Medications · Diabetes · Practical Health
If you've just started taking metformin, or you've been on it for a while and something's changed, it helps to know what's actually normal.
Metformin is the medicine most people with type 2 diabetes are prescribed first, and while most people tolerate it well over time, the first few weeks can be genuinely uncomfortable. This guide covers what's expected, what to watch for over the longer term, and the rare but serious signs that need urgent attention.
The common side effects — and why they usually settle
Feeling sick, being sick, diarrhoea, stomach ache, and a reduced appetite are the most common side effects, according to NHS guidance. If you're a few weeks into taking metformin and your stomach hasn't quite forgiven you yet, you're not doing anything wrong — this is the normal, well-recognised way most people's bodies react at first.
If the stomach symptoms are still bothering you after a few weeks, or diarrhoea keeps happening, it's worth mentioning to your GP or pharmacist rather than just putting up with it. A slow-release version of metformin exists precisely for people whose stomachs don't settle with the standard tablets, and switching can make a real difference.
Vitamin B12 deficiency — the long-term one worth knowing about
This one doesn't get talked about as much as the stomach symptoms, but it's worth knowing, especially if you've been on metformin for a while.
Taking metformin at higher doses or over a long period can lower your vitamin B12 levels. The signs are easy to mistake for something else entirely — feeling very tired, muscle weakness, a sore or red tongue, mouth ulcers, changes in vision, or skin that looks unusually pale or slightly yellow. None of these scream “vitamin deficiency” on their own, which is part of why it's worth actually mentioning to your GP rather than assuming it's just tiredness or getting older.
The good news is this is straightforward to check and straightforward to fix. Your doctor can test your B12 level with a simple blood test, and if it's come down, supplements usually sort it out. This is one of the things a diabetes annual review should pick up on — worth reading our guide on what to expect at that review if you haven't already.
Does metformin affect your kidneys?
This is one of the questions people ask most, and it's worth being precise about, because a lot of what's online gets it slightly wrong.
What actually happens is more of a safety precaution than a direct harm: metformin is cleared from your body by the kidneys, so if kidney function drops, the drug can build up rather than being cleared properly, and that raises the risk of a rare but serious complication (more on that below). Because of this, if your kidneys aren't working as well as they should, your doctor will simply tell you to stop taking metformin and switch you to a different medicine — not because it's damaged anything, but because it's no longer the safest fit.
This is exactly why kidney function is checked regularly for anyone on metformin, usually as part of routine diabetes monitoring. It's not a sign anything's gone wrong — it's the system working as it should.
Does metformin cause weight loss?
According to the NHS, metformin doesn't cause weight gain, unlike some other diabetes medicines — which is a genuinely useful thing about it, but it isn't the same as being a weight-loss treatment. Some people do lose a modest amount of weight while taking it, but it isn't prescribed for that purpose, and it isn't something to expect or rely on. If weight loss is something you're hoping for, that's worth a separate, honest conversation with your GP about what's actually appropriate for you.
Lactic acidosis — rare, but worth recognising
This is the one serious side effect worth being able to spot, even though it's genuinely uncommon. Lactic acidosis happens when lactic acid builds up in the blood, and it's considered a medical emergency.
It's rare enough that most people on metformin will never encounter it, and it almost always happens alongside another serious problem — significantly reduced kidney function, liver disease, heavy alcohol use, or a serious illness involving dehydration or a lack of oxygen in the body. On its own, in someone whose kidneys and liver are working normally, the risk is extremely low.
Symptoms to know
Unusual weakness or fatigue, fast or difficult breathing, muscle cramps, and — if it progresses — confusion or increasing difficulty coordinating.
If you notice these, especially in combination and especially if you've been unwell, unwell enough to be dehydrated, or have known kidney or liver problems, stop taking metformin and get medical help straight away — call 111 for advice, or 999 if it feels severe.
The “sick day rule” most people don't know about
Here's something worth knowing before you need it, because it isn't widely talked about and it's genuinely useful.
The rule
If you're unwell with vomiting, diarrhoea, or anything that puts you at risk of dehydration, the advice is to temporarily stop taking metformin. This is a precaution — dehydration combined with metformin raises the risk of the lactic acidosis mentioned above, so pausing the medicine while you're unwell is simply the safer path.
You can start taking it again about 48 hours after you're eating and drinking normally, with no lasting harm from the short gap.
If you're ever unsure whether this applies to you, it's always fine to check with a pharmacist or your GP practice — this is exactly the kind of question they're used to answering.
Drug and lifestyle interactions to know
A few things genuinely change how metformin behaves in your body, and it's worth knowing them upfront rather than finding out the hard way.
Increases the risk of low blood sugar and, alongside other risk factors, lactic acidosis. That doesn't mean cutting it out completely for most people, but it's a genuine reason to mention how much you drink to your GP honestly — not to be judged, just so they have the full picture when they're thinking about your risk.
Used in some imaging procedures, contrast dye can temporarily affect kidney function, so metformin is usually paused before these procedures and for at least 48 hours afterwards. If you're having any kind of imaging that involves a contrast injection, mention that you're on metformin — most departments will ask, but it's worth flagging yourself too.
Can change how your body handles blood sugar, so your doctor may need to make a small adjustment to your metformin dose if you've recently started one.
If you're taking several other medicines alongside metformin, it's worth reading our guide on medication management, which covers how to keep track of everything and when a proper review is worth requesting.
How Hea fits in
A lot of what matters with metformin — how your stomach's actually been over the past few weeks, whether the tiredness feels different from usual, whether something's changed since a dose adjustment — is hard to remember accurately by the time you're sitting in front of your GP.
Hea checks in each morning through WhatsApp with a short, friendly question about how you're feeling — your energy, your sleep, anything on your mind. Over time, that builds into a genuine picture you can bring to your GP or your next annual review, rather than trying to recall three weeks of “I've felt a bit off” from memory.
Hea won't interpret your blood test results or tell you whether a symptom is metformin-related — that's exactly what your GP and your regular blood tests (kidney function, B12) are there for. What it can do is help you notice and describe a pattern clearly, which tends to make those conversations far more useful.
Describe a pattern clearly, not "I've felt a bit off"
Hea checks in each morning through WhatsApp and builds a genuine picture over time — ready to bring to your GP or your next annual review.
See how Hea worksFrequently asked questions
Are metformin side effects the same for everyone?
No. Many people experience mild stomach upset in the first few weeks and nothing further after that. Others notice little at all. Long-term effects like B12 deficiency tend to depend on dose and how long you've been taking it, so your experience genuinely depends on your own situation and treatment.
Should I stop taking metformin if I feel sick?
For ordinary stomach upset in the first few weeks, no — this usually settles, and it's worth persevering unless your GP advises otherwise. But if you're vomiting, have diarrhoea, or are at risk of dehydration for any reason, the advice is to pause metformin temporarily and restart about 48 hours after you're eating and drinking normally again.
Can metformin cause kidney damage?
Not directly. Metformin doesn't damage healthy kidneys, but because it's cleared from the body by the kidneys, reduced kidney function means the drug can build up, which raises other risks. This is why kidney function is checked regularly, and why your doctor may switch you to a different medicine if your kidney function changes.
Is metformin a weight-loss drug?
No. It doesn't cause weight gain, unlike some other diabetes medicines, and some people lose a modest amount of weight while taking it — but it isn't prescribed for weight loss, and it shouldn't be relied on for that purpose.
Sources
- NHS, About metformin — nhs.uk/medicines/metformin/about-metformin
- NHS, Side effects of metformin — nhs.uk/medicines/metformin/side-effects-of-metformin
- NHS, Common questions about metformin — nhs.uk/medicines/metformin/common-questions-about-metformin
- NHS, Taking metformin with other medicines and herbal supplements — nhs.uk/medicines/metformin/taking-metformin-with-other-medicines-and-herbal-supplements
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