Metformin Side Effects and Contraindications
Few medicines have been used as widely or for as long as metformin. Most people tolerate it well, but a sizeable minority struggle with digestive side effects, and certain health conditions make it unsafe. Interest in metformin has also spread to the gym, where it is discussed as a ‘longevity’ or ‘body composition’ drug. Our editorial team reviews its real side-effect profile and contraindications.
The most common problem: the gut
Gastrointestinal symptoms are by far the most frequent reason people struggle with metformin. Diarrhoea, nausea, abdominal cramps, bloating and loss of appetite are reported in a substantial proportion of new users, typically in the first weeks of treatment.
The mechanisms are not fully understood. Metformin reaches high concentrations in the gut wall, alters bile acid handling, increases intestinal glucose use and changes the gut microbiome. Bonnet and Scheen summarised these factors in their review of metformin intolerance.
In most people the symptoms ease with time. Doctors in the UK usually start at a low dose taken with food and increase it gradually over several weeks. If problems persist, a modified-release tablet, which releases the drug more slowly, is often better tolerated.
A minority remain intolerant despite these measures. Persistent diarrhoea is not something to simply put up with; it can cause dehydration, which in turn affects kidney function and the safety of the drug itself.
Vitamin B12, taste changes and other effects
Long-term metformin use reduces absorption of vitamin B12. The effect builds up over years, and the MHRA now classes reduced B12 levels as a common side effect. Deficiency may present as fatigue, anaemia or nerve symptoms such as tingling feet, which can be wrongly attributed to diabetic neuropathy.
Some users notice a metallic taste, particularly in the early weeks. This is usually harmless and often fades. Skin reactions, including rash and itching, are reported but are uncommon.
Metformin rarely causes hypoglycaemia when used alone, because it does not stimulate insulin secretion. The risk rises when it is combined with sulfonylureas or insulin, or when someone exercises hard after skipping meals.
Modest weight loss or weight neutrality is typical, which is one reason the drug is favoured in people with type 2 diabetes and excess weight. It is not, however, a weight-loss medicine in the conventional sense.
Lactic acidosis: rare but serious
Lactic acidosis is the side effect most often associated with metformin in the public mind. It is a build-up of lactate in the blood that makes it acidic, and it can be life-threatening. With metformin it is very rare, and a Cochrane review by Salpeter and colleagues found no clear increase in risk compared with other diabetes treatments in trial populations without major risk factors.
The danger arises when metformin accumulates or when the body is already producing excess lactate. Typical situations include severe dehydration, acute kidney injury, sepsis, shock, heart failure decompensation, severe liver disease and heavy alcohol intake.
Symptoms are non-specific: vomiting, abdominal pain, muscle cramps, rapid breathing, profound weakness and drowsiness. Anyone on metformin who develops these features, especially during an acute illness, needs urgent medical assessment.
This is why NHS ‘sick day rules’ often advise temporarily stopping metformin during vomiting, diarrhoea or fever with poor fluid intake, and restarting once recovered, following advice from a GP or diabetes nurse.
Contraindications: who should not take it
The summary of product characteristics and the BNF list situations in which metformin must not be used or needs particular care. The main ones are summarised below.
| Situation | Status |
|---|---|
| eGFR below 30 mL/min/1.73 m² | Contraindicated |
| eGFR 30–45 | Use with caution; dose reduced and reviewed |
| Diabetic ketoacidosis | Contraindicated |
| Acute conditions causing tissue hypoxia (e.g. shock, respiratory failure, recent heart attack) | Contraindicated |
| Severe hepatic impairment, acute alcohol intoxication | Contraindicated |
| Iodinated contrast or major surgery | Usually paused temporarily |
Pregnancy is a special case. Metformin is used in the UK for gestational diabetes and in some women with polycystic ovary syndrome, but only under specialist supervision, and insulin remains the main alternative.
Older adults are not excluded, yet age-related decline in kidney function means monitoring becomes more important. Frailty, low body weight and poor appetite all shift the balance of benefit and risk.
Alcohol deserves a specific mention. Moderate drinking is not forbidden, but binge drinking increases both lactate production and the risk of hypoglycaemia when combined with other diabetes medicines.
Metformin and training: what the trials show
In fitness circles metformin is sometimes taken without diabetes in the hope of better body composition or slower ageing. In the UK it is a prescription-only medicine, and using it without a prescription bypasses the safety checks described above.
There is also evidence that it may blunt some benefits of exercise. In the MASTERS trial, older adults taking metformin gained less muscle mass from progressive resistance training than those taking placebo. Konopka and colleagues reported that metformin reduced improvements in aerobic fitness and mitochondrial adaptations in older adults doing aerobic training.
These findings do not mean that people with diabetes should avoid exercise or stop their medicine. For them, the benefits of glucose control and physical activity together clearly outweigh a possible attenuation of training gains.
For healthy, active people, however, the evidence argues against using metformin as a performance or physique aid. Key points to remember:
- benefits in people without diabetes are not established;
- training adaptations may be reduced;
- gut side effects and B12 depletion still apply;
- hydration problems during endurance events may raise the risk of complications.
Editorial conclusions
Metformin is generally safe, but stomach upset is common and B12 depletion develops silently over years.
Lactic acidosis is rare and mostly linked to kidney impairment, dehydration, severe illness or alcohol excess, which is why sick day rules and kidney monitoring matter.
For healthy people who train, there is no good case for off-label use, and some trials suggest it can blunt adaptations to exercise.
Related reading on our site: blood tests while taking metformin, the side effects of semaglutide and the history of rosuvastatin.
References
- Bailey CJ, Turner RC. Metformin. N Engl J Med. 1996;334(9):574–579.
- Salpeter SR, Greyber E, Pasternak GA, Salpeter EE. Risk of fatal and nonfatal lactic acidosis with metformin use in type 2 diabetes mellitus. Cochrane Database Syst Rev. 2010;(4):CD002967.
- Bonnet F, Scheen A. Understanding and overcoming metformin gastrointestinal intolerance. Diabetes Obes Metab. 2017;19(4):473–481.
- Lipska KJ, Bailey CJ, Inzucchi SE. Use of metformin in the setting of mild-to-moderate renal insufficiency. Diabetes Care. 2011;34(6):1431–1437.
- 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.
- Walton RG, Dungan CM, Long DE, et al. Metformin blunts muscle hypertrophy in response to progressive resistance exercise training in older adults: the MASTERS trial. Aging Cell. 2019;18(6):e13039.
- Konopka AR, Laurin JL, Schoenberg HM, et al. Metformin inhibits mitochondrial adaptations to aerobic exercise training in older adults. Aging Cell. 2019;18(1):e12880.
