Metformin and Alcohol: Lactic Acidosis, Hypos and What the Label Actually Says

UK sources do not tell people on metformin to give up alcohol, but they are not relaxed about it either. The NHS says limit it. The licensed product information goes further, listing acute alcohol intoxication and alcoholism among the conditions in which metformin should not be used at all. The reason behind both is lactic acidosis.

What the UK sources actually say

The NHS page and the label address different readers: one is for someone having a drink at the weekend, the other is a contraindication, the strongest category a label has. Our reading is that these are aimed at opposite ends of one range rather than in conflict, and that the NHS page does not say so.

The NHS metformin page also never names lactic acidosis in its alcohol answer. It says “serious side effects” and leaves the reader to find out elsewhere which one is meant. That is a verified absence, not an inference.

And NICE NG28, the current UK guideline for type 2 diabetes, says nothing about metformin and alcohol and never mentions lactic acidosis. Its one relevant recommendation, 1.3.6, is dietary: “Individualise recommendations for carbohydrate and alcohol intake, and meal patterns. Make reducing the risk of hypoglycaemia a particular aim for people using insulin or an insulin secretagogue.” Note where NICE puts the alcohol risk: on hypos, not acidosis.

The labels do not agree with each other either

We checked five UK metformin labels on the electronic Medicines Compendium. All five carry the same section 4.3 contraindication and the same section 4.5 sentence. Only one, an oral solution, adds: “Avoid consumption of alcohol and alcohol-containing medicinal products.”

Whether your label tells you to avoid alcohol outright therefore depends on whose product you were dispensed. Our observation is that this is generic labelling drift rather than a clinical distinction, since nothing else in the alcohol wording differs.

Why alcohol makes lactic acidosis more likely

Lactic acidosis is a build-up of lactate in the blood. The label calls it “a very rare, but serious metabolic complication, most often occurring at acute worsening of renal function or cardiorespiratory illness or sepsis”. Very rare, in labelling terms, means fewer than 1 in 10,000.

A 2025 case report in Cureus sets out the mechanism: metabolising ethanol raises the NADH to NAD+ ratio in the liver, which “promotes the conversion of pyruvate to lactate, inhibits pyruvate dehydrogenase activity, and suppresses hepatic gluconeogenesis, thereby impairing lactate clearance”. Metformin “exerts similar redox-dependent effects by inhibiting mitochondrial complex I and suppressing hepatic gluconeogenesis”. Two things pushing the same lever, and the reason alcohol also lowers blood sugar.

How rare is it, actually

Two current estimates do not match.

The Cochrane review goes further than the label. Pooling 347 comparative trials and cohort studies, it “revealed no cases of fatal or nonfatal lactic acidosis in 70,490 patient-years of metformin use or in 55,451 patients-years in the non-metformin group”, and concluded “there is no evidence from prospective comparative trials or from observational cohort studies that metformin is associated with an increased risk of lactic acidosis”.

The figure usually quoted from it, 4.3 cases per 100,000 patient-years, is widely misread. It is not a measured rate. Zero cases were observed. It is the statistical ceiling of what the rate could have been given that zero, and the ceiling for people not taking metformin was higher, at 5.4.

Against that, a 2024 review in World Journal of Diabetes gives “3 to 10 cases per 100000 patient-years based on a meta-analysis of 65 studies”, and is blunt about what happens when it occurs: “MALA is not benign, with mortality exceeding 10% according to several studies.” The crude figures it cites run from roughly one in ten to better than one in three.

They do not agree. Our observation is that Cochrane’s populations were trial and cohort participants, while the label’s alcohol warning targets people who are acutely intoxicated, alcohol dependent or have impaired liver function. A ceiling from the first group does not describe the second. No source we found reconciles them, so we assert nothing further.

The same review adds the point most coverage omits: metformin “by itself usually does not lead to MALA” unless it accumulates, typically because of severe renal impairment.

The “even modest amounts” case, read properly

The Cureus report is titled “Metformin-Associated Lactic Acidosis Induced by Even Modest Amounts of Alcohol”, and the title has travelled further than the case.

The man in it had moderately reduced kidney function, had become dehydrated, and was on a dose the authors state exceeded what his kidney function allowed. The authors are careful: “although excessive alcohol intake is recognized as a precipitant, the threshold quantity capable of inducing MALA remains unclear.”

Their review of previously published alcohol-associated cases found seven, of which six survived. Seven cases is not an incidence figure. Our reading is that the case supports one conclusion: the risk sits in the combination of alcohol with reduced kidney function and dehydration, not in alcohol alone.

The low blood sugar problem people do not expect

Metformin on its own does not cause hypoglycaemia. The label says so twice, in sections 4.4 and 4.7: “Metformin monotherapy does not cause hypoglycaemia and therefore has no effect on the ability to drive.”

Diabetes UK explains that the liver normally releases stored glucose when blood sugar drops, “but alcohol stands in the way of the liver’s ability to do this effectively”, and that “your risk of having a hypo doesn’t go away after you stop drinking, it increases and can last up to 24 hours”. The NHS lists drinking “a lot of alcohol, particularly without eating” among the causes of low blood sugar.

The label requires caution when metformin is used alongside insulin or other oral antidiabetics, and NICE 1.3.6 singles out those same people.

Hypo symptoms include confusion, dizziness, sweating, shaking and slurred speech, and the NIAAA-published review of alcohol and medication interactions is plain: “Diabetics who consume alcohol also must be alert to the fact that the symptoms of mild intoxication closely resemble those of hypoglycaemia.” Diabetes UK says it from the other direction: “It’s not uncommon for some people to mistake having a hypo for being drunk.”

Someone slurring and unsteady after a few drinks gets left to sleep it off. NHS advice is the opposite: call 999 for someone with severe low blood sugar who has been drinking. Outside the UK, use your local emergency number. Diabetes UK also advises wearing medical identification.

What lactic acidosis feels like, and why alcohol hides it

The patient leaflet lists the symptoms as “vomiting, stomach ache (abdominal pain), muscle cramps, a general feeling of not being well with severe tiredness, difficulty in breathing, reduced body temperature and heartbeat”, and instructs: “Stop taking Metformin film-coated tablets and contact a doctor or the nearest hospital immediately if you experience some of the symptoms of lactic acidosis, as this condition may lead to coma.”

That instruction is the leaflet’s, and it applies to that emergency. It is not a reason to stop metformin in any other circumstance. The NHS routes it the same way: call 999 if you are taking metformin and think you might be experiencing lactic acidosis.

The awkward overlap is that vomiting, abdominal pain and severe tiredness also describe a heavy night. Our reading is that this is the practical reason the label singles out alcohol: drinking raises the risk and disguises the presentation.

Two things the guidance does not join up

Metformin can lower vitamin B12. The MHRA advised in June 2022 that this is a common adverse reaction and set out risk factors for monitoring. Alcohol is not among them, and we are not going to add it. Our extension, labelled as ours rather than the MHRA’s finding, is that sustained heavy drinking and poor nutritional intake tend to arrive together, and that this is worth raising with a prescriber.

The second is dehydration. The label instructs that “in case of dehydration (severe diarrhoea or vomiting, fever or reduced fluid intake), metformin should be temporarily discontinued and contact with a health care professional is recommended”. The NHS says the same about illness. Our observation is that a heavy drinking session ending in vomiting and a day of not keeping fluids down produces precisely that state, and that most people would not connect the two. That is a conversation with a pharmacist or prescriber, not something to work out from a web page.

If the drinking is the part that is hard to change

“Alcoholism” is the word the label uses, and it is doing real work. It sits in the contraindications.

Alcohol-related liver disease “does not always cause symptoms in the early stages”, and the NHS says the liver can usually recover at the first stage if you stop drinking, while at the third the damage cannot usually be reversed.

If cutting down has stopped being straightforward, the early signs of alcohol dependence, what a functioning alcoholic actually looks like and what helps with cravings are more useful reading than another page about metformin. If you have been drinking heavily every day, the NHS warns that it can be very dangerous to stop suddenly, so speak to a clinician first.

The Orchid Recovery is a residential addiction and mental health treatment centre in Hang Dong District, Chiang Mai, Thailand, for international English speaking adults. If drinking has become hard to control, our pages on alcohol addiction treatment and medically supervised alcohol detox explain what that involves, our clinical team is listed here, and you can speak to us if it would help. Most people who read this page will not need us, and that is the right outcome.

We have also written about pregabalin and alcohol, where the mechanism is sedation, and whether people who have been dependent can ever drink again.

Sources

• NHS, Metformin
• NHS, Common questions about metformin
• NHS, Who can and cannot take metformin
• NHS, Side effects of metformin
• NHS, Low blood sugar (hypoglycaemia)
• NHS, Alcohol-related liver disease
• electronic Medicines Compendium, Metformin 500 mg Tablets, Summary of Product Characteristics
• electronic Medicines Compendium, Metformin 500 mg film-coated tablets, Summary of Product Characteristics
• electronic Medicines Compendium, Metformin 500mg prolonged release tablets, Summary of Product Characteristics
• electronic Medicines Compendium, Metformin Hydrochloride 500mg/5ml Oral Solution, Summary of Product Characteristics
• electronic Medicines Compendium, Metformin 500 mg, Patient Information Leaflet
• NICE, Type 2 diabetes in adults: management (NG28)
• MHRA Drug Safety Update, Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk, June 2022
• Salpeter SR et al, Risk of fatal and nonfatal lactic acidosis with metformin use in type 2 diabetes mellitus, Cochrane Database of Systematic Reviews
• See KC, Metformin-associated lactic acidosis: a mini review of pathophysiology, diagnosis and management in critically ill patients, World Journal of Diabetes, 2024
• Miyata Y et al, Metformin-Associated Lactic Acidosis Induced by Even Modest Amounts of Alcohol: A Case Report, Cureus, 2025
• Weathermon R and Crabb DW, Alcohol and Medication Interactions, Alcohol Research and Health, 1999
• Binder C and Bendtson I, Endocrine emergencies. Hypoglycaemia, Baillière’s Clinical Endocrinology and Metabolism, 1992
• Diabetes UK, Alcohol and diabetes
• UK Chief Medical Officers’ low risk drinking guidelines