What it's actually good for
Vitamin B12 (cobalamin) is required for red blood cell formation, DNA synthesis, and maintaining the myelin sheath around nerves. Unlike most vitamins, it's made only by bacteria and archaea, which is why it's essentially absent from plant foods and why strict vegans and vegetarians are a genuine at-risk group. It's also one of the few supplements where the deficiency-correction story is airtight and the enhancement story mostly isn't.
Real deficiency is not rare. Beyond vegans, older adults lose stomach acid and intrinsic factor needed to release B12 from food (atrophic gastritis affects an estimated 8-9% of adults 65 and older), and metformin — taken by millions of people with type 2 diabetes — measurably depletes B12 with long-term use. If you're in one of these groups, correcting a deficiency is one of the higher-confidence moves in supplementation.
What the research says
Deficiency correction (Grade A). This is the strongest and least controversial part of the story. Deficiency causes megaloblastic anemia, glossitis, and — if it goes on long enough — irreversible neurological damage. A 2022 meta-analysis of 17 studies found metformin users had nearly triple the odds of B12 deficiency (OR 2.95) compared to non-users, and risk climbed with dose and duration. A 2018 Cochrane review found that high-dose oral B12 (1,000-2,000 mcg/day) works about as well as intramuscular injections for normalizing serum levels in deficient people — useful to know if a clinician suggests avoiding the needle.
Cardiovascular disease and stroke (Grade A, and the answer is no). B vitamins reliably lower homocysteine, and observational studies link high homocysteine to heart disease and stroke — a reasonable hypothesis that supplementing would prevent them. It didn't hold up. A 2013 meta-analysis of 18 RCTs covering more than 57,000 people found no significant reduction in stroke risk from B-vitamin supplementation, and the NIH's fact sheet states plainly that current evidence shows no association between B12 supplementation and cardiovascular disease. It's a textbook case of a biomarker moving without the clinical outcome following.
Energy and cognition in non-deficient people (Grade C). This is where marketing outruns the data. A 2021 systematic review and meta-analysis of 16 RCTs (6,276 participants, mostly older adults without overt deficiency) found no evidence that B12 supplementation improves cognitive function or depressive symptoms. Fatigue specifically has barely been studied in trials — only one included study measured it — so there's no pooled verdict, just an absence of supporting evidence. The authors' own framing was blunt: the results contrasted sharply with popular marketing claims. If your levels are already adequate, more B12 is unlikely to make you feel more energetic.
How much, and which form
The RDA is a modest 2.4 mcg/day, easily met by animal foods in a typical omnivorous diet. People supplementing preventively (vegans, older adults) commonly use 500-1,000 mcg/day; confirmed deficiency is usually treated with 1,000-2,000 mcg/day oral or a short injection course, followed by retesting. Cyanocobalamin is the cheapest and most common form; methylcobalamin is marketed as more "active," but the NIH fact sheet notes no evidence that absorption meaningfully differs by form in people with normal gut function — the dose matters more than the form.
Safety & interactions
B12 has no established upper intake limit — it's water-soluble, and excess is excreted without documented harm at high doses. The one real caution: taking high-dose folate without knowing your B12 status can mask the anemia that would otherwise flag a B12 deficiency, letting silent neurological damage continue. Metformin and long-term acid-reducing medications (PPIs, H2 blockers) reduce absorption over time, so periodic level checks make sense for people on either long-term. This is informational, not medical advice — check with a clinician before starting.
How we picked the brand
A B12 product earns a spot when it uses the methylcobalamin (active) form, passes independent third-party testing (NSF Certified for Sport or equivalent), skips unnecessary fillers, and states dose clearly rather than burying B12 inside an unrelated blend.