Endocrine / Diabetes
Metformin
Biguanide — First-line oral medicine for type 2 diabetes that does not usually cause hypoglycaemia.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Metformin is the usual first medicine for type 2 diabetes. It lowers blood glucose without typically causing hypoglycaemia or weight gain, and has a long track record of safety and cardiovascular benefit.
How it works
It reduces the amount of glucose the liver releases and improves the body's sensitivity to insulin, lowering blood glucose without stimulating extra insulin release — which is why it rarely causes hypos on its own.
In practice
In practice the two things that keep metformin safe are renal monitoring and "sick day" advice: hold it during acute illness with dehydration, and review the dose as kidney function falls. Start low and titrate, or use the modified-release form, to limit the gut upset that otherwise drives people to stop. It stays first-line because it lowers glucose without hypos and has a strong long-term safety record.
Examples
Practical use
How to take it & use it well
- Take it with or just after meals to reduce stomach upset, which is common when you first start.
- Swallow modified-release tablets whole and do not crush or chew them.
- Take it at the same times each day, building it into your meal routine so you do not forget.
- If you miss a dose, take it with your next meal, but skip it if it is nearly time for the next one. Do not double up.
- Stay well hydrated, and pause the medicine and seek advice if you become acutely unwell with vomiting, diarrhoea or dehydration, as advised by your team.
- Keep up regular blood tests for kidney function and vitamin B12, as your doctor recommends.
Common uses
- Type 2 diabetes
- Polycystic ovary syndrome (off-label, specialist context)
- Prevention of diabetes in selected high-risk people
Monitoring
- Renal function before starting and at least annually (more often if impaired)
- HbA1c to assess control
- Vitamin B12 on long-term use if symptoms suggest deficiency
Weighing it up
Advantages & disadvantages
Advantages
- It lowers blood sugar without usually causing low blood sugar on its own.
- It does not cause weight gain and may help with modest weight control.
- It is a well established first-choice medicine for type 2 diabetes with strong long-term evidence.
- It is inexpensive and widely used on the NHS.
- It may have benefits for heart health in people with diabetes.
Disadvantages
- It commonly causes stomach upset, nausea or diarrhoea, especially when starting, though a slow-release form can help.
- It can reduce vitamin B12 absorption over the long term.
- It cannot be used if kidney function is very poor and needs pausing during certain illnesses or scans.
- It is taken regularly long term and works best alongside diet and lifestyle changes.
- Rarely it can cause a serious build-up of acid in the blood, mainly when other illness is present.
Key safety principles
What to watch for
- Reduce or stop in significant renal impairment because of the rare risk of lactic acidosis; it is contraindicated below a renal threshold.
- Withhold during acute illness with dehydration, and around contrast imaging or major surgery as advised.
- Gastrointestinal upset is common when starting — a modified-release form or slow titration helps.
Key interactions
What to avoid or check alongside
- Drinking a lot of alcohol increases the small risk of a serious side effect called lactic acidosis and is best avoided.
- Some X-ray dyes used in scans can affect the kidneys, so metformin may be paused around the time of the scan.
- Combining it with other diabetes medicines or insulin can increase the chance of low blood sugar.
- Water tablets and some blood pressure medicines can affect kidney function and blood sugar control.
- Steroids can raise blood sugar and may reduce how well it controls your diabetes.
- Becoming dehydrated through illness can affect the kidneys and how the medicine is handled.
Patient & carer advice
- Take with or after food to reduce stomach upset
- Stop and seek advice if you become acutely unwell with vomiting or dehydration ("sick day" rules)
- It works best alongside diet and activity changes
Use with
Related clinical calculators
Dose and risk decisions for this class often depend on renal function, weight or bleeding/stroke risk. These tools help:
Answers
Metformin: frequently asked questions
Why does it upset my stomach?
Stomach upset is common when starting. Taking it with food and building up slowly helps. If it persists, ask about a slow-release version, which is often gentler.
Will it cause low blood sugar?
On its own metformin rarely causes low blood sugar. The risk rises if it is combined with insulin or certain other diabetes medicines.
Can I drink alcohol with it?
Occasional moderate drinking is usually fine, but heavy or binge drinking raises the risk of a rare but serious side effect, so keep within recommended limits.
Do I need to stop it for an operation or scan?
Sometimes. It may be paused around scans using contrast dye or major surgery to protect the kidneys. Your team will tell you when to stop and restart.
Is it safe in pregnancy?
Metformin can be used in pregnancy in some situations, including gestational diabetes, under medical supervision. Discuss your treatment with your doctor if you are pregnant or planning to be.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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