Metformin: What It Does, How to Start It, and How to Use It Well

Metformin is one of the most commonly prescribed medicines for people with type 2 diabetes — and for good reason. It has been used for decades, is well studied, and for many people it becomes the foundation on which long-term diabetes treatment is built.

Yet most people are started on metformin with very little explanation. They are told it is “first-line”, given a prescription, and then left with very practical questions: What does it actually do? Why this medication? Why does it upset the stomach? How should it be started? And how does it fit with everything else I’m doing?

This page is designed to answer those questions clearly and calmly. It brings together the science, the real-world experience, and the bigger picture of how metformin fits into diabetes care.

If you’ve just been prescribed metformin, you may also want to read our step-by-step guide: Starting metformin: what you really need to know.

What metformin actually does (in plain language)

Metformin improves the way your body handles glucose (sugar). Unlike some diabetes medications, it does not force the pancreas to make more insulin. Instead, it supports the body systems that are already under strain.

In practical terms, metformin works in three main ways:

• It reduces the amount of glucose the liver produces, particularly overnight and between meals. This is a major reason fasting blood sugar levels often improve.
• It helps the body respond better to its own insulin, meaning it improves insulin resistance, which is one of the core problems in type 2 diabetes.
• It slightly reduces glucose absorption from food, helping to soften post-meal blood sugar rises.

This is why metformin is often described as treating the underlying problem rather than just the numbers. It supports insulin sensitivity instead of simply reacting to high glucose readings.

Who metformin is commonly prescribed for

Metformin is most often used in people with type 2 diabetes, particularly early in the condition. For many people, it is started at diagnosis alongside changes to diet, physical activity, and weight management.

It is also used in several other situations:

• Prediabetes — especially in higher-risk individuals. Large studies such as the Diabetes Prevention Program (DPP) showed that metformin reduced progression to type 2 diabetes by around 30%, and by over 50% in younger people with higher body weight (DPP study).
• Insulin resistance, when lifestyle measures alone are not enough to stabilise glucose levels.
• Polycystic ovary syndrome (PCOS), where insulin resistance plays a central role.
• Type 1 diabetes with insulin resistance (“double diabetes”), where metformin may be added under specialist care.

Metformin is not required for everyone with prediabetes, but for people at higher risk — such as those with rising sugars, previous gestational diabetes, or significant insulin resistance — it can be a valuable protective tool when combined with lifestyle change.

How metformin is started, common side effects, and vitamin B12

One of the most important things to understand about metformin is that how it is started matters. Many side effects can be minimised with the right approach.

Metformin is usually started at a low dose and increased gradually over several weeks. Taking it with food and increasing slowly allows the gut to adapt. A more detailed, practical guide is available here: How to start metformin safely.

Stomach upset and diarrhoea

The most common side effects are gastrointestinal — nausea, bloating, and diarrhoea. These symptoms are usually temporary and improve with dose adjustment or switching to an extended-release form.

We explain this in depth here: Why metformin causes diarrhoea — and what helps.

Vitamin B12 and long-term use

With long-term use, metformin can reduce vitamin B12 absorption in some people. This does not mean it is unsafe, but it does mean monitoring matters.

Symptoms such as fatigue, tingling, or numbness should always be checked. You can read more here: Vitamin B12 deficiency and metformin.

Using metformin with insulin and the bigger picture

Metformin is commonly continued even when insulin is added. The reason is simple: insulin lowers glucose directly, while metformin improves insulin sensitivity and reduces liver glucose output. Together, they often allow lower insulin doses and more stable control.

If you are using insulin, you may find this helpful: Basal insulin explained.

International guidelines consistently recommend metformin as first-line therapy for type 2 diabetes, including guidance from NICE, the ADA, and the EASD (NICE NG28, ADA/EASD consensus).

Medication alone, however, is never the full answer. Most people do best when they understand how food, activity, weight, medications, and monitoring fit together — not as rules, but as a system.

Free education session:
Spend 30–40 minutes with a diabetes specialist learning how treatment decisions are made and how to stabilise blood sugar levels in real life. Watch the free diabetes masterclass.