Metformin
medicationA cheap, decades-old diabetes pill that some people ask about for weight and healthy aging.
12 providers
No. 066supplement
Vitamin B12
Vitamin B12 is a supplement that is sold over the counter.
Also known as Cobalamin, Cyanocobalamin, Methylcobalamin, B12 shots
Vitamin B12 is needed to make red blood cells and keep nerves healthy. Deficiency can cause anemia, fatigue, and nerve changes such as numbness or tingling. Older adults, vegans, people with pernicious anemia or past stomach surgery, and people taking metformin or acid reflux drugs are at higher risk.
Is Vitamin B12 FDA-approved?
Sold without a prescription. Supplements are not reviewed by the FDA before sale, so third-party testing matters. Oral B12 is sold as a dietary supplement. B12 injections and the nasal form are prescription products, often used for pernicious anemia.
As of October 7, 2026.
Key takeaways
- Vitamin B12 is essential for red blood cells and nerves. Deficiency causes anemia, fatigue and nerve damage that can appear years later.
- Older adults, vegans and vegetarians, people with pernicious anemia or stomach surgery, and people on metformin or acid reflux drugs are at higher risk.
- NIH has set no upper limit for B12, and no supplement form has been shown to work better than another.
- A Cochrane review found oral B12 raised blood levels about as well as injections.
How does Vitamin B12 work?
B12 is needed to make DNA and red blood cells and to maintain the protective coating on nerves. Without it, red cells grow large and fragile (megaloblastic anemia) and nerves can be damaged.
Absorbing B12 from food depends on stomach acid and a protein called intrinsic factor. That is why low stomach acid, pernicious anemia, stomach surgery and some medicines make deficiency more likely, and why some people need injections.
Should people on metformin take vitamin B12?
The ADA Standards of Care 2026 say metformin use is associated with a higher risk of B12 deficiency and worse neuropathy symptoms, suggesting periodic testing of B12 levels, and advise monitoring and repleting as appropriate. ADA Standards of Care in Diabetes 2026, Section 9: Pharmacologic approaches to glycemic treatment
NIH notes that metformin and acid-suppressing drugs may lower B12 levels. NIH ODS: Vitamin B12 fact sheet for health professionals
What does the evidence say about Vitamin B12?
Strong human evidenceSupplementation reliably corrects deficiency, and a Cochrane review found oral and injected B12 raised blood levels similarly.
What do clinicians say about Vitamin B12?
Clinicians see B12 as a simple, safe fix for a real deficiency, best guided by a blood test in people with risk factors or symptoms.
NIH lists older adults, people with pernicious anemia or GI disorders, people who have had GI surgery, vegetarians, and infants of mothers on vegan diets as groups at risk of deficiency.
NIH ODS: Vitamin B12 fact sheet for health professionalsNIH has set no tolerable upper intake level for B12 because it is generally considered safe even at high intakes.
NIH ODS: Vitamin B12 fact sheet for health professionalsA Cochrane review of 3 trials found low-quality evidence that oral and intramuscular B12 normalize blood levels similarly, with oral costing less.
Wang et al.: Oral vitamin B12 vs intramuscular vitamin B12 for deficiency (Cochrane 2018)MedlinePlus says B12 tests are often ordered with folate, and that methylmalonic acid and homocysteine tests can help find the cause of a deficiency.
MedlinePlus: Vitamin B test
Summaries of published guidelines, drug labels, regulators and reviews, each attributed to its source. DiscoverMedica does not employ clinicians.
Is Vitamin B12 right for you, and who should avoid it?
Often considered for
- Adults with a confirmed low B12 level or deficiency symptoms
- Older adults, vegans and vegetarians, and people after weight-loss or stomach surgery
- People taking metformin or long-term acid reflux medicine, guided by testing
Not a fit, or needs extra care
- Anyone with numbness or tingling should get tested rather than self-treat, so the cause is found
- People with pernicious anemia usually need clinician-directed treatment, often injections
Only a licensed clinician who knows your history can decide whether it is right for you.
What are the risks and side effects of Vitamin B12?
- NIH has set no upper limit for B12 because it is considered safe even at high intakes.
- Metformin, proton pump inhibitors and H2 blockers can lower B12 levels over time.
- Nerve damage from deficiency can occur even without anemia, so numbness or tingling deserves a blood test.
- NIH says no supplement form, including methylcobalamin, has been shown to work better than the others.
What should you ask a clinician about Vitamin B12?
- Should my B12 be checked, given my diet, age or medicines?
- Do I need injections, or would oral B12 work for me?
These save with the compound when you add it to My Journal, so they’re on the printout for your visit.
How do people buy Vitamin B12?
No prescription is needed. Supplement brands and retailers sell it online and in stores.
Check with a clinician or pharmacist first if you take prescription medication or have a chronic condition.
Choose a product with independent testing, such as NSF Certified for Sport or USP Verified, because FDA does not check supplements before they are sold.
Labs you may be asked for
- Vitamin B12 level
- Complete blood count
- Folate
- Methylmalonic acid or homocysteine when results are borderline
Check your levels first
Some services let you order common lab tests yourself, so the results are ready for your visit. Your clinician decides which labs, if any, you need, and what the results mean.
- Function HealthAnnual membership for broad lab panels, drawn at Quest sites or at home.Visit
- Quest HealthOrder individual lab tests from Quest Diagnostics online, without a doctor’s visit.Visit
From each service’s own site, checked September 23, 2026.
Where some people buy it
- ThorneSells a vitamin B12 supplement direct to consumers.
- iHerbCarries vitamin B12 from many brands, in several forms.
- Quest HealthNot a seller of supplements. Sells a vitamin B12 blood test you can order without a doctor visit.
From each provider’s own site, checked October 7, 2026. Programs change, so confirm the details before you sign up.
Where can I get Vitamin B12?
16 providers list Vitamin B12 on DiscoverMedica, lowest listed price first.
$7.50listed price†
Buy at Life Extension $19.95listed price†
Buy at Nordic Naturals $20.50listed price†
Buy at Pure Encapsulations † Prices as listed by the provider when we checked; they change.
Vitamin B12: questions, answered
What are the side effects of vitamin B12?
Oral B12 is very well tolerated. NIH has set no upper intake limit because it is considered safe even at high intakes. The bigger risk is the opposite: an untreated deficiency can cause anemia and lasting nerve damage. If you take metformin or acid reflux drugs long term, ask whether your B12 should be checked.
What are the symptoms of low vitamin B12?
NIH lists fatigue, weakness, megaloblastic anemia, a sore tongue, palpitations and neurological changes such as numbness or tingling in the hands and feet. Nerve damage can happen even without anemia, and symptoms can take years to appear. A blood test, often with folate and sometimes methylmalonic acid, confirms the diagnosis.
Is methylcobalamin better than cyanocobalamin?
Not according to current evidence. NIH says supplements usually contain cyanocobalamin, with methylcobalamin, adenosylcobalamin and hydroxycobalamin also sold, and that research has not shown any form of supplemental B12 to be better than the others. Choose a product with independent quality testing rather than paying more for a specific form.
Are B12 shots better than pills?
For most people with deficiency, pills appear to work. A 2018 Cochrane review found oral and injected B12 raised blood levels similarly, though the evidence quality was low. Injections and nasal B12 need a prescription and are commonly used for pernicious anemia or severe deficiency, where absorption from the gut is impaired.
Does metformin lower vitamin B12?
It can. The ADA Standards of Care 2026 say metformin use is associated with a higher risk of B12 deficiency and worse nerve symptoms, and suggest periodic testing of B12 levels. NIH also lists metformin and acid-reducing drugs among medicines that may lower B12. Ask your clinician whether testing makes sense for you.
Important safety information
Key risks
- NIH has set no upper limit for B12 because it is considered safe even at high intakes.
- Metformin, proton pump inhibitors and H2 blockers can lower B12 levels over time.
- Nerve damage from deficiency can occur even without anemia, so numbness or tingling deserves a blood test.
- NIH says no supplement form, including methylcobalamin, has been shown to work better than the others.
Talk to a clinician first, or avoid it, if
- Anyone with numbness or tingling should get tested rather than self-treat, so the cause is found
- People with pernicious anemia usually need clinician-directed treatment, often injections
This is not a complete list of risks. Talk to a licensed clinician or pharmacist about your own situation. If you have a severe reaction, call 911. You can report side effects to the FDA through MedWatch at 1-800-FDA-1088.
Sources
- NIH ODS: Vitamin B12 fact sheet for health professionals
- NIH ODS: Vitamin B12 fact sheet for consumers
- Wang et al.: Oral vitamin B12 vs intramuscular vitamin B12 for deficiency (Cochrane 2018)
- ADA Standards of Care in Diabetes 2026, Section 9: Pharmacologic approaches to glycemic treatment
Also checked for this guide
Guide reviewed October 7, 2026.
Educational information only, not medical advice. It does not replace a conversation with a licensed clinician.
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