of the most under-diagnosed nutritional issues, especially in vegans, adults over 50, long-term PPI users, and metformin users. The 5,000 mcg dose looks like overkill against the 2.4 mcg RDA — but the math of B12 absorption (saturable at intrinsic factor, ~1-2% efficiency by passive diffusion) makes 5,000 mcg the practical dose for correction. Methylcobalamin or cyanocobalamin both work; methyl- is preferred for MTHFR carriers. If you have neurological symptoms, get tested before supplementing to avoid masking. PuraVigor's B12 5,000 mcg is methylcobalamin, sublingual, $24 for a 90-day supply.
B12 5,000 mcg, 90 lozenges — $24 at the apothecary.
§.RXStudies cited.
- Allen, 2009 — How common is vitamin B12 deficiency? Am J Clin Nutr
- Stabler, 2013 — Vitamin B12 deficiency (NEJM clinical practice)
- Reinstatler et al., 2012 — Metformin and B12 deficiency
- Lam et al., 2013 — PPIs and B12 deficiency (JAMA)
- Andrès et al., 2004 — Oral cobalamin therapy effectiveness
- Pawlak et al., 2014 — B12 status in vegetarians and vegans