Metformin and vitamin B12: what it does, and what to do

If you take metformin, you may have read that it drains your vitamin B12. That is partly true. Metformin is linked to lower B12 levels, and the UK medicines regulator now lists this as a common side effect that may affect up to 1 in 10 people. What it is not is a reason to stop taking it.

Key takeaways

• Metformin is linked to lower vitamin B12. It may affect up to 1 in 10 people taking it
• Higher doses carry more risk. Whether longer use adds to that is less certain than it is often stated
• Do not stop or change your metformin because of a B12 result. The UK regulator advises continuing it while the B12 is corrected
• Low B12 and diabetic nerve problems feel similar, which is why it gets missed
• B12 comes from animal foods. Daal, sabzi and roti do not supply it

Does metformin lower your vitamin B12?

Yes, on average it does. Pooling four trials, people on metformin had B12 levels about 57 pmol/L lower than those not taking it, after six weeks to three months of use. The UK regulator lists low B12 as a common side effect, possibly affecting up to 1 in 10 people.

Dose matters. Reviews consistently find that higher daily doses go with lower B12 levels. Duration is less settled than you might expect. The regulator lists longer treatment as a risk factor., but several studies, including one in Pakistan, found no clear link with how many years someone had been taking it.

There is one more honest complication. A lower blood B12 reading does not always mean your body is short of B12. Some research following people for over four years found signs of real deficiency building up. Other work found the blood number falling while the active form of B12 stayed normal, and concluded the standard blood test is a poor marker here. These studies disagree, and nothing settles it yet. Most pages you will read state this more confidently than the evidence allows.

Why it happens, in plain terms

The leading explanation is that metformin interferes with how the end of your small intestine absorbs B12. That step needs calcium, and metformin appears to disturb it.

Treat that as the current best explanation rather than a settled fact. Reviews describe several proposed mechanisms and do not agree on one. What matters practically is that this is about absorption, not about eating too little.

How common it is, and who is most at risk

In one study of 260 Pakistani adults with type 2 diabetes taking metformin for over a year, about 36% had a low B12 level. That is a single hospital in one city, so read it as an indication rather than a national figure.

It also needs context that is often left out. Low B12 is already common in Pakistan without any metformin involved. The national nutrition survey found around 20% of women aged 15 to 49 were deficient. That is a different group from the one above, so the two numbers cannot be compared directly. No study has measured Pakistani adults with diabetes who are not taking metformin, so nobody can currently say how much of the local burden is the medicine and how much is the diet.

The regulator names the factors that raise your risk: a higher daily dose, other medicines that reduce stomach acid, and eating little or no food from animals. People who eat mostly plant foods do have lower B12 levels.

Signs that are worth mentioning to your doctor

Tiredness, tingling or numbness in the hands and feet, and pins and needles can all point to low B12.

Here is the difficulty. Those are also the symptoms of diabetic nerve damage, and the two are easily confused. A doctor cannot tell them apart by listening to you. They need a blood test.

So treat this section as a reason to ask for a test, not as a checklist to diagnose yourself. If any of this is new or getting worse, say so at your next appointment.

Getting tested, and what happens if your level is low

The regulator advises testing when deficiency is suspected, for example if someone has anaemia or new nerve symptoms, and considering periodic checks for people with risk factors. It does not set a fixed interval, so how often you are checked is a judgement your doctor makes. The usual test is a serum B12 level.

If your level is low, the important part is what does not happen. The regulator is direct: correct the B12 and “continue metformin therapy for as long as it is tolerated”. Its advice to patients is plainer still: “you can keep taking metformin while vitamin B12 levels are being corrected” and “do not stop your treatment without first discussing this with your doctor”.

Tablets can raise B12 levels, and in people with deficiency generally they work about as well as injections. That research was not done in people whose deficiency came from metformin, so it is suggestive rather than settled. Which form you need, and how much, is your prescriber’s decision and not something to work out from an article.

One more honest point. If nerve symptoms have already started, it is not clear that correcting B12 reverses them. A careful review of treatments for diabetic nerve pain looked at the B12 studies and did not include it among the treatments it supports. That is not a reason to ignore a low level. It is a reason to act on it early rather than late.

Vitamin B12 in food you already eat

B12 comes from animal foods. In a typical Pakistani kitchen that means kaleji (liver), beef and mutton, chicken, fish such as rahu, anda, dahi, milk and cheese. Liver is the richest of these by a wide margin.

Plant foods do not supply usable B12. Daal, chana, sabzi, roti and rice contain none worth counting. This is the one nutrient where a mostly vegetarian diet leaves a real gap, and it adds to the metformin effect.

How much do you need? Guideline bodies do not fully agree, so the source matters. FAO and WHO set 2.4 micrograms a day for adults., while EFSA, the European body, sets 4 micrograms. Both are small amounts, and both are met by eating animal foods regularly.

We cannot give you B12 figures for specific Pakistani dishes. The Indian food composition tables, the most recent regional source, contain no B12 data at all, and the Pakistani tables are not machine readable. Anyone publishing exact B12 numbers for roti or kaleji is using data from somewhere else, usually America.

What to do next

If you take metformin, mention B12 at your next appointment, especially if you feel unusually tired or have tingling in your hands or feet. Ask whether a test is worth doing. Keep eating animal foods regularly if you eat them at all.

Do not change or stop your metformin because of anything you have read here. That decision belongs to you and your doctor together.

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