A woman in her sixties came to the counter with her monthly metformin refill and a question she had been turning over for months. "My feet tingle at night, and I am so tired I can barely get through the afternoon. My doctor says it is my sugar. Could the metformin be doing this?" This scene is a composite of several conversations, and I hear some version of it regularly.

It could be part of the picture. Long-term metformin can lower vitamin B12 levels, and low B12 can cause tingling, tiredness and other symptoms. But diabetes can also affect the nerves, and several other conditions cause the same complaints. So the right step is not to guess. A simple blood test usually settles the question, and low B12 is very treatable once it is found.

What the regulators say about metformin and B12

The UK Medicines and Healthcare products Regulatory Agency (MHRA) published advice on this in June 2022. It describes vitamin B12 deficiency as a common side effect of metformin that may affect up to 1 in 10 people who take it. It says the risk increases with a higher metformin dose, with longer treatment, and in people with certain risk factors, and that the medicine should be continued for as long as it is tolerated and not contraindicated.

The US metformin prescribing information takes a similar line. It reports that in 29-week trials, about 7% of people whose B12 level started out normal developed a below-normal level. It advises checking blood counts once a year and vitamin B12 at intervals of two to three years. The two sources differ in detail, so the right testing schedule for you is one to agree with your prescriber.

Who is more likely to be affected

According to the MHRA, these factors raise the risk:

a vitamin B12 level at the lower end of normal before you start
conditions that reduce absorption, such as older age, stomach surgery, Crohn's disease and other inflammatory bowel conditions, or autoimmune conditions
a strict vegan diet, or some vegetarian diets
medicines that reduce absorption, including proton pump inhibitors and colchicine
a rare inherited condition that affects B12 absorption or transport

The NHS also lists anticonvulsant medicines and proton pump inhibitors among those that can affect how much B12 your body absorbs. It notes that B12 and folate deficiency are more common in older people, affecting around 1 in 10 people aged 75 or over.

Symptoms to notice

The NHS lists these symptoms of vitamin B12 deficiency: extreme tiredness, a lack of energy, pins and needles, a sore and red tongue, mouth ulcers, muscle weakness, problems with your vision, and problems with memory, understanding and judgement. Psychological changes can also occur, from low mood or worry to confusion. The MHRA advises people on metformin to get medical advice for new or worsening extreme tiredness, a sore and red tongue, pins and needles, or pale or yellow skin.

Be honest with yourself about the overlap. Diabetes can cause numbness and tingling in the feet, and tiredness has many causes. The point is not to blame the metformin. It is to check whether a fixable cause is hiding among the possibilities.

Metformin's own label and the NHS medicine page both list vitamin B12 deficiency as a possible effect, which is one more reason to ask the question rather than wait.

While you wait for results, look after your feet. Check them daily for cuts, blisters or redness, wear shoes that fit, and tell your team about any sore that is slow to heal. Numbness can hide an injury, so this simple habit matters whatever the cause turns out to be.

How it is checked

The NHS says B12 deficiency can often be diagnosed from symptoms and a blood test. The MHRA advises testing whenever deficiency is suspected, for example in someone with megaloblastic anemia, a type of anemia linked to low B12, or a nerve problem that has appeared for the first time. It also suggests considering regular monitoring for people with risk factors.

Keep a copy of every B12 and blood count result you receive. Numbers are easier to interpret over time than in isolation, and a falling trend can be spotted early. Ask your prescriber what schedule suits you. Bring a written list of your symptoms, including when the tingling happens and what makes it better or worse. That detail often helps the test results make sense.

What the B12 test involves

The B12 test is a standard blood test. It is often taken alongside a full blood count, because the metformin label asks for yearly blood counts and the MHRA lists megaloblastic anemia, a type of anemia linked to low B12, as a reason to test. A number on its own can be borderline, so your prescriber will read it alongside your symptoms and history. If the result is borderline, or your symptoms are clear, further tests may be suggested before any treatment decision. Ask the lab or your prescriber for any specific instructions before the sample is taken.

If the result is low

If your level is low, your prescriber will decide how to treat it. The MHRA advice is to give corrective treatment in line with current clinical guidelines, and to continue metformin as long as it is tolerated. The NHS says low B12 can be treated with injections or tablets, depending on the cause. Please do not stop metformin on your own, and do not start high-dose B12 tablets before you know your level. Once you have the result, your prescriber can choose the form, the dose and the follow-up.

Two more metformin safety points

Alcohol and kidney function both matter with this medicine. The metformin label says alcohol potentiates the effect of metformin on lactate metabolism, and advises patients to avoid excessive alcohol. The NHS medicine page says to limit how much alcohol you drink while taking metformin. The label also advises a kidney function (eGFR) check at least once a year, and more often in people at higher risk of kidney problems, such as older adults, because kidney function affects how safely the medicine can be used.

If you are unwell with vomiting, diarrhea or fever, the NHS warns there is a risk of becoming dehydrated. Call your prescriber or pharmacist for advice on whether to pause doses, or follow the sick day plan they have already given you. Do not pause them on your own.

Questions to ask your doctor or pharmacist

Could my tingling, tiredness or memory changes be from low vitamin B12?
Should I have a vitamin B12 blood test, and how often should I repeat it on metformin?
Do any of my other medicines, such as stomach acid tablets, affect B12 absorption?
If my level is low, what treatment do you recommend, and how will we check it?
Is my kidney function up to date, and does my dose still suit it?
Should I limit alcohol with this medicine, and how much?

Frequently asked questions

Does everyone on metformin need a B12 test?

Not automatically, but many people benefit from a plan. The label suggests checks at intervals of two to three years, and the MHRA suggests regular monitoring for people with risk factors. Your prescriber can decide the schedule based on your risks and symptoms.

Will taking B12 tablets stop the tingling?

It may help if the tingling is caused by low B12, but the cause needs confirming first. Tingling can also come from nerve damage related to diabetes, which needs its own assessment.

Should I stop metformin if I am worried?

No. Stopping metformin without advice can affect your blood sugar. Bring your concerns to your prescriber, who can weigh the benefits against any risk and check your levels.

Can diet alone keep my B12 up?

For most people, a varied diet supplies enough B12, but strict vegan diets and some absorption problems can lower levels. If you follow a restrictive diet, talk to your prescriber or a dietitian about a safe plan.

If you'd like the full guide, my book The Type 2 Diabetes Food List and Meal Plan explains what to eat and gives a plain-English look at how common diabetes medicines work, so you can ask better questions at your next appointment.

Sources

UK Medicines and Healthcare products Regulatory Agency. Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk (Drug Safety Update, June 2022)
DailyMed. Metformin hydrochloride tablets (Zydus) prescribing information, revised 07/2026
NHS. Vitamin B12 or folate deficiency anaemia
NHS. Metformin