Reviewed
Reviewed on 2026-09-19 against the evidence rules: every claim carries a grade, every claim and every safety flag carries a source, and every citation was re-resolved against the record it names. The read was delegated by the owner on 2026-09-18 and signed by the Lead on his behalf (D-295) — it is a delegated review, not the owner's own reading of the page. Every claim on it carries an evidence grade and a named weakest link. It is educational and is not medical advice.
Last reviewed
Sixteen randomised trials in people who are not deficient found nothing. The honest content here is who should have their level checked, and that is a conversation with a clinician rather than a purchase.
Safety
No data in your group
The trials above deliberately excluded people with overt deficiency, so they say nothing about that group — which is the group for whom B12 matters. If you eat no animal products, are over sixty, or take metformin or an acid-suppressing drug long term, your question is not answered on this page and is worth asking a clinician.
Who should not
Supplementing B12 can normalise a blood count while a neurological problem continues underneath, which is why an unexplained symptom is a reason to be tested rather than a reason to self-dose. Interpret a serum B12 result with a clinician, not a chart.
Adverse effects
The Cochrane review notes that most of the included study reports did not mention adverse events at all, or made only general statements about them. An absence of reported harm in that literature is weaker evidence of safety than it looks.
What the evidence says, and how strong it is
A grade belongs to one claim, in one population, for one preparation — never to a compound, a food or a page. Open a grade to see the four links between the evidence and the thing you would actually do, and which of them is doing the least work.
Across 16 randomised trials and 6,276 participants who were not overtly deficient, B12 supplementation showed no effect on any subdomain of cognitive function and no effect on measures of depression. Meta-regression found nothing that predicted a treatment effect either.
Evidence grade A: Evidence supportsIn whom: Adults without overt B12 deficiency or advanced neurological disease
What form: Vitamin B12 alone, or with folic acid and B6, across 16 randomised trials
Measured as: Any subdomain of cognitive function, and depressive symptoms
Evidence does not support
Why this grade
- Weakest link
- Population — the people studied are not the people reading. Sixteen trials in people deliberately selected as not overtly deficient — which is exactly the group that takes B12 for concentration, and exactly the group it did nothing for.
- What was actually studied
- People without advanced neurological disorders or overt B12 deficiency, 16 randomised trials (n = 6276)
- Detail worth knowing
- No evidence of an effect of B12 alone or of a B complex on any subdomain of cognitive function, and no effect on depression measures. Meta-regression found no predictor of treatment effect.
A Cochrane review pooled 14 studies and 27,882 participants and concluded that B vitamins probably have little or no effect on global cognitive function at any point up to five years, with pooled differences between −0.03 and 0.06, and may have none at five to ten years.
Evidence grade B: May supportIn whom: Cognitively healthy adults, mostly in their sixties and seventies
What form: Folic acid, vitamin B12, vitamin B6 or a combination, against placebo
Measured as: Global cognitive function over 5 and over 5–10 years
Evidence does not support
Why this grade
- Weakest link
- Population — the people studied are not the people reading. The 27,882 participants are mostly in their sixties and seventies, so the null is measured a long way from a thirty-year-old.
- What was actually studied
- Cognitively healthy people aged 40 and over, 28 studies (n = 83000)
- Detail worth knowing
- B vitamins: little or no effect on global cognition to 5 years (SMD −0.03 to 0.06) and probably none at 5–10 years. Vitamin D3 400 IU/day with calcium in 4,143 participants: −0.1 MMSE points (95% CI −0.81 to 0.61) after a mean 7.8 years. Only two of the 28 studies had a mean baseline age under 60.
Only two of the 28 studies in the Cochrane review had a mean baseline age under 60, and the meta-analysis of B12 specifically was in people presenting with complaints rather than in healthy young adults. There is no adequate trial in this group to grade, searched on 2026-08-23.
Evidence grade D: Human evidence insufficientIn whom: Healthy adults under 60 eating a mixed diet
What form: Vitamin B12 supplementation at any dose
Measured as: Any cognitive outcome
Why this grade
- Weakest link
- Population — the people studied are not the people reading. Only two of the 28 pooled studies had a mean baseline age under 60, and the B12 meta-analysis recruited people presenting with complaints; a healthy young adult on a mixed diet is in neither.
- What was actually studied
- Cognitively healthy people aged 40 and over, 28 studies (n = 83000); People without advanced neurological disorders or overt B12 deficiency, 16 randomised trials (n = 6276)
- Detail worth knowing
- B vitamins: little or no effect on global cognition to 5 years (SMD −0.03 to 0.06) and probably none at 5–10 years. Vitamin D3 400 IU/day with calcium in 4,143 participants: −0.1 MMSE points (95% CI −0.81 to 0.61) after a mean 7.8 years. Only two of the 28 studies had a mean baseline age under 60.
- No evidence of an effect of B12 alone or of a B complex on any subdomain of cognitive function, and no effect on depression measures. Meta-regression found no predictor of treatment effect.
Everything above, and where it comes from
- Markun S, Gravestock I, Jäger L, Rosemann T, Pichierri G, Burgstaller JM (2021). Effects of Vitamin B12 Supplementation on Cognitive Function, Depressive Symptoms, and Fatigue: A Systematic Review, Meta-Analysis, and Meta-Regression. Nutrients 13(3):923
- Rutjes AW, Denton DA, Di Nisio M, et al. (2018). Vitamin and mineral supplementation for maintaining cognitive function in cognitively healthy people in mid and late life. Cochrane Database of Systematic Reviews 2018(12):CD011906.pub2
The one thing worth doing
If you already have blood tests for something else, ask whether B12 is on the panel. Knowing costs nothing extra; guessing costs the price of a supplement that sixteen trials say will do nothing for you.
Regulatory note — this note has not yet been reviewed by a lawyer. In the United States, supplements are sold under DSHEA: a structure/function claim is not reviewed by the FDA before it appears, and the label must carry the words "This statement has not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease." In the EU and the UK, only health claims on the authorised register may be used in advertising, and a food or supplement advertisement may not say a product prevents, treats or cures a disease. Whether an evidence page that carried a purchase link would itself count as advertising under those rules is an open question that counsel, not this page, will answer; there are no purchase links here. Nothing on this page is medical advice.