Vitamin B12 (methylcobalamin vs cyanocobalamin)
Summary
Vitamin B12 supplements most often contain cyanocobalamin; methylcobalamin, adenosylcobalamin and hydroxocobalamin are also sold. NIH ODS finds no evidence that absorption differs by form. Oral B12 at 1,000–2,000 mcg/day raised low blood levels about as well as injections (grade C). In older adults with low B12 but no symptoms, a year of supplementation did not improve nerve or cognitive function.
Key facts
- Cyanocobalamin (most common in supplements), methylcobalamin, adenosylcobalamin, hydroxocobalamin; oral and sublingual[1]
- ODS: no evidence that absorption rates vary by form[1]
- Raising low B12 levels: high-dose oral ≈ injection (C); thinking and nerve function in older adults with low B12 ↔ (no effect); heart events with B-vitamin combinations ↔ (no effect)[2][5][6]
- About 50% at 1–2 mcg, about 2% at 500 mcg and 1.3% at 1,000 mcg[1]
- RDA 2.4 mcg (2.6 pregnancy, 2.8 breastfeeding); no UL set[1]
- 5 graded human outcomes on this page
- October 9, 2026
How it works
EstablishedB12 is a cofactor for methionine synthase, which converts homocysteine to methionine, and for methylmalonyl-CoA mutase (established). Methylcobalamin and adenosylcobalamin are the active coenzyme forms; cyanocobalamin and hydroxocobalamin are converted to them in the body (established).[1]
What the human research says
Human trials only, one row per outcome. The form column shows what each trial actually tested. Grades follow our evidence-grading policy.
At a glance: strongest evidence per outcome
| Outcome | Grade | Effect | Strongest evidence | Rows |
|---|---|---|---|---|
| Energy | Grade C | No effect | Systematic review · n = 153 · 2018 | 3 rows |
| Heart | No effect | No effect | Meta-analysis · n = 71,422 · 2017 | 1 row |
| Focus | No effect | No effect | RCT · n = 191 · 2015 | 1 row |
Every graded row
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Raising low blood B12: high-dose oral vs injection | No effect | Grade C | Systematic review · n = 153 · 2018 3 RCTs in people with vitamin B12 deficiency, mean ages 38.6–72 Form: oral vitamin B12 vs intramuscular vitamin B12 · Dose: 1,000–2,000 mcg/day oral · Duration: 3–4 months At 1,000 mcg/day, no clinically relevant difference in serum B12 versus injections (2 trials); at 2,000 mcg/day (1 trial), serum B12 was 680 pg/mL higher with oral. Oral treatment cost less in one trial. | 29543316 (opens PubMed)Source [2] |
| B12 status marker (methylmalonic acid) by oral dose | Improved | Grade C | RCT · n = 120 · 2005 Older people with mild vitamin B12 deficiency Form: cyanocobalamin · Dose: 2.5, 100, 250, 500 or 1,000 mcg/day · Duration: 16 weeks Methylmalonic acid fell 16%, 16%, 23%, 33% and 33% across doses. Daily doses of 647–1,032 mcg produced 80–90% of the maximum estimated reduction. | 15911731 (opens PubMed)Source [3] |
| Raising low blood B12: sublingual vs swallowed tablet | No effect | Grade D | RCT · n = 30 · 2003 Adults with low serum cobalamin Form: cobalamin, sublingual vs oral (and an oral B complex) · Dose: 500 mcg/day · Duration: 4 weeks Serum B12 rose significantly in all groups with no difference between sublingual and oral routes. | 14616423 (opens PubMed)Source [4] |
| Major vascular events with homocysteine-lowering B vitamins | No effect | No effect | Meta-analysis · n = 71,422 · 2017 15 RCTs in adults with and without existing cardiovascular disease Form: vitamins B6, B9 (folic acid) and/or B12, alone or combined · Dose: Varied across trials · Duration: 1 to 7.3 years No difference in myocardial infarction (RR 1.02) or death from any cause (RR 1.01), high-quality evidence. A small reduction in stroke (RR 0.90) was also reported. | 28816346 (opens PubMed)Source [6] |
| Nerve and cognitive function in older adults with low B12 | No effect | No effect | RCT · n = 191 · 2015 Adults aged 75+ with moderate vitamin B12 deficiency and no anemia (OPEN trial) Form: crystalline vitamin B12 · Dose: 1 mg (1,000 mcg)/day · Duration: 12 months Serum B12 rose 177% and homocysteine fell 17%, but there was no effect on nerve conduction, other neurologic outcomes or cognitive function. | 26135351 (opens PubMed)Source [5] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Cyanocobalamin | Varies[8][1] | Converted in the body to methylcobalamin and adenosylcobalamin. No head-to-head human data in our sources; ODS reports no evidence that absorption differs by form.[1] | Generally considered safe even at high doses; no UL set.[1] | Dose-response of B12 status markers; most oral-vs-injection and cognition trials |
| Methylcobalamin | Varies[8] | An active coenzyme form. No head-to-head human absorption data versus cyanocobalamin in our sources; ODS reports no evidence of a difference.[1] | No form-specific tolerance data in our sources.[1] | ODS cites 1,000 mcg/day oral methylcobalamin normalizing B12 after gastric bypass surgery |
| Hydroxocobalamin and adenosylcobalamin | Varies[1] | Hydroxocobalamin is converted to active forms; adenosylcobalamin is an active form. No comparative human data in our sources.[1] | No form-specific data in our sources.[1] | Hydroxocobalamin is mainly a prescription injection |
Studied doses, daily needs and limits
Doses studied in human trials ranged from 2.5 to 2,000 mcg/day of oral vitamin B12, for 4 weeks to 12 months. Because absorption drops sharply above 1–2 mcg, trials in people with low B12 status mostly tested 500–2,000 mcg/day.[3][2][4][5][1]
Intake reference: Adult RDA: 2.4 mcg/day; 2.6 mcg in pregnancy and 2.8 mcg while breastfeeding. No UL has been set.[1] No UL because of low potential for toxicity; the body does not store excess amounts. NIH ODS fact sheet (updated July 2, 2025).
Recommended intake and upper limit by age and sex
| Age | Male | Female | Pregnancy | Lactation | Upper limit (UL) |
|---|---|---|---|---|---|
| 0–6 months (AI) | 0.4 mcg | 0.4 mcg | — | — | Not set |
| 7–12 months (AI) | 0.5 mcg | 0.5 mcg | — | — | Not set |
| 1–3 years | 0.9 mcg | 0.9 mcg | — | — | Not set |
| 4–8 years | 1.2 mcg | 1.2 mcg | — | — | Not set |
| 9–13 years | 1.8 mcg | 1.8 mcg | — | — | Not set |
| 14–18 years | 2.4 mcg | 2.4 mcg | 2.6 mcg | 2.8 mcg | Not set |
| 19+ years | 2.4 mcg | 2.4 mcg | 2.6 mcg | 2.8 mcg | Not set |
Interactions
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
Medicines that can lower levels
- MetforminNIH fact sheetMay reduce B12 absorption and serum levels; a meta-analysis of 4 trials found serum B12 about 57 pmol/L lower after 6 weeks to 3 months.[1][7]
- Acid-reducing drugsNIH fact sheetCan interfere with absorption of B12 from food by reducing stomach acid, which can lead to deficiency.[1]
Who should be careful
- Groups at higher risk of low B12
- Older adults, people with pernicious anemia or stomach and intestinal conditions, people who have had GI surgery, vegans and vegetarians, and infants of vegan mothers are more likely to have low B12. Low B12 should be diagnosed and managed with a clinician, sometimes with injections.[1]
- Pregnancy and breastfeeding
- Needs rise slightly (RDA 2.6 mcg in pregnancy, 2.8 mcg while breastfeeding). Infants of mothers on vegan diets can develop deficiency early.[1]
- Kidney
- Methylmalonic acid, a test used to confirm low B12, also rises with reduced kidney function, which can complicate results.[1]
- Medicines
- Long-term metformin or acid-reducing drugs can lower B12 status; ODS advises discussing B12 status with a clinician.[1]
- Before you start
- Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[1]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
Cost per studied dose, without the sales pitch
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Cost per studied dose
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Articles about Vitamin B12 (methylcobalamin vs cyanocobalamin)
Plain-English answers to common questions, built on the evidence above.
Frequently asked questions
Is methylcobalamin better than cyanocobalamin?
There is no good evidence that it is. ODS states no evidence shows absorption differs by form; cyanocobalamin is converted to methylcobalamin and adenosylcobalamin in the body. We found no head-to-head human trials comparing the two on health outcomes. Methylcobalamin and cyanocobalamin differ in molecular weight by under 1%.[1][8]
Why do B12 supplements contain 500 or 1,000 mcg when the RDA is 2.4 mcg?
Absorption drops steeply as the amount rises: about 50% at 1–2 mcg but only about 2% at 500 mcg and 1.3% at 1,000 mcg. In a dose-finding trial in older adults with mild deficiency, about 650–1,000 mcg/day was needed to largely normalize a key status marker.[1][3]
Is sublingual B12 better than a swallowed tablet?
Evidence suggests no difference. A small trial found 500 mcg/day raised low B12 equally by either route, and ODS reports no difference in efficacy.[4][1]
Can oral B12 replace injections?
In a Cochrane review of 3 small trials, 1,000–2,000 mcg/day oral B12 normalized serum levels about as well as injections over 3–4 months, at lower cost. The evidence is low quality, and some conditions still call for injections, so decisions belong with a clinician.[2]
Does B12 give you more energy?
Only if you are low. ODS concludes B12 supplements do not appear to improve energy or athletic performance in people with adequate B12 status.[1]
Can you take too much B12?
No UL has been set because toxicity is low. Some observational studies link high blood B12 with cancer, but ODS describes the overall evidence as mixed and says more research is needed.[1]
Sources
- [1]Vitamin B12: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2025. fact-sheet
- [2]Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency.. Cochrane Database Syst Rev, 2018. systematic-review
- [3]Oral cyanocobalamin supplementation in older people with vitamin B12 deficiency: a dose-finding trial.. Arch Intern Med, 2005. RCT
- [4]Replacement therapy for vitamin B12 deficiency: comparison between the sublingual and oral route.. Br J Clin Pharmacol, 2003. RCT
- [5]Effects of vitamin B-12 supplementation on neurologic and cognitive function in older people: a randomized controlled trial.. Am J Clin Nutr, 2015. RCT
- [6]Homocysteine-lowering interventions for preventing cardiovascular events.. Cochrane Database Syst Rev, 2017. meta-analysis
- [7]Association between metformin and vitamin B12 deficiency in patients with type 2 diabetes: A systematic review and meta-analysis.. Diabetes Metab, 2016. meta-analysis
- [8]Cyanocobalamin and methylcobalamin: molecular weights and formulas. PubChem, NCBI, 2026. database
What changed
- First draft. Every evidence-row PMID checked with NCBI E-utilities (esummary + efetch abstract) and re-read in an independent adversarial pass.
- Added intake table from the NIH ODS fact sheet (checked Oct 9 2026), cost-per-studied-dose method, labeled analysis derived from cited rows.