Evidence explainerVitamin B12 (methylcobalamin vs cyanocobalamin)

B12 After 50 and on Metformin: Why Levels Drop, and What the Trials Say About Fixing Them

Absorbing B12 gets harder with age, and two of the most prescribed drug types make it harder still. Here is who is at risk, why pills come in 1,000 mcg, and what a year of supplements did and did not do.

By Kymata Health editorial teamPublished October 9, 2026Last reviewed October 9, 2026

Medical review pending. Written by our editorial team from the studies cited below; a credentialed clinician has not reviewed it yet. Educational only, not medical advice.

Key takeaways

  • Between 3% and 43% of community-dwelling older adults have low B12 depending on the definition used, especially those with atrophic gastritis [1].
  • Metformin lowered serum B12 by about 57 pmol/L after 6 weeks to 3 months in a meta-analysis of 4 trials [2].
  • Acid-reducing drugs (PPIs, H2 blockers) can also reduce B12 absorption from food [1].
  • In older adults with mild deficiency, oral doses around 647 to 1,032 mcg/day produced 80% to 90% of the maximum improvement in a key B12 marker [3].
  • In adults 75+ with low B12 but no symptoms, a year of 1,000 mcg/day raised levels but did not improve nerve or cognitive function [4].

Safety at a glance

From our Vitamin B12 (methylcobalamin vs cyanocobalamin) monograph. Published interactions and cautions only, each with its source.

Interactions

  • Metformin. May 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 drugs. Can 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]

Full Vitamin B12 (methylcobalamin vs cyanocobalamin) safety section

The evidence, graded

Pulled from our monograph evidence tables. Grades follow our evidence-grading policy.

  • Grade CImprovedB12 status marker (methylmalonic acid) by oral doseVitamin B12 (methylcobalamin vs cyanocobalamin) · RCT · n=120 · 2005 · PMID 15911731Methylmalonic 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.
  • Grade CNo effectRaising low blood B12: high-dose oral vs injectionVitamin B12 (methylcobalamin vs cyanocobalamin) · Systematic review · n=153 · 2018 · PMID 29543316At 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.
  • No effectNo effectNerve and cognitive function in older adults with low B12Vitamin B12 (methylcobalamin vs cyanocobalamin) · RCT · n=191 · 2015 · PMID 26135351Serum B12 rose 177% and homocysteine fell 17%, but there was no effect on nerve conduction, other neurologic outcomes or cognitive function.

Why B12 gets harder to absorb with age

B12 in food is bound to protein, and stomach acid plus a protein called intrinsic factor are needed to free and absorb it. Atrophic gastritis, which reduces both, affects about 2% of the general population but 8% to 9% of adults aged 65 and older [1]. Depending on the definition, 3% to 43% of community-dwelling older adults have low B12 [1].

NIH also lists people with pernicious anemia, digestive conditions or past GI surgery, and vegans and vegetarians among the groups more likely to run low [1].

Metformin and acid reducers

Metformin is the most widely used oral diabetes drug, and a 2016 meta-analysis of 4 trials found it lowered serum B12 by about 57 pmol/L after 6 weeks to 3 months of use [2]. The authors concluded it is prudent to monitor B12 in people taking metformin [2].

Proton pump inhibitors and H2 blockers reduce stomach acid, which can interfere with absorbing B12 from food [1]. If you take either long term, NIH suggests discussing your B12 status with a clinician [1].

Why the pills are 1,000 mcg when the RDA is 2.4

Absorption falls steeply with dose: about 50% of a 1 to 2 mcg dose, but only about 2% at 500 mcg and 1.3% at 1,000 mcg [1]. Passive absorption at high doses works even without intrinsic factor, which is why large oral doses can help people who absorb poorly.

A 2005 dose-finding trial in 120 older people with mild deficiency tested 2.5 to 1,000 mcg/day for 16 weeks [3]. Methylmalonic acid, a sensitive marker of B12 status, fell 16% at the lowest doses and 33% at 500 and 1,000 mcg; doses of 647 to 1,032 mcg/day produced 80% to 90% of the maximum estimated reduction [3].

A Cochrane review of 3 small trials found 1,000 to 2,000 mcg/day by mouth raised low B12 about as well as injections over 3 to 4 months, at lower cost in one trial [5]. The evidence was low quality, and some conditions still call for injections, which is a decision for your clinician [5]. Our B12 injections guide explains how prescription options work.

What fixing a low number did not do

In the OPEN trial, 191 adults aged 75 and older with moderate B12 deficiency but no symptoms took 1,000 mcg/day or placebo for 12 months [4]. Serum B12 rose 177% and homocysteine fell 17%, yet nerve conduction, other neurologic measures and cognitive function did not improve [4].

And for energy, NIH concludes B12 supplements do not appear to help people whose B12 status is already adequate [1]. Test first, then decide with your clinician. For form choice, see methylcobalamin vs cyanocobalamin.

Quick answers

Should everyone on metformin take B12?

The metformin meta-analysis authors recommended monitoring B12 levels [2]; whether to supplement depends on your results and your clinician.

Can you take too much B12?

No upper limit has been set because toxicity is low [1]. NIH notes some observational studies link high B12 with cancer but calls the evidence mixed [1].

Keep reading

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References

  1. [1]Vitamin B12: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2025.
  2. [2]Association between metformin and vitamin B12 deficiency in patients with type 2 diabetes: A systematic review and meta-analysis. Diabetes Metab, 2016. PMID 27130885
  3. [3]Oral cyanocobalamin supplementation in older people with vitamin B12 deficiency: a dose-finding trial. Arch Intern Med, 2005. PMID 15911731
  4. [4]Effects of vitamin B-12 supplementation on neurologic and cognitive function in older people: a randomized controlled trial. Am J Clin Nutr, 2015. PMID 26135351
  5. [5]Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database Syst Rev, 2018. PMID 29543316

Vitamin B12 (methylcobalamin vs cyanocobalamin) and other dietary supplements are not intended to diagnose, treat, cure or prevent any disease, and these statements have not been evaluated by the FDA. Talk to your clinician or pharmacist before starting a supplement, especially if you take prescription medicines. This page has no product or affiliate links.

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