Vitamin B12 (Cobalamin)Vitamin

• Correcting a diagnosed deficiencyLikely helps

  This is the use with settled evidence behind it. NIH's Office of Dietary Supplements notes deficiency is usually corrected with intramuscular B12, which bypasses the gut entirely [1].
  
  A 2018 Cochrane review of 3 RCTs (153 participants) found daily oral doses of 1,000–2,000 mcg normalised serum B12 about as well as injections [1].
  
  That evidence was rated low quality, so the oral-versus-injection question is settled less firmly than the deficiency-correction question itself. ^[1]

• Energy and athletic performanceNot shown to help

  In people whose B12 status is already adequate, supplementation did nothing for energy, athletic performance or endurance in the trials NIH ODS reviewed [1].
  
  That is the exact claim B12 is marketed on. The measured benefit appears only where a real deficit is being corrected [1]. ^[1]

• Cognition and dementiaNot shown to help

  RCTs running 1–2 years, using B12 alone or alongside folic acid and B6, found no improvement in cognitive test scores in older adults [1].
  
  That held whether or not participants had dementia, mild cognitive impairment or Alzheimer's — even though the same supplements reliably lowered homocysteine [1].
  
  Observational studies are messier. Some link low B12 to worse cognition and others do not, per NIH ODS [1]. ^[1]

• Heart attack and strokeNot shown to help

  A Cochrane review of 15 studies (71,422 participants) found B12, alone or with other B vitamins, did not reduce heart attacks or death rates in at-risk people [1].
  
  Separate meta-analyses found no significant association between B12 intake and coronary heart disease or stroke risk [1].
  
  Homocysteine falls; the cardiovascular endpoints do not move with it. ^[1]

• Diabetic and peripheral neuropathyPossibly helps

  Two reviews of overlapping trial sets reach opposite conclusions, and this page is not resolving that for you.
  
  Jayabalan and Low's 2016 systematic review of 4 RCTs (363 patients) found no clinical benefit from oral B12, with most studies showing no nerve-conduction improvement [7].
  
  Karedath and colleagues' 2022 meta-analysis of RCTs found significant reductions in neuropathic symptom and pain scores, though no effect on cholesterol markers [8]. ^[7]

• Cancer riskPossibly helps

  The evidence points three ways at once, and NIH ODS says more research is needed to sort it out [1].
  
  One large cohort found men on 55 mcg/day or more for about 10 years had 40% higher lung cancer risk. Women in the same study showed no association [1].
  
  In one RCT, colorectal cancer incidence was 3.4% on 500 mcg B12 plus 400 mcg folic acid versus 2% on placebo, over 2–3 years [1]. The folic acid component may account for that.
  
  Other cohorts point the other way, linking lower B12 to higher gastric, colorectal and prostate cancer risk [1].
  
  A meta-analysis of 18 RCTs (74,498 participants, 20–2,000 mcg/day) found little or no effect on cancer incidence, cancer deaths or all-cause mortality [1]. ^[1]

• Megaloblastic anemia — large, abnormally nucleated red blood cells, usually felt as fatigue and weakness
• Low counts of white blood cells, red blood cells or platelets
• Glossitis — a sore, inflamed tongue
• Heart palpitations, pale skin and weight loss
• Infertility
• Numbness and tingling in the hands and feet
• Balance problems
• Confusion, poor memory, and dementia in severe or prolonged cases
• Nerve symptoms can appear with no anemia at all, so a late diagnosis risks permanent damage
• Depression is associated with low B12 or low intake in some studies, less firmly than the rest
• In infants of B12-deficient mothers: failure to thrive, developmental delays and megaloblastic anemia
• Overt classic deficiency is uncommon. Marginal status of 200–300 pg/mL without symptoms is far more common, reaching up to 40% in some Western populations^[1]

• CyanocobalaminGeneral supplementation, and the reference form behind most of the dosing research .

  Elemental content

  Inactive until the body converts it. The most common form in oral supplements [1]

  Absorption

  About 50% of a dose is absorbed below 1–2 mcg, the binding capacity of intrinsic factor [1].
  
  At pharmacologic doses it collapses. Roughly 2% of a 500 mcg dose and 1.3% of a 1,000 mcg dose get absorbed [1].

  Best for

  General supplementation, and the reference form behind most of the dosing research [1].

• Methylcobalamin and adenosylcobalaminPeople who prefer the pre-converted form.

  Elemental content

  The two metabolically active coenzyme forms, used directly by the body's enzymes [1]

  Absorption

  The marketing claim is that methylcobalamin absorbs better than cyanocobalamin.
  
  NIH's Office of Dietary Supplements states that no evidence indicates absorption rates differ by supplemental form [1].

  Best for

  People who prefer the pre-converted form. On the measured numbers, no absorption advantage has been shown [1].

• Sublingual tabletsAnyone who dislikes swallowing tablets.

  Elemental content

  The same cobalamin forms, made to dissolve under the tongue

  Absorption

  No difference in efficacy has been found between oral and sublingual preparations [1].

  Best for

  Anyone who dislikes swallowing tablets. It offers nothing the oral version does not, on current evidence [1].

• Intramuscular injection (prescription)Deficiency from pernicious anemia or malabsorption, where oral routes are unreliable .

  Elemental content

  Cyanocobalamin or hydroxocobalamin, given by injection [4]

  Absorption

  Injected, so it skips the stomach acid, intrinsic factor and ileal steps entirely [1].

  Best for

  Deficiency from pernicious anemia or malabsorption, where oral routes are unreliable [1].

• Nasal gel spray (prescription)People needing a non-injection route under prescription .

  Elemental content

  A prescription intranasal formulation

  Absorption

  Effective at raising blood B12 concentrations, per NIH ODS [1].

  Best for

  People needing a non-injection route under prescription [1].

• Hydroxocobalamin IV, 5 g (Cyanokit)Cyanide poisoning, under emergency medical supervision.

  Elemental content

  An FDA-approved antidote for known or suspected cyanide poisoning [5]

  Absorption

  Given intravenously in emergency care. Route and dose have nothing in common with nutritional supplementation [5].

  Best for

  Cyanide poisoning, under emergency medical supervision. Listed here only so the name is not confused with a B12 supplement [5].

• High-dose safety

  B12 has not been shown to cause harm even at high supplemental doses, and no upper intake limit has been set [2].
  
  That is why 500–1,000 mcg products sit on shelves routinely [1]. ^[2]

• Absorption disorders, not diet

  Pernicious anemia — an autoimmune loss of intrinsic factor — is the most common cause of clinically evident B12 deficiency worldwide [1].
  
  Atrophic gastritis affects 8–9% of adults 65 and over, cutting both stomach acid and intrinsic factor [1].
  
  Prior gastrointestinal surgery, celiac disease and Crohn's disease all impair absorption regardless of how much B12 is eaten [1]. ^[1]

• Vegans, vegetarians and older adults

  Naturally occurring B12 is exclusively animal-derived, so people eating little or no animal food carry meaningfully higher deficiency risk [1].
  
  Fortified cereals, fortified nutritional yeast and supplements are the usual route around that [1].
  
  Among community-dwelling older adults, reported deficiency ranges from 3% to 43% depending on the definition used [1]. ^[1]

• The blood tests disagree with each other

  Prevalence estimates swing wildly — 3% versus 43% in older adults — depending on which serum cutoff and biomarker a study picks [1].
  
  NIH ODS calls methylmalonic acid the most sensitive marker. It also rises on its own with kidney disease and with age [1]. ^[1]

• Metformin

  Metformin, the first-line drug for prediabetes and type 2 diabetes, may reduce B12 absorption and significantly lower serum B12 with regular use [1].
  
  Because of that, people on long-term metformin are sometimes advised to have their B12 checked periodically [1]. ^[1]

• Stomach acid reducers

  Proton pump inhibitors such as omeprazole and lansoprazole, and H2 blockers such as cimetidine and ranitidine, suppress the acid that frees B12 from dietary protein [1].
  
  Over time that can produce deficiency. It hits food-bound B12 far harder than crystalline supplement B12, which skips that release step [1]. ^[1]

Naturally occurring B12 comes from animal foods only — fish, meat, poultry, eggs and dairy. Plant foods carry none unless they are fortified [1].

Organ meat and shellfish dominate: beef liver, 3 oz — 70.7 mcg (2,944% DV), clams 17 mcg (708% DV), oysters 14.9 mcg (621% DV) [1].

Fortified nutritional yeast runs 8.3–24 mcg per ¼ cup depending on brand, up to 1,000% DV. It is the concentrated vegan-friendly option [1].

Mid-range per 3 oz: Atlantic salmon 2.6 mcg, canned light tuna 2.5 mcg, 85% lean ground beef 2.4 mcg [1].

Smaller amounts: 2% milk at 1.3 mcg per cup (54% DV), one large cooked egg at 0.5 mcg (19% DV) [1].

Bioavailability is not flat across foods — roughly three times higher from dairy than from meat, fish or poultry [1].

Supplemental B12 is about 50% more bioavailable than food-bound B12, since it skips the acid-and-enzyme release step [1].

Cleveland Clinic family medicine physician Dr. Rodolfo Perez-Gallardo suggests taking B12 on an empty stomach with a glass of water, since it is water-soluble [3].

He also suggests the morning, on the reasoning that B12 can be energising and a late dose risks interfering with sleep [3].

That is clinical guidance, not a result from a controlled timing trial. NIH's Office of Dietary Supplements makes no time-of-day recommendation at all [3].