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Metformin for Prediabetes: Do You Need It?

In the DPP, metformin cut diabetes risk by 31%, less than lifestyle change at 58%. Dr. Sayantan Banerjee explains who benefits most, side effects, and US and UK guidance.

PREDIABETES REMISSION

Dr. Sayantan Banerjee

10/31/20267 min read

If you have prediabetes, your doctor may have mentioned metformin. Or you may have read that it is a cheap, safe, decades-old drug that "prevents diabetes". Both things are partly true. The more useful question is whether it is right for you, and what it can and can't do.

This article explains how metformin works, what the main trial found, which groups benefit most, what US and UK guidance says, what side effects to expect, and how it fits alongside lifestyle change.

What metformin is and how it works

Metformin has been used to treat type 2 diabetes for more than 60 years, and in the UK since the late 1950s. It is now one of the most widely prescribed medicines in the world, and it is inexpensive as a generic.

Its main action is on the liver. Metformin reduces the amount of glucose the liver releases into the blood, especially overnight and between meals, which lowers fasting glucose. It also modestly improves the body's response to insulin and acts in the gut, where it can increase the release of GLP-1, a gut hormone involved in glucose control and fullness.

On its own, metformin rarely causes low blood sugar, because it doesn't force the pancreas to release extra insulin. Many people lose a small amount of weight on it, or at least don't gain.

What the trial showed

The main evidence for metformin in prediabetes comes from the US Diabetes Prevention Program, which started in the late 1990s. It included 3,234 adults with raised glucose. They were randomly assigned to metformin 850 mg twice a day, an intensive lifestyle programme, or placebo.

After an average of 2.8 years, metformin reduced the risk of developing type 2 diabetes by 31% compared with placebo. The lifestyle programme reduced it by 58%. Put another way, about 14 people needed to take metformin for three years to prevent one case of diabetes, compared with about 7 for the lifestyle programme.

The long-term follow-up

After the main trial, the DPP Outcomes Study kept following participants for many years. Over about 15 years, metformin still lowered diabetes rates by around 18% compared with placebo. The lifestyle group's benefit shrank too, to around 27%, as people aged and some weight returned.

Long-term analyses haven't shown that metformin reduced heart attacks or strokes in people with prediabetes over two decades of follow-up. Larger trials are testing this question further. So, for now, the main proven benefit is delaying or preventing type 2 diabetes.

In the long-term follow-up, vitamin B12 deficiency was more common in people taking metformin, which is why monitoring B12 is now recommended.

Who benefits most

The DPP found that metformin helped some groups much more than others:

People with a BMI of 35 or higher. In this group, metformin worked nearly as well as lifestyle change.
Younger adults, aged 25 to 44 at the start. Metformin helped people over 60 much less.
Women who had gestational diabetes in a previous pregnancy. In this group, metformin and lifestyle change reduced risk by similar amounts, around half.

People with higher fasting glucose levels at the start also tended to benefit more.

What US guidance says

The American Diabetes Association's Standards of Care say metformin should be considered for preventing type 2 diabetes in people with prediabetes, especially those with a BMI of 35 or more, those under 60, and women with a history of gestational diabetes. People with rising A1C despite lifestyle changes may also be considered.

Metformin isn't FDA-approved specifically for prediabetes, so it is prescribed off-label. This is common, legal and well established.

The ADA also recommends checking vitamin B12 periodically in anyone taking metformin long term.

What UK guidance says

In the UK, metformin isn't licensed for prediabetes either. NICE guidance on preventing type 2 diabetes in people at high risk suggests considering metformin for adults whose blood glucose keeps rising despite taking part in an intensive lifestyle programme, or who can't take part in one, for example because of disability or other health conditions.

In practice, most NHS patients with non-diabetic hyperglycaemia are offered the Healthier You programme first, rather than tablets. Some doctors may suggest metformin sooner for women who had gestational diabetes, or for people with a high BMI.

How it is usually taken

Doses vary, so follow your prescriber's instructions. A common approach is to start low, for example 500 mg once a day with the evening meal, and increase slowly over a few weeks. The dose used in the DPP was 850 mg twice a day.

Taking metformin with food, and increasing the dose gradually, reduces stomach side effects. The extended-release version, called modified-release or slow-release (SR) in the UK and XR or ER in the US, is often easier on the stomach and can be taken once a day.

Side effects to know about

Metformin is one of the most studied drugs in the world and is generally safe, but side effects are common at first.

Stomach and bowel effects. Nausea, loose stools, diarrhoea, bloating, wind and a metallic taste are the most common, especially in the first few weeks. They often settle. If they don't, a lower dose or the extended-release form usually helps.

Vitamin B12. Long-term use can lower B12 levels. Low B12 can cause tiredness, anaemia, and numbness or tingling in the hands and feet. Ask for a check every few years, or sooner if you have symptoms.

Low blood sugar. Rare on metformin alone, but possible if it is combined with other glucose-lowering medicines or with very low food intake.

Lactic acidosis. A rare but serious build-up of acid in the blood. It is mainly a risk in people with severe kidney disease, severe liver disease, serious heart or lung failure, severe dehydration, or heavy alcohol use. Your doctor will check your kidney function before starting and regularly afterwards.

Temporary pauses. Metformin may need to be paused if you become dehydrated, for example with severe vomiting or diarrhoea, before some X-ray scans with contrast dye, and around some operations. Your doctor or pharmacist will advise.

Should you ask for it?

Here is a reasonable way to think about it.

If your BMI is 35 or above, you are under 60, or you had gestational diabetes, metformin is worth discussing alongside lifestyle change, not instead of it.

If your A1C keeps rising despite a real effort with diet and exercise, it is reasonable to ask.

If you are over 60 and your BMI is lower, the evidence suggests lifestyle change will do far more for you than metformin.

If you start it, treat it as support. The DPP showed the biggest drop in risk came from moving more and losing a modest amount of weight.

What about other medicines?

GLP-1 medicines such as semaglutide and tirzepatide bring many people with prediabetes back to normal glucose in trials, largely through weight loss. They aren't usually prescribed for prediabetes alone, but they may be for people who qualify on the basis of weight. In the US, that usually means a BMI of 30 or more, or 27 or more with a weight-related condition. In the UK, NHS eligibility is more limited and depends on BMI and other conditions. That conversation is worth having with your doctor if your BMI is high.

Other diabetes drugs, such as pioglitazone and acarbose, have also reduced diabetes risk in trials, but side effects and cost mean they are rarely used for prevention.

Supplements promoted as "natural metformin", such as berberine, are not equivalent and can interact with other medicines. Check with a pharmacist before taking them.

Questions to ask your doctor

Based on my BMI, age and history, would metformin help me?
Should I try a lifestyle programme first?
What dose would I start on, and how would it increase?
Would the extended-release version suit me better?
How often should my kidney function and B12 be checked?
How long would I take it, and how will we judge whether it is working?

Metformin in PCOS and pregnancy

Polycystic ovary syndrome. Metformin is often used in women with PCOS, particularly those with insulin resistance or prediabetes. It can help with glucose and, in some women, with periods and ovulation. It is usually used alongside lifestyle changes, and decisions depend on whether a woman is trying to conceive. Discuss this with your GP, gynaecologist or endocrinologist.

Pregnancy. Metformin is sometimes used to treat gestational diabetes and type 2 diabetes in pregnancy. It isn't routinely used for prediabetes in pregnancy, where diet, activity and close monitoring are the usual approach. If you take metformin and become pregnant or plan to, talk to your doctor promptly.

After gestational diabetes. As mentioned above, women with a history of gestational diabetes benefited from metformin in the DPP about as much as from lifestyle change. This is one group in whom guidelines suggest considering it.

Metformin myths

Myth: Metformin is an anti-ageing drug. You may have read claims that metformin slows ageing or extends lifespan. These ideas come mainly from observational studies and animal research. A large trial to test this question has been planned, but there is no proof yet that metformin extends life in people without diabetes. It isn't recommended for this purpose.

Myth: Metformin damages the kidneys. It doesn't. It is cleared by the kidneys, so doses need adjusting if kidney function is reduced, and it is usually stopped if kidney function falls very low.

Myth: Metformin is a weight-loss drug. Some people lose a little weight on it, but the effect is small compared with lifestyle change or GLP-1 medicines.

Myth: Once you start, you can never stop. If your glucose returns to normal, your doctor may consider stopping it, with follow-up tests.

What happens if you stop?

In the DPP, when metformin was stopped briefly at the end of the main trial, some of its benefit disappeared within weeks, as glucose rose again. This suggests metformin works mainly while you take it, rather than permanently changing the underlying condition. Lifestyle changes, by contrast, address the causes.

How my book can help

The Prediabetes Exit Strategy focuses on the lifestyle side, which is where most of the benefit lies. You can read more on the book page. If you already have type 2 diabetes and are wondering about coming off metformin, my article on stopping metformin safely covers that.

Frequently asked questions

Should I take metformin for prediabetes? It depends on your BMI, age and history. Ask your doctor, especially if your BMI is 35 or more or you had gestational diabetes.

Can metformin reverse prediabetes? It can help bring glucose down, but in the DPP lifestyle change was nearly twice as effective.

How long does metformin take to work for prediabetes? Glucose often improves within a few months. A1C reflects about three months, so retest after that.

Will metformin help me lose weight? Some people lose a small amount, typically a few kilograms at most. It isn't a weight-loss drug.

Can I stop metformin if my glucose becomes normal? Possibly. Discuss it with your doctor, who may want to see stable results first.

Is metformin safe long term? For most people, yes, with regular kidney and B12 checks.

Can I drink alcohol on metformin? Moderate drinking is usually fine. Heavy drinking raises the risk of side effects, including lactic acidosis.

What time of day should I take metformin? Usually with meals, often with the evening meal when starting. Follow your prescriber's advice.

Does metformin interact with other medicines? It can interact with some, including certain medicines that affect the kidneys. Tell your pharmacist about everything you take.

Is metformin expensive? No. It is a low-cost generic in both the US and the UK, and is covered by most insurance plans and NHS prescriptions.