Blood Tests While Taking Metformin: What to Monitor and Why
Metformin is the most widely prescribed glucose-lowering medicine in the UK and has a long safety record. Even so, it is not a ‘take and forget’ tablet. A handful of routine blood tests tell your GP whether the drug is doing its job, whether it is still safe for your kidneys and whether it is quietly draining your vitamin B12 stores. Our editorial team explains each test in plain terms.
Why monitoring matters with metformin
Metformin lowers blood glucose mainly by reducing glucose output from the liver and improving the body’s response to insulin. It does not usually cause hypoglycaemia on its own, which is one reason it is the first-line choice in NICE guidance for type 2 diabetes. Its safety, however, depends heavily on how well the kidneys clear it from the body.
The drug is excreted unchanged in the urine. If kidney function falls, metformin can accumulate, and in rare circumstances this contributes to lactic acidosis, a serious metabolic emergency. Regular kidney tests are therefore the backbone of safe prescribing rather than an optional extra.
Monitoring also answers a simpler question: is the treatment working? Glucose control changes over time as diabetes progresses, weight shifts and lifestyle evolves. A tablet that was sufficient three years ago may no longer be enough, and only blood results reveal that objectively.
Finally, long-term metformin use is linked to lower vitamin B12 levels. Because B12 deficiency develops slowly and its symptoms overlap with diabetic nerve damage, it can be missed unless someone deliberately looks for it.
HbA1c: the measure of glucose control
Glycated haemoglobin, or HbA1c, reflects average blood glucose over roughly the previous two to three months. It is reported in the UK in mmol/mol. For many adults with type 2 diabetes NICE suggests an individual target, often around 48 mmol/mol when managed with lifestyle and a single drug such as metformin, though targets are personalised.
When treatment is started or changed, HbA1c is typically rechecked after about three to six months. Once results are stable, a check every six months is common practice. Testing sooner than about three months rarely helps, because red blood cells need time to turn over before the value reflects a new pattern.
HbA1c can mislead in certain situations. Anaemia, recent blood loss, haemoglobin variants, pregnancy and advanced kidney disease can all distort the reading. In those cases clinicians may lean more on finger-prick or continuous glucose data.
A falling HbA1c is reassuring, but an unexpectedly rapid drop combined with weight loss should prompt a conversation with your GP, since it may reflect reduced food intake, illness or other changes worth reviewing.
Kidney function: eGFR and creatinine
Kidney function is estimated from serum creatinine, adjusted for age and sex, to give the estimated glomerular filtration rate (eGFR) in mL/min/1.73 m². The British National Formulary and MHRA guidance link metformin use directly to this number, so it is checked before treatment begins and at least once a year afterwards.
In practice, people with normal kidney function are usually tested annually, while those with reduced eGFR or additional risk factors may be checked two or more times a year. As eGFR falls, the dose is reviewed and often reduced; below 30 mL/min/1.73 m² metformin is generally not used.
| Test | What it shows | Typical frequency in stable patients |
|---|---|---|
| HbA1c | Average glucose over 2–3 months | Every 3–6 months after changes, then about 6-monthly |
| eGFR / creatinine | Kidney clearance of metformin | Before starting, then at least yearly; more often if reduced |
| Vitamin B12 | Possible deficiency with long-term use | If symptoms or risk factors; many practices check periodically |
| Full blood count | Anaemia, including B12-related | Often with annual review |
| Liver function tests | Liver health, fatty liver | Annually or as clinically indicated |
Temporary situations matter too. Dehydration from vomiting or diarrhoea, iodinated contrast for scans and major surgery can all reduce kidney function suddenly. NHS ‘sick day rules’ often advise pausing metformin during such episodes and restarting once eating and drinking normally, but this should be agreed with your own clinician.
Because creatinine depends on muscle mass, very muscular people and those taking creatine supplements may see a mildly raised value without true kidney damage. It is worth telling your GP about supplements so the result is interpreted correctly.
Vitamin B12 and full blood count
Metformin interferes with the calcium-dependent absorption of B12 in the final part of the small intestine. In the Diabetes Prevention Program Outcomes Study, Aroda and colleagues found that long-term metformin use was associated with a higher rate of biochemical B12 deficiency. In 2022 the MHRA issued a drug safety update stating that B12 deficiency is a common side effect, particularly at higher doses and with longer treatment.
The MHRA advises testing B12 in patients with symptoms of deficiency and considering periodic checks in those with risk factors. Symptoms worth reporting include:
- tingling, numbness or burning in the hands and feet;
- unusual tiredness, breathlessness or pale skin;
- a sore, red tongue or mouth ulcers;
- memory problems, low mood or poor concentration;
- unsteadiness when walking.
A full blood count may show enlarged red blood cells (raised MCV) or anaemia, although B12 deficiency can damage nerves well before the blood count changes. That is why B12 itself, sometimes alongside active B12 or methylmalonic acid, is measured.
Vegetarians, vegans, older adults and people taking acid-suppressing medicines such as proton pump inhibitors face a higher baseline risk, so the combination with metformin deserves particular attention.
Liver tests, lipids and the annual review
Liver function tests are not specifically required because of metformin, which is not generally toxic to the liver. They are part of routine diabetes care because non-alcoholic fatty liver disease is common in type 2 diabetes. Severe liver impairment, however, is a reason to avoid metformin because of the lactic acidosis risk.
The annual diabetes review in England also includes cholesterol, blood pressure, urine albumin-to-creatinine ratio, foot checks and retinal screening. These are not metformin tests as such, but they reflect the overall cardiovascular and kidney risk that diabetes treatment aims to reduce.
Bring a list of all medicines and supplements to the review. Some products, including high-dose biotin, can interfere with laboratory assays, while others such as herbal ‘blood sugar’ supplements can add to the glucose-lowering effect.
If you are physically active, mention training load too. Heavy exercise in the 24–48 hours before a blood test can transiently raise creatinine and liver enzymes such as AST and ALT, which may otherwise trigger unnecessary concern.
Editorial conclusions
Metformin is safe and effective for most people, but its safety depends on regular checks of kidney function, and its effectiveness is judged by HbA1c.
Long-term users should be alert to symptoms of vitamin B12 deficiency and ask about testing, especially if they follow a plant-based diet or take acid-suppressing medicines.
Results should always be interpreted in context: hydration, recent exercise, muscle mass and supplements can all shift numbers without signalling disease.
If this topic is useful, see also our articles on the side effects and contraindications of metformin, on blood tests during N-acetylcysteine use and on how the history of statins shaped modern cardiovascular prevention.
References
- UK Prospective Diabetes Study (UKPDS) Group. Effect of intensive blood-glucose control with metformin on complications in overweight patients with type 2 diabetes (UKPDS 34). Lancet. 1998;352(9131):854–865.
- Knowler WC, Barrett-Connor E, Fowler SE, et al. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med. 2002;346(6):393–403.
- Aroda VR, Edelstein SL, Goldberg RB, et al. Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab. 2016;101(4):1754–1761.
- de Jager J, Kooy A, Lehert P, et al. Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency: randomised placebo controlled trial. BMJ. 2010;340:c2181.
- Inzucchi SE, Lipska KJ, Mayo H, Bailey CJ, McGuire DK. Metformin in patients with type 2 diabetes and kidney disease: a systematic review. JAMA. 2014;312(24):2668–2675.
- National Institute for Health and Care Excellence. Type 2 diabetes in adults: management (NG28). London: NICE; 2015 (updated).
- Medicines and Healthcare products Regulatory Agency. Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk. Drug Safety Update. 2022.
