What Is Metformin?
Metformin is a prescription medication belonging to the biguanide class of drugs. It is most commonly used as a first-line treatment for type 2 diabetes and works by reducing the amount of glucose produced by the liver, improving the body's sensitivity to insulin, and slowing glucose absorption from the intestines. It does not cause the pancreas to produce more insulin, which reduces the risk of hypoglycaemia (low blood sugar) compared to some other diabetes medications.
What Is It Used For?
The primary approved use of metformin is managing blood glucose levels in adults with type 2 diabetes, often alongside dietary and lifestyle changes. It may also be used:
- To prevent type 2 diabetes in people with prediabetes who are at high risk
- As part of treatment for polycystic ovary syndrome (PCOS) to address insulin resistance
- In combination with other diabetes medications or insulin
Note: Use for PCOS and prediabetes is considered "off-label" in some countries, meaning it is used based on clinical evidence but not formally licensed for those indications everywhere.
How Is Metformin Taken?
Metformin is taken orally and comes in several forms:
- Immediate-release tablets — typically taken two to three times daily with meals
- Extended-release tablets — taken once daily, usually with the evening meal; often better tolerated digestively
- Liquid solution — available for those who have difficulty swallowing tablets
Dosage is usually started low and gradually increased to minimise side effects, with your doctor adjusting the dose based on your blood glucose response and kidney function.
Common Side Effects
The most frequently reported side effects are gastrointestinal and include:
- Nausea or vomiting
- Diarrhoea
- Stomach upset or cramping
- Metallic taste in the mouth
These side effects are most common when starting treatment or increasing the dose. Taking metformin with food and using the extended-release form can help reduce these effects for many people.
Serious but Rare Risk: Lactic Acidosis
A rare but serious risk associated with metformin is lactic acidosis — a build-up of lactic acid in the bloodstream. This risk increases if you have impaired kidney function, liver disease, heart failure, or if you are severely dehydrated. Symptoms include muscle pain, weakness, trouble breathing, stomach pain, and feeling cold. Seek immediate medical attention if you experience these.
For this reason, metformin is not recommended for people with significantly reduced kidney function, and kidney function should be monitored regularly during treatment.
Important Interactions and Precautions
| Situation | Guidance |
|---|---|
| Contrast dye (for imaging scans) | May need to temporarily stop metformin; consult your doctor |
| Alcohol | Heavy drinking increases lactic acidosis risk; limit intake |
| Surgery or major procedures | Often paused temporarily; follow medical team's instructions |
| Vitamin B12 | Long-term use may reduce B12 absorption; monitoring advised |
Key Takeaways
- Metformin is generally well-tolerated and effective for managing blood glucose in type 2 diabetes.
- Take it with food to reduce digestive side effects.
- Have your kidney function tested regularly while on this medication.
- Never stop or adjust your dose without speaking to your doctor first.
- Inform any treating clinician — including dentists and radiologists — that you take metformin.
This article is for informational purposes only and is not a substitute for advice from your prescribing doctor or pharmacist.