Metformin
How metformin works, why the side effects settle, and the B12 monitoring people are rarely told about.
Last revised
Usually the first medicine offered for type 2 diabetes. It lowers blood glucose mainly by reducing production in the liver, and does not itself cause low blood sugar.
How it works
Metformin reduces hepatic glucose production and improves insulin sensitivity in muscle. Unlike insulin or sulfonylureas it does not stimulate insulin release, so on its own it does not cause hypoglycaemia. It is weight neutral or associated with modest weight loss, which is part of why it is preferred first line.
Getting through the first fortnight
Nausea, diarrhoea, abdominal discomfort and a metallic taste are common when starting and usually settle within a couple of weeks. Starting at a low dose and increasing slowly, and taking it with or just after food, substantially reduces this. Where symptoms persist, a modified-release preparation is often much better tolerated and is worth asking about rather than abandoning the medicine.
Vitamin B12
Long-term metformin use is associated with reduced vitamin B12 absorption, and deficiency can develop over years. It matters because B12 deficiency causes anaemia and nerve damage whose symptoms — numbness and tingling in the feet — can be mistaken for diabetic neuropathy. Periodic B12 testing is advised, particularly in anyone developing those symptoms.
When to pause it
Metformin should be stopped temporarily during acute illness with dehydration — vomiting, diarrhoea, high fever — and around some procedures involving contrast dye, because of a rare risk of lactic acidosis when kidney function is compromised. This is standard sick-day guidance and worth knowing in advance rather than looking up while unwell.
At a glance
| Term | Detail |
|---|---|
| Class | biguanide |
| Main action | reduces liver glucose production, improves insulin sensitivity |
| Causes hypoglycaemia alone | no |
| Effect on weight | neutral or modest loss |
| Common early side effects | nausea, diarrhoea, metallic taste; usually settle in 1-2 weeks |
| Better tolerated option | modified-release preparation |
| Long-term monitoring | vitamin B12, and kidney function |
| Pause during | dehydrating illness, and around contrast procedures |
When to get help
Seek urgent medical help for severe nausea and vomiting with abdominal pain, rapid or deep breathing, muscle cramps, extreme tiredness or feeling cold — a rare but serious reaction called lactic acidosis. Stop metformin and contact your diabetes team or GP if you become dehydrated through vomiting or diarrhoea. Report new numbness or tingling in the feet, which warrants a B12 check.
Where to go next
Where this comes from
- NICE - Type 2 diabetes in adults: management
- NHS - Metformin
- Diabetes UK - metformin and sick day rules
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to a GP, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 999.
