# Metformin And Low B12: What To Check And What Not To Stop

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Last modified: 2026-10-22T09:42:05+00:00
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**Metformin can lower vitamin B12**, especially with longer use, higher doses or other risk factors. But fatigue or tingling doesn't prove that your B12 is low, and not everyone taking metformin needs a large B12 supplement. New symptoms deserve assessment and, when appropriate, blood testing. Don't stop metformin on your own. The [UK medicines regulator advises](https://www.gov.uk/drug-safety-update/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk) that B12 deficiency can be corrected while metformin continues when it is tolerated and otherwise appropriate.

**Contact your clinician promptly for new weakness, balance changes, or new or worsening numbness or tingling.** Don't wait to see whether a supplement makes these symptoms disappear. Sudden or severe weakness or difficulty walking needs urgent medical assessment. B12 deficiency is one possible explanation for nerve symptoms, but diabetes-related nerve damage and other causes also need consideration.

## Why Metformin Can Lower B12

Vitamin B12 helps keep nerves healthy and helps your body make red blood cells, which carry oxygen. Too little B12 can lead to anemia, a shortage of healthy red blood cells, as well as nerve problems. The [NIH vitamin B12 fact sheet](https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/) identifies metformin as a medicine that can reduce B12 absorption.

The likelihood of a low level isn't the same for everyone. Factors worth discussing include:

- A higher metformin dose or longer time taking it.
- A vegan diet, particularly without a reliable source of B12.
- Older age.
- Stomach or intestinal conditions that interfere with absorption.
- Medicines that reduce stomach acid.

These are risk factors, not guarantees that you will develop a deficiency. They help explain why two people taking metformin may need different testing and follow-up.

**What One Long-Term Trial Found**

A [2010 placebo-controlled trial](https://www.bmj.com/content/340/bmj.c2181) followed 390 adults with type 2 diabetes who were already using insulin. Participants received metformin at 850 mg three times daily or placebo for 4.3 years. Compared with placebo, metformin was associated with a 19% average reduction in B12 and a 7.2-percentage-point higher risk of B12 deficiency.

That metformin amount describes the study, not a dose recommendation. These findings don't predict the risk for every metformin user or show that a symptom appearing a few days after starting metformin is caused by B12 deficiency. The results concern B12 levels and deficiency risk, not proof that a supplement will relieve any particular person's fatigue or tingling.

## Which Symptoms Are Worth Checking?

Unusual tiredness, a sore tongue, pins and needles, or numbness are worth mentioning to your clinician. They can occur with low B12, but none is specific enough to diagnose it. Fatigue has many possible causes. Tingling in someone with diabetes may reflect diabetic neuropathy, meaning nerve damage related to diabetes, B12 deficiency, or another problem.

Don't assume that new foot symptoms are simply an expected part of diabetes. Explain when they began, whether they're spreading or worsening, and whether you've noticed weakness or a change in walking or balance. That information helps your clinician decide what examination and testing are needed.

**Hypothetical example:** Someone who has taken metformin for several years, eats no animal foods and develops tingling in both feet has several reasons to discuss B12 testing. Those details still don't establish the diagnosis. If testing doesn't explain the symptoms, the next step is further assessment, not automatically taking more B12.

## What A B12 Blood Test Can And Cannot Tell You

A blood B12 test is a useful starting point when symptoms or risk factors raise concern. The result needs to be considered alongside your symptoms, medical history and the laboratory's reference range. It doesn't identify every cause of tiredness or tingling, or explain by itself why B12 might be low.

If the result is borderline or unclear, your clinician may consider methylmalonic acid, usually shortened to MMA. This substance can rise when the body doesn't have enough usable B12. Kidney function also affects MMA, so an elevated result isn't automatically proof of B12 deficiency. The [NIH explains these testing limits](https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/).

How Two B12-Related Tests Compare

| Test | What it helps show | Main limitation |
| --- | --- | --- |
| Blood B12 | The amount of B12 circulating in the blood. | An uncertain result may need additional interpretation. |
| MMA | Whether an unclear B12 result may reflect a deficiency. | Reduced kidney function can also raise MMA. |

You don't need to choose a diagnostic cutoff yourself. Ask what your result means in your situation and whether it explains your symptoms well enough, or whether more evaluation is needed.

The [MHRA's June 2022 advice](https://www.gov.uk/drug-safety-update/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk) recommends testing when anemia or new nerve symptoms suggest deficiency, and considering periodic monitoring for people with risk factors. This is UK guidance, not a US testing requirement or a universal schedule. Your symptoms, risk factors and previous results should inform the testing conversation rather than a fixed interval recommended for everyone.

For a fuller explanation of testing, see [B12 blood tests explained](https://supplementexplained.com/labs/b12-testing-explained/) before trying to interpret a single result on its own.

Our [B12 marker guide](https://supplementexplained.com/quality/b12-lab-marker-decision-map/) also explains why an extra marker can help in some cases but still needs context.

## Do You Need A Supplement With Metformin?

Not automatically. Whether you need B12 depends on your intake, test results, symptoms and ability to absorb it. Metformin use alone isn't a reason to assume that a high-dose supplement is necessary. Diet matters, especially if you eat no animal foods, but dietary changes shouldn't replace assessment of new nerve symptoms or a confirmed deficiency.

### Routine Intake And Replacement Are Different

For most adults, the recommended dietary allowance is **2.4 mcg a day**. That is an amount intended to meet ordinary nutritional needs, not a dose for replacing B12 in someone with an established deficiency. A replacement plan may use a much larger amount for a specific reason.

Only a small fraction of a large oral amount is absorbed. That helps explain why some replacement products contain far more than 2.4 mcg, but it doesn't mean everyone needs a large amount. B12 has no established upper intake limit; that isn't a reason to take unlimited amounts or keep increasing them when symptoms persist. The [NIH fact sheet describes intake needs and absorption](https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/).

### Oral B12 And Injections

Whether oral B12 is suitable or injections are needed depends on the cause and severity of the deficiency. A lozenge isn't a substitute for an assessment when significant nerve symptoms are present. Ask why a particular route and amount fit your situation and how the response will be checked.

Cyanocobalamin and methylcobalamin are both forms used in supplements. The NIH reports no evidence that absorption rates differ by supplemental form. Methylcobalamin hasn't been proved superior simply because it is the methyl form. Likewise, a sublingual product, which is held under the tongue, isn't proved more effective than a swallowed oral product. Choose the form and amount that fit the agreed replacement plan.

## An Optional B12 Label To Discuss

**NOW Vitamin B-12 1000 mcg, 250 Lozenges** is one clearly labeled oral option to discuss if a clinician has recommended B12 replacement and that strength fits the plan. Its practical role is supplying a known amount of B12 in a chewable format. It isn't automatic prevention for everyone taking metformin, and it isn't a treatment for every cause of tingling.

The [manufacturer's label](https://www.nowfoods.com/products/supplements/vitamin-b-12-1000-mcg-lozenges) lists 1,000 mcg of B12 as cyanocobalamin per lozenge. It also supplies 100 mcg of folic acid, listed as 170 mcg dietary folate equivalents, or DFE. The matching 250-lozenge bottle is manufacturer SKU 0468, UPC 733739004680. These are product amounts, not a personal replacement recommendation.

The current manufacturer directions describe one lozenge daily with a meal, chewed or dissolved in the mouth and then swallowed. Those directions are not an individualized plan for a confirmed deficiency; use the amount and follow-up agreed with your clinician.

There are listing differences to be aware of. Although [HerbsPro](https://www.herbspro.com/products/b-12-3) places the product in a methylcobalamin category, its Supplement Facts identify cyanocobalamin, matching the manufacturer. The manufacturer's current inactive ingredients include xylitol and stevia, while the retailer shows an older list that includes fructose and sorbitol. The active ingredient, strength and pack size match, but the listings don't establish which inactive-ingredient version will ship. Check the bottle if particular sweeteners or other ingredients matter to you.

The added folic acid doesn't establish that this combination improves metformin-related symptoms more than B12 alone. Show the full label when reviewing other vitamins you take. The useful reasons to consider this option are its stated B12 amount and format, not a demonstrated advantage over other suitable B12 products. Don't change the timing of metformin or other prescribed medicines to fit a supplement.

![NOW Vitamin B-12 1000 mcg, 250 lozenges, actual product bottle](https://supplementexplained.com/wp-content/uploads/2026/10/se-twelve-guides-20261022/t23.webp)

Optional Product

### NOW Vitamin B-12 1000 mcg, 250 Lozenges

1,000 mcg of cyanocobalamin per lozenge; 250 lozenges. A known strength and chewable format to discuss if B12 replacement has been recommended.

Also contains 100 mcg folic acid. Manufacturer and retailer list different sweetener versions; check the bottle. Not a treatment for every cause of tingling.

[Check at HerbsPro](https://www.jdoqocy.com/click-101886474-15733504?url=https%3A%2F%2Fwww.herbspro.com%2Fproducts%2Fb-12-3)

Affiliate link: we may earn a commission if you buy. The retailer sets price and availability.

## What To Expect After A Deficiency Is Found

Replacement aims to correct the deficiency and help problems that are actually caused by low B12. It won't necessarily explain or relieve all fatigue, numbness or tingling. Someone can have low B12 and another cause of symptoms at the same time, which is why follow-up needs to consider both blood results and how the person is doing.

There isn't a reliable promise that symptoms will disappear within a few weeks. Nerve problems may take longer to improve, and symptom recovery isn't guaranteed. The amount of B12, route of replacement and follow-up should be chosen for the person's cause and severity of deficiency, rather than copied from a supplement label or another person's experience.

Before leaving the visit, make sure you understand when results and symptoms will be reviewed, whether replacement is expected to continue, and what changes should prompt earlier contact. Ongoing metformin use and other absorption risks may remain relevant even after a low level has been corrected.

If symptoms persist or worsen during replacement, contact your clinician rather than increasing the amount or adding a supplement stack. More B12 isn't a substitute for checking whether another problem is contributing.

## Questions To Bring To Your Next Visit

Bring your medicine and supplement list, including metformin amounts, acid-reducing medicines and any B12 you've already started. Note when symptoms began and how they have changed. If you're considering a product, bring the bottle or a clear photo of its Supplement Facts and ingredients.

- Do my symptoms warrant B12 testing now, and what other causes should we consider?
- If my B12 result is unclear, would MMA help, and how does my kidney function affect its interpretation?
- If replacement is needed, why is oral B12 or an injection the better fit?
- What amount should I use, for how long, and when will we review blood results and symptoms?
- Does this product's B12 and added folic acid fit with my other supplements?
- Which symptom changes mean I should contact you before the planned follow-up?

## Sources

- [UK Medicines and Healthcare products Regulatory Agency: Metformin and reduced vitamin B12 levels, June 20, 2022.](https://www.gov.uk/drug-safety-update/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk)
- [NIH Office of Dietary Supplements: Vitamin B12 Fact Sheet for Health Professionals.](https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/)
- [BMJ: Long-term metformin treatment and vitamin B12 deficiency, randomized placebo-controlled trial, 2010.](https://www.bmj.com/content/340/bmj.c2181)
- [NOW Foods: Vitamin B-12 1000 mcg Lozenges, manufacturer label and product information.](https://www.nowfoods.com/products/supplements/vitamin-b-12-1000-mcg-lozenges)
- [HerbsPro: NOW B-12 1000 mcg, 250 Lozenges, retailer label and ingredient listing.](https://www.herbspro.com/products/b-12-3)

General information, not individual medical advice. [How We Review](https://supplementexplained.com/how-we-review-evidence/).