# Taking metformin? Put vitamin B12 on your review list

*Draft • Research checked 4 October 2026 • AI-assisted DomDNA editorial content. No independent clinical review or publication is claimed. General education, not individual medical advice.*

A medicine review is often crowded. There are repeat prescriptions to arrange, results to discuss and perhaps a symptom you nearly forgot to mention. Vitamin B12 deserves a place in that conversation for some people taking metformin.

This is a monitoring question, not a reason to stop a useful medicine. Keep taking prescribed treatment as directed while asking your clinician whether a B12 assessment is appropriate. An online article cannot decide the balance of treatment for you.

## What the safety advice says

In 2022, the UK Medicines and Healthcare products Regulatory Agency updated advice about reduced B12 levels with metformin. Risk increases with factors including higher doses and longer treatment. The regulator advises testing when deficiency is suspected, including in people with anaemia or new neuropathy, and considering periodic monitoring for people with risk factors. [MHRA safety update](https://www.gov.uk/drug-safety-update/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk).

The advice supports a specific conversation. It does not say that everyone taking metformin has a deficiency or needs the same supplement. The regulator also advises continuing metformin while a deficiency is corrected when the medicine remains tolerated and appropriate.

If you have noticed a new symptom, describe it directly. Do not wait until you can prove it is related to B12. Equally, do not use a possible explanation from an article to rule out other causes before an assessment.

## The trial behind part of the evidence

A randomised placebo-controlled trial followed 390 insulin-treated adults with type 2 diabetes for 4.3 years. B12 deficiency was more frequent in the metformin group. The absolute difference at the end was 7.2 percentage points, using the study's biochemical definition. [Original trial abstract](https://pubmed.ncbi.nlm.nih.gov/20488910/).

That is evidence about a measured outcome in a defined group. It is not a personal prediction for a younger person, a different dose or a different clinical situation. It also does not prove that every episode of tiredness in a metformin user is caused by B12 deficiency.

Keeping the population attached to the result makes the finding more useful. “This has been studied” and “this explains my symptom” are separate conclusions. The first can justify raising the question; the second needs clinical assessment.

## A supplement label cannot answer the cause

The NIH Office of Dietary Supplements describes several reasons for low B12, including dietary intake and problems with absorption. Assessment can involve a blood B12 measurement and, in some circumstances, additional markers. Laboratory methods and medical context affect interpretation. [NIH fact sheet](https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/).

That is why a shopping decision is a poor substitute for a review. A product's dose, format or “methylated” label does not establish whether it fits the reason for your result. Tell the clinician about supplements you already take, including the amount and when you started.

Keep the container or a photograph of its ingredients available. “A vitamin tablet” can mean several different things. Accurate information lets the conversation start with what is actually being taken, rather than a guessed product category.

## A review note you can finish today

Write down when you started metformin, your current prescribed dose and any recent changes. Add the symptoms you want to discuss, with rough dates. Include previous B12 results if you have them, but do not delay asking for help because an old report is hard to find.

You can then ask: “Given my treatment and history, should we check B12 now or include it in future monitoring?” A second useful question is who will follow up the result. A test without a clear follow-up plan can leave the same uncertainty sitting in your inbox.

There is no need to arrive with a preferred diagnosis or a supplement order. A concise medication history and an honest symptom description are enough to begin. The aim is a review that connects the medicine, the person and the result.

[Explore DomDNA’s free educational health quiz](https://domdna.com/quiz). It offers answer-based lifestyle education, not B12 testing, DNA analysis, prescribing or a medication-change plan.