Metformin can lower vitamin B12 levels in some people, particularly with higher doses, longer treatment and when other vitamin B12 deficiency risk factors are present. The MHRA now classifies reduced vitamin B12 as a common side effect of metformin and advises testing when deficiency is suspected. It also advises considering periodic monitoring in people who have additional risk factors. This does not mean everyone taking metformin needs the same blood-test schedule, and it does not mean you should stop metformin. The safest approach is to recognise the risk, watch for symptoms and discuss testing with the healthcare professional managing your diabetes or metformin treatment.
Why Can Metformin Affect Vitamin B12 Levels?
Vitamin B12 is needed for normal red blood cell formation and healthy nervous-system function. Metformin appears to reduce vitamin B12 status through effects on the gastrointestinal system and absorption. The MHRA notes that the risk rises with increasing metformin dose and longer treatment duration. NICE also lists metformin as a recognised risk factor when deciding whether vitamin B12 deficiency should be suspected and tested.
How Common Is Low Vitamin B12 with Metformin?
The MHRA updated metformin product information after evidence suggested that vitamin B12 deficiency is more common than previously recognised in people taking the medicine. The safety update states that decreased vitamin B12 levels are a common adverse effect and may affect up to 1 in 10 people taking metformin. That figure does not mean one in ten people will develop severe symptoms or require injections. Some people have a low level without obvious symptoms, while others can develop anaemia or neurological problems.
Who Is More Likely to Need Monitoring?
Long-term use matters, but duration is only one part of the risk assessment. The MHRA advises greater caution when the metformin dose is higher, treatment has continued for longer, or the person already has factors that make vitamin B12 deficiency more likely. Examples include vitamin B12 levels that were already at the lower end of normal, older age, stomach or bowel conditions that reduce absorption, previous gastrointestinal surgery, strict vegan or some vegetarian diets, and other medicines that can impair vitamin B12 absorption such as proton pump inhibitors or colchicine.
Does Everyone on Long-Term Metformin Need Regular B12 Tests?
There is no single NHS or MHRA rule saying that every person on metformin must have vitamin B12 checked at a fixed interval such as every six or twelve months. Instead, the MHRA advises clinicians to consider periodic vitamin B12 monitoring in people who have risk factors for deficiency. NICE reinforces metformin as a risk factor and recommends testing when a person has at least one common symptom or sign together with at least one common risk factor. Your GP or diabetes team can decide whether routine periodic testing is appropriate for you based on your dose, treatment duration, previous results, symptoms and other risks.
Symptoms Worth Discussing with a Healthcare Professional
Possible vitamin B12 deficiency symptoms include unexplained or worsening tiredness, weakness, a sore or red tongue, mouth ulcers, pale or yellow skin, pins and needles, numbness, muscle weakness, balance problems, vision changes and changes in memory, thinking or mood. These symptoms are not specific to B12 deficiency. Diabetes itself, diabetic neuropathy, iron or folate deficiency, thyroid problems and many other conditions can cause similar symptoms, which is why assessment and blood testing are more reliable than self-diagnosis.
Metformin, Pins and Needles and Diabetic Neuropathy
Pins and needles or numbness deserve particular attention in someone taking metformin because vitamin B12 deficiency can affect the nervous system, while diabetes can also cause peripheral neuropathy. The MHRA specifically advises testing vitamin B12 when deficiency is suspected, for example in someone presenting with new-onset neuropathy or megaloblastic anaemia. Do not assume new nerve symptoms are simply a normal part of diabetes, and do not treat them with supplements or injections without appropriate assessment.
Should You Stop Metformin If Your B12 Is Low?
Do not stop metformin because you are worried about vitamin B12. The MHRA advises that metformin can usually be continued while vitamin B12 deficiency is corrected, provided the medicine remains tolerated and is not otherwise contraindicated. Stopping or reducing metformin without medical advice could worsen diabetes control or remove another benefit for which it was prescribed. If vitamin B12 deficiency is confirmed, the clinician can treat the deficiency and separately review whether the metformin prescription still remains appropriate.
How Is Vitamin B12 Deficiency Tested?
If vitamin B12 deficiency is suspected, a clinician may arrange a total vitamin B12 or active B12 blood test and interpret it alongside your symptoms, medical history and other test results. NICE advises that diagnostic blood samples should usually be taken before vitamin B12 replacement is started where the clinical situation allows, because supplements or injections can increase measured B12 concentrations and make later results harder to interpret. Urgent treatment should not be delayed when a clinician suspects a serious neurological or haematological problem.
What If Your B12 Result Is Low or Borderline?
A low or borderline vitamin B12 result does not automatically tell you what treatment you need. NICE uses thresholds and, in some situations, additional tests such as methylmalonic acid or homocysteine to clarify uncertain results. The underlying cause also matters. In a person taking metformin, the medicine may be contributing, but diet, autoimmune gastritis, bowel disease, surgery or another medicine may also be relevant.
How Is Medicine-Induced Vitamin B12 Deficiency Treated?
When vitamin B12 deficiency is judged to be caused by a medicine, NICE recommends offering either oral or intramuscular vitamin B12 replacement, based on clinical judgement and the person’s preference. Treatment may need to continue while the causative medicine is still being taken. If the medicine is later stopped or changed and the person’s deficiency symptoms have resolved, the need for ongoing vitamin B12 replacement should be reviewed. This is one reason an injection should not be treated as an automatic answer to metformin use.
When a Private B12 Injection Assessment May Be Relevant
If you have recent blood results, a confirmed deficiency or an established treatment plan, Biscot Pharmacy’s private vitamin B12 injection service in Luton may be able to support treatment when a pharmacist confirms that the service is clinically appropriate. The service is not a substitute for diagnosing metformin-related vitamin B12 deficiency. If you have not been tested, your symptoms are new or neurological, or the cause of a low result is unclear, the pharmacist may recommend GP assessment or blood testing instead. A consultation does not guarantee that an injection will be given.
Can You Prevent Metformin-Related B12 Deficiency?
Do not rely on high-dose vitamin B12 supplements purely because you take metformin unless a healthcare professional has advised them or you have another reason for supplementation. A balanced diet containing reliable sources of vitamin B12 is important, but diet cannot always overcome medicine-related or absorption-related problems. If you follow a vegan or restricted diet, use suitable fortified foods or supplements as advised and make sure your clinician knows about that additional risk factor when deciding whether monitoring is appropriate.
When Should You Seek Medical Advice?
Arrange medical advice if you develop persistent or unexplained tiredness, a sore or red tongue, pale or yellow skin, pins and needles, numbness, weakness, balance problems or other possible deficiency symptoms while taking metformin. Prompt assessment is particularly important for new or progressive neurological symptoms, because vitamin B12 deficiency can cause nerve damage and some complications may not fully reverse if treatment is delayed. Use NHS 111 for urgent advice when the problem is not immediately life-threatening, and call 999 for a medical emergency.
Conclusion
Metformin and vitamin B12 monitoring is about identifying people whose risk is high enough to justify testing, not about giving every long-term user the same schedule. The MHRA advises checking vitamin B12 when deficiency is suspected and considering periodic monitoring in people with risk factors, with higher doses and longer treatment increasing the likelihood of low levels. Keep taking metformin unless the clinician responsible for your treatment advises otherwise, and seek assessment if possible deficiency symptoms develop.
Written by the Biscot Pharmacy Editorial Team.
Medically reviewed by Faisal Shafiq, Superintendent Pharmacist (GPhC 2067224).
Frequently Asked Questions
Does metformin cause vitamin B12 deficiency?
Metformin can commonly reduce vitamin B12 levels and can contribute to vitamin B12 deficiency. The risk is higher with larger doses, longer treatment duration and additional deficiency risk factors.
How often should vitamin B12 be checked if I take metformin?
There is no single fixed testing interval for everyone. The MHRA advises considering periodic monitoring in people with risk factors and testing vitamin B12 when deficiency is suspected. Your clinician should decide the interval based on your circumstances.
What symptoms should I watch for while taking metformin?
Possible vitamin B12 deficiency symptoms include extreme or unexplained tiredness, a sore or red tongue, pale or yellow skin, pins and needles, numbness, weakness, balance problems and cognitive changes. These symptoms can have other causes and need assessment.
Should I stop metformin if my vitamin B12 is low?
Do not stop metformin without medical advice. The MHRA says metformin can generally be continued while vitamin B12 deficiency is corrected, provided it remains tolerated and is not otherwise contraindicated.
Does low vitamin B12 mean I need an injection?
Not necessarily. NICE says medicine-induced vitamin B12 deficiency can be treated with either oral or intramuscular replacement depending on clinical judgement and patient preference. The cause, symptoms and test results matter.
Can metformin-related B12 deficiency cause pins and needles?
Yes, vitamin B12 deficiency can cause neurological symptoms such as pins and needles or numbness. Diabetes can also cause neuropathy, so new or worsening symptoms should be assessed rather than assumed to have one cause.
Can a pharmacy check whether a B12 injection is suitable?
A pharmacist can assess suitability for a private B12 injection service, but unexplained symptoms or suspected metformin-related deficiency may need blood testing or GP assessment first. A pharmacy consultation does not guarantee treatment.
Disclaimer
This article provides general information and is not a personal diabetes, metformin or vitamin B12 treatment plan. Do not stop, reduce or change metformin or start vitamin B12 injections based on this article. Testing and treatment depend on your symptoms, dose, treatment duration, other risk factors, blood results and the clinician responsible for your care.
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