Metformin and Low B12: Why Does It Happen and What Should Be Checked?

Yes. Metformin can lower vitamin B12 levels, and the UK medicines regulator (MHRA) now describes low B12 as a common side effect, affecting up to 1 in 10 people who take it. The risk rises with higher doses and longer treatment, so people on metformin for years are the group most often discussed.

This guide is for adults who take metformin and have a low or borderline B12 result, or who wonder whether their symptoms could be linked. It explains the link and the usual monitoring. It cannot tell you whether your own symptoms are caused by low B12.

The NHS says B12 deficiency should be diagnosed and treated as soon as possible, because some nerve problems can become permanent, and the risk grows the longer it goes untreated. Contact your doctor promptly rather than waiting for a routine review if you notice:

• New or spreading pins and needles or numbness, especially in the hands or feet.
• Problems with balance, coordination or walking, or new muscle weakness.
• Changes in vision, memory, understanding or mood, including confusion.
• New problems controlling the bladder or bowels.
• Breathlessness, a racing or pounding heartbeat, or feeling faint, which can happen with anaemia.

The MHRA says the exact mechanism is likely to involve several factors. These include changes in how the gut moves, bacterial overgrowth in the bowel, and reduced uptake of B12 in the small intestine. The effect builds slowly, which is why it is linked with long-term use rather than the first weeks of treatment.

The MHRA lists other factors that add to the risk: a starting B12 level at the low end of normal, older age, stomach or bowel conditions such as gastrectomy or Crohn's disease, autoimmune conditions, strict vegan or some vegetarian diets, and some rare inherited conditions. It also names medicines that can reduce B12 absorption, including proton pump inhibitors and colchicine.

The MHRA lists the main signs as extreme tiredness, a sore and red tongue, pins and needles, and pale or yellow skin. Other possible effects include low mood, irritability, memory or thinking problems, mouth ulcers and changes in vision or movement.

Nerve symptoms are the ones people with diabetes often ask about. The American Diabetes Association notes that metformin is associated with a higher risk of B12 deficiency and with worsening of neuropathy symptoms. Diabetes can also damage nerves on its own, so tingling feet do not prove that B12 is the cause. The MHRA advises a blood test because these symptoms can come from diabetes or other conditions.

It helps to write down which symptoms you have, when they began and whether they are getting worse. Note where any numbness or tingling is felt and whether it affects one side or both. This gives the clinician a clearer picture than a general description at the appointment.

Guidance differs slightly by country, but the direction is similar. The MHRA advises testing B12 when deficiency is suspected, for example with new nerve symptoms or a type of anaemia with large red cells, and considering periodic B12 checks in people with risk factors. The American Diabetes Association's Standards of Care suggest periodic testing of B12 levels in people taking metformin.

Neither source sets one fixed interval for everyone. How often your B12 is checked depends on your risk factors, symptoms and previous results, so ask your own care team what plan applies to you and when your next check is due.

If you have already had a B12 test, keep a copy of the result with its date and reference range. A series of results over time shows the trend, which is more helpful at a review than a single remembered number.

A low B12 result in someone on metformin makes the medicine a likely contributor, but clinicians still consider other causes. The NHS lists pernicious anaemia (an autoimmune condition that blocks B12 absorption), diet, stomach surgery, bowel conditions such as Crohn's disease and other medicines, including proton pump inhibitors and nitrous oxide. More than one cause can be present at once.

B12 deficiency does not always show up on a full blood count. If your blood count was normal, see our guide on B12 deficiency without anaemia. If your report shows large red blood cells, the guide on a high MCV blood test explains what that marker means. Reference ranges for B12 differ between laboratories, so compare your result with the range printed on your own report.

Do not stop or change metformin, or any other prescribed medicine, without talking to the prescriber. The NHS warns that blood sugar may rise again if metformin is stopped, and MedlinePlus also advises not to stop it without talking to your doctor. The MHRA advises that low B12 is treated according to clinical guidelines and that metformin can continue while levels are corrected, as long as it is tolerated and suitable.

Treatment for low B12 is decided by your clinician, based on the cause and how low the level is. This guide does not recommend any supplement, product or dose.

Bring your B12 result with its unit, reference range and date, any earlier results, and a list of all medicines and supplements you take.

• Is my B12 low enough to explain my symptoms, or could diabetes or something else be involved?
• Do I have other risk factors, such as diet, stomach or bowel problems, or another medicine that reduces B12?
• Do I need any further blood tests to confirm the result or look for another cause?
• How often should my B12 be checked while I take metformin?
• Which new symptoms should make me contact you before my next review?

On SameCase, patients and verified doctors share de-identified cases. Reading how others on long-term metformin described their symptoms, results and follow-up can help you notice details worth mentioning and shape your own questions before an appointment. Other people's cases cannot diagnose you or predict your result, and we do not offer treatment advice. Your own clinician remains the person to interpret your tests.