Vitamin B12: evidence grade B, dosage & worth it

What it does

Vitamin B12 (cobalamin) is an essential cofactor for two enzymes: methionine synthase, which recycles homocysteine to methionine and keeps folate cycling for DNA synthesis, and methylmalonyl-CoA mutase, needed for normal nerve myelin maintenance. Without enough of it, red blood cell production falters (megaloblastic anaemia) and nerves are damaged, sometimes irreversibly. It comes only from animal foods, fortified foods, or supplements, and absorption depends on stomach acid and intrinsic factor, both of which decline with age and common medications.

What the research says

Corrects B12 deficiency and its anaemia and neurological symptoms in people who are actually deficient
Evidence: strong. This is established medicine rather than a supplement claim: deficiency causes megaloblastic anaemia and can cause irreversible nerve and spinal cord damage if untreated. Oral replacement works even in many malabsorption cases: a Cochrane review (Wang et al., 2018) of oral versus intramuscular B12 in deficient patients found oral doses of 1,000 to 2,000 mcg normalized blood levels comparably, with a small evidence base. The Harvard Health Publishing B12 overview states the clinical picture plainly: deficiency produces anaemia, numbness and tingling, balance problems, and memory loss, and treatment reverses most of it if caught in time.
Lowers homocysteine, and combined B vitamins slowed brain atrophy in older people with high homocysteine and mild cognitive impairment
Evidence: moderate. The real signal is narrow. VITACOG (Smith et al., PLoS One 2010) randomized 271 people over 70 with mild cognitive impairment to folic acid 0.8 mg, B12 0.5 mg, and B6 20 mg for 24 months: brain atrophy slowed by about 30 percent on average (0.76 vs 1.08 percent per year), and by about 53 percent in those with homocysteine above 13 micromol/L, with less atrophy tracking better cognition. That is one trial, a combined B-vitamin intervention, in a high-homocysteine subgroup. The larger prevention picture is null: B-PROOF (van Wijngaarden et al., Neurology 2014) gave 2,919 older adults with elevated homocysteine B12 500 mcg plus folic acid 400 mcg for 2 years and found no effect on cognitive decline despite a 48 percent homocysteine drop. Homocysteine lowering is proven; cognitive benefit outside the VITACOG niche is not.
Provides energy, focus, or mood benefits in people who are not deficient
Evidence: weak. This is the claim the marketing makes and the evidence does not support. Reviews of B12 supplementation in people with normal status find no consistent improvement in fatigue, cognition, or mood; the 'B12 shot for energy' effect in replete people has no controlled-trial basis. If fatigue resolves after starting B12, the likely explanation is that a real, often unmeasured deficiency was present. Harvard Health's scorecard framing captures the split: B12 earns its place for deficiency risk groups, not as a general tonic.

B12 splits cleanly in two, and the grade averages an A-grade use with a D-grade use. For deficiency, which is genuinely common in vegans, older adults with atrophic gastritis, and people on metformin or long-term acid suppression, supplementation is not optional wellness; it prevents anaemia and irreversible neurological damage, and oral high-dose replacement is proven comparable to injections for most patients. That is as strong as supplement evidence gets. Against that, the popular claims fail: in non-deficient people there is no good trial evidence for energy, mood, or general cognitive enhancement, and the big prevention trial (B-PROOF, n = 2,919) found no cognitive benefit despite large homocysteine reductions. The one bright spot, VITACOG's slowing of brain atrophy, was a combined B-vitamin trial in a high-homocysteine MCI subgroup and has not been replicated at scale. A foundational nutrient everyone at risk should cover, a booster nobody replete should expect anything from: B.

Grades move when the evidence moves.

This grade for Vitamin B12 rests on the record above. When a new result changes that record, the grade changes with it, and the Thursday email is where the change lands first: the week's longevity and biotech news, in plain English, linked back to the reasoning on this page.

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Every grade follows a published rubric, and sponsors never change one. When the evidence moves a grade, Thursday is where you hear it first, before you renew a stack that stopped earning its cost.

What the experts say

Every position links to its source. We include skeptics alongside supporters.

Bryan Johnson
Source, 2026-09-10 Takes 125 mcg/day of vitamin B12 in his Blueprint Essential Capsules, in the morning with food, per the published Blueprint protocol (compiled in our expert-stacks.json). A food-level insurance dose rather than a deficiency-treatment dose.

Is it worth it?

A-

At $3 to $8 a month, this is one of the cheapest decisive interventions in the database for anyone in a risk group (vegan, over 60, metformin, long-term PPI), where deficiency is common and the harm of missing it is irreversible nerve damage. The A- rather than A reflects applicability: for a young omnivore with normal levels, the same bottle buys nothing measurable. Derived under the October 2026 worth-it model at integration.

Typical cost

$3-8/month

Common form, not a brand recommendation

Dosage contextestablished

The RDA is 2.4 mcg/day for adults, but dose depends on the job. Insurance dosing for at-risk but not-yet-deficient people: 25 to 250 mcg/day (Johnson's 125 mcg sits here, roughly 50x the RDA, reflecting that only about 1 percent of a large oral dose is absorbed passively). Treating established deficiency: 1,000 to 2,000 mcg/day orally, which the Cochrane comparison found comparable to injections for normalizing levels in most patients; injections remain standard where neurological symptoms or severe malabsorption are present. Form: cyanocobalamin is the cheapest, most stable, and best studied; methylcobalamin is the active coenzyme form and is widely marketed as superior, but the NIH states there is no evidence that absorption from supplements varies by form, and there is no good human evidence methylcobalamin outperforms cyanocobalamin for raising status in most people.

Worth-it leaderboard

Vitamin B12 ranks #6 for worth-it.

That rank comes from the gated model on the leaderboard: evidence grade B- or better, purchasable, and useful beyond a single occasion, then scored on evidence against monthly cost.

See the full worth-it leaderboard →

When the evidence moves a grade, this ranking moves with it. Thursday is where you hear it first.

Safety notes

One of the safest supplements in the database at ordinary doses: the NIH Office of Dietary Supplements states no tolerable upper intake level has been set and B12 is generally considered safe even at large doses. Three cautions are worth printing. First, acne: Kang et al. (Sci Transl Med 2015) showed B12 supplementation represses B12 synthesis genes in the skin bacterium Propionibacterium acnes and increases its production of inflammatory porphyrins, and one of ten healthy supplemented subjects developed acne within a week; stopping usually resolves it. Second, high-dose signals from data not designed to prove them: in B-PROOF, incident cancer was reported by 63 participants on B12 plus folic acid versus 42 on placebo (HR 1.56, 95 percent CI 1.04 to 2.31) in a trial not designed to assess cancer, and in the US VITAL cohort of about 77,000 adults, men taking more than 55 mcg/day of B12 from individual supplements for ten years had roughly double the lung cancer risk, strongest in current smokers. Neither is a causal finding; both argue against casual megadosing. Third, high-dose folic acid taken alongside can correct the anaemia while B12 nerve damage progresses silently, so the two should be assessed together, and unexplained deficiency can signal pernicious anaemia or another absorption disorder that needs diagnosis, not just a bigger supplement.

Longevity Lab Report

Vitamin B12 news, graded when it happens.

The grade on this page stays free. The email carries the news on vitamin b12: expert moves, dose changes, and what is working, in plain English every Thursday.

Frequently asked questions

Is Vitamin B12 safe?

One of the safest supplements in the database at ordinary doses: the NIH Office of Dietary Supplements states no tolerable upper intake level has been set and B12 is generally considered safe even at large doses. Three cautions are worth printing. First, acne: Kang et al. (Sci Transl Med 2015) showed B12 supplementation represses B12 synthesis genes in the skin bacterium Propionibacterium acnes and increases its production of inflammatory porphyrins, and one of ten healthy supplemented subjects developed acne within a week; stopping usually resolves it. Second, high-dose signals from data not designed to prove them: in B-PROOF, incident cancer was reported by 63 participants on B12 plus folic acid versus 42 on placebo (HR 1.56, 95 percent CI 1.04 to 2.31) in a trial not designed to assess cancer, and in the US VITAL cohort of about 77,000 adults, men taking more than 55 mcg/day of B12 from individual supplements for ten years had roughly double the lung cancer risk, strongest in current smokers. Neither is a causal finding; both argue against casual megadosing. Third, high-dose folic acid taken alongside can correct the anaemia while B12 nerve damage progresses silently, so the two should be assessed together, and unexplained deficiency can signal pernicious anaemia or another absorption disorder that needs diagnosis, not just a bigger supplement.

What is the right dose of Vitamin B12?

The RDA is 2.4 mcg/day for adults, but dose depends on the job. Insurance dosing for at-risk but not-yet-deficient people: 25 to 250 mcg/day (Johnson's 125 mcg sits here, roughly 50x the RDA, reflecting that only about 1 percent of a large oral dose is absorbed passively). Treating established deficiency: 1,000 to 2,000 mcg/day orally, which the Cochrane comparison found comparable to injections for normalizing levels in most patients; injections remain standard where neurological symptoms or severe malabsorption are present. Form: cyanocobalamin is the cheapest, most stable, and best studied; methylcobalamin is the active coenzyme form and is widely marketed as superior, but the NIH states there is no evidence that absorption from supplements varies by form, and there is no good human evidence methylcobalamin outperforms cyanocobalamin for raising status in most people.

Is Vitamin B12 worth taking?

We grade Vitamin B12 A- for worth-it at a typical cost of $3-8 per month. At $3 to $8 a month, this is one of the cheapest decisive interventions in the database for anyone in a risk group (vegan, over 60, metformin, long-term PPI), where deficiency is common and the harm of missing it is irreversible nerve damage. The A- rather than A reflects applicability: for a young omnivore with normal levels, the same bottle buys nothing measurable. Derived under the October 2026 worth-it model at integration.

What do experts say about Vitamin B12?

Bryan Johnson: Takes 125 mcg/day of vitamin B12 in his Blueprint Essential Capsules, in the morning with food, per the published Blueprint protocol (compiled in our expert-stacks.json). A food-level insurance dose rather than a deficiency-treatment dose.