Metformin for Weight Loss: What the Evidence Shows

Metformin for weight loss is an off-label idea. Metformin is FDA-approved only to improve blood sugar control in type 2 diabetes, not for weight management, and in trials it has produced modest average weight loss at most. In the Diabetes Prevention Program, adults with raised blood sugar who took metformin lost about 2.1 kg on average over 2.8 years, compared with 0.1 kg on placebo and 5.6 kg with an intensive lifestyle program. GLP-1 drugs approved for weight management produced average losses of about 15% of body weight or more in their main trials. Metformin also carries a boxed warning for lactic acidosis, a serious side effect.

Peptide Labs does not sell metformin, Ozempic or any prescription drug. This article is general information, not medical advice. For background on the hormone that newer weight drugs copy, see what GLP-1 is.

What metformin is and what it is approved for

Metformin is a biguanide, a different class from GLP-1 drugs, first approved in the U.S. in 1995. The immediate-release tablets are approved, with diet and exercise, to improve blood sugar control in adults and children 10 and older with type 2 diabetes; the extended-release tablets are approved for adults only. According to the label, metformin lowers the amount of glucose the liver makes, reduces glucose absorption from the gut and improves the body’s response to insulin, without raising insulin secretion. It is taken by mouth with meals.

Using an approved drug for a purpose not on its label is off-label use. The FDA says prescribers may generally do this when medically appropriate, but the agency has not found the drug safe and effective for that use. Weight loss and diabetes prevention are both off-label uses of metformin.

Does metformin cause weight loss?

On average, a little, and not in every trial. The current label reports body weight from three of the trials behind its approval:

• 29-week trial in adults with obesity and type 2 diabetes: average weight change was -1.4 lb on metformin (141 people) and -2.4 lb on placebo (145 people), so no weight advantage.
• 29-week trial in adults no longer controlled on glyburide: people switched to metformin alone lost 8.4 lb on average vs 0.7 lb on glyburide, but they had also stopped glyburide, which complicates the comparison.
• 16-week trial in children 10 to 16: -3.3 lb on metformin vs -2.0 lb on placebo.

The label claims no weight benefit. A 2019 review noted that large cohort studies link metformin to weight loss; the clearest long-term data on metformin weight loss come from a prevention trial.

Metformin for weight loss in the Diabetes Prevention Program

The Diabetes Prevention Program (DPP), sponsored by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and published in NEJM in 2002, enrolled 3,234 adults with raised fasting and post-meal blood sugar who did not have diabetes. Their average age was 51 and average BMI 34. They were randomly assigned to placebo, metformin, or an intensive lifestyle program aiming for at least 7% weight loss and 150 minutes of activity a week, and were followed for an average of 2.8 years.

DPP result (average 2.8 years)	Placebo	Metformin	Lifestyle program
Average weight loss	0.1 kg	2.1 kg	5.6 kg
New diabetes cases per 100 person-years	11.0	7.8	4.8
Reduction in diabetes vs placebo		31%	58%
Digestive side effects per 100 person-years	30.7	77.8	12.9

The lifestyle program was significantly more effective than metformin. According to NIDDK, metformin was most effective in women with a history of gestational diabetes, in people aged 25 to 44, and in people with a BMI of 35 or higher.

In the DPP Outcomes Study, a 2012 analysis found weight 2.06% lower on metformin vs 0.02% on placebo during the blinded trial, and 2.0% vs 0.2% over 7 to 8 years of open-label follow-up. Loss was larger in people who took metformin consistently, digestive symptoms faded over time, and no significant safety issues were found. By 15 years, metformin had lowered new diabetes by 18% vs placebo.

How metformin might affect appetite: GDF15 and Lac-Phe

In a 2020 Nature paper led by the University of Cambridge, metformin raised blood levels of GDF15, a hormone that reduces food intake through a receptor in the brainstem, in two randomized trials in people. In mice, metformin prevented weight gain on a high-fat diet, but not in mice lacking GDF15 or its receptor, while its glucose-lowering effect remained. Some authors worked for NGM Biopharmaceuticals, a drug company.

In 2024, two Nature Metabolism papers pointed to Lac-Phe, a molecule linked to reduced appetite. A Stanford-led team found that mice unable to make Lac-Phe did not eat less or lose weight on metformin, and a Trinity College Dublin team found higher Lac-Phe in people taking metformin across seven studies. These findings suggest a mechanism, mostly from animals; they do not make the average weight loss any larger.

Other situations where weight on metformin has been studied

Polycystic ovary syndrome (PCOS). A 2015 review of 12 randomized trials in 608 women with PCOS found that adding metformin to lifestyle changes lowered BMI by about 0.73 points more than lifestyle alone after 6 months, with more regular periods. The authors noted small, short trials with risk of bias. Metformin’s label does not include PCOS, but it notes that metformin may restore ovulation in some women who do not ovulate, so the chance of an unplanned pregnancy should be discussed.

Weight gain linked to antipsychotic medicines. A 2016 review of 12 placebo-controlled trials in 743 people with schizophrenia or schizoaffective disorder found about 3.3 kg less weight with metformin than with placebo. This is a research finding, not an approved use.

Metformin vs Ozempic and other GLP-1 drugs

Metformin vs Ozempic is a common search, but the two are usually partners rather than rivals. Ozempic is semaglutide, a once-weekly GLP-1 receptor agonist approved for type 2 diabetes, including heart and kidney risk reduction in defined groups; it is not approved for weight management. In SUSTAIN 7, the Novo Nordisk trial comparing Ozempic with Trulicity, all 1,201 participants were already taking metformin. We did not find a large trial comparing metformin alone with semaglutide alone for weight.

Trials in different groups give a rough sense of scale. In STEP 1, Wegovy (semaglutide for weight management) lowered weight by 14.9% vs 2.4% over 68 weeks in 1,961 adults without diabetes. In SURMOUNT-1, Zepbound (tirzepatide) lowered weight by 15.0% to 20.9% vs 3.1% over 72 weeks. Both weight trials were funded by the drugs’ makers.

	Metformin	GLP-1 drugs for weight (Wegovy, Zepbound)
Type of drug	Biguanide tablet	Peptide hormone mimics, mostly weekly injections
FDA-approved for weight management	No (off-label)	Yes
Main approved use	Type 2 diabetes, adults and children 10+	Chronic weight management, plus other uses
How it is thought to work	Less glucose from the liver; possibly GDF15 and Lac-Phe	Act on GLP-1 receptors (tirzepatide also on GIP receptors) that affect appetite and stomach emptying
Average weight change in key trials	About 2.1 kg (about 2%) over 2.8 years in the DPP	About 15% to 21% over 68 to 72 weeks
Boxed warning	Lactic acidosis	Thyroid C-cell tumors seen in rodents
Most common side effects	Diarrhea, nausea and vomiting, gas	Nausea, diarrhea, vomiting, constipation

Averages from different trials cannot be compared directly. Our guides to Wegovy vs Ozempic, semaglutide vs tirzepatide and GLP-1 side effects cover those drugs, and our overview of Ozempic alternatives lists the FDA-approved weight-management options.

Metformin and berberine

Berberine, a plant compound sold as a dietary supplement, is often marketed as a natural metformin. It is not an FDA-approved drug. A 2012 review of 14 trials in 1,068 people with type 2 diabetes found berberine was not better than metformin or other diabetes drugs, and the trials were generally of low quality. A 2026 analysis of 23 trials found an average weight difference of only 0.88 kg. Because berberine can add to the glucose-lowering effect of diabetes medicines, including metformin, combining them should be discussed with a prescriber. See the benefits of berberine and our berberine label guide, which compares it with metformin.

Side effects and label warnings

Key safety points from the metformin label (revised July 2026):

• Lactic acidosis (boxed warning): cases have been fatal. Risk factors include kidney impairment, age 65 or older, contrast imaging, surgery, low-oxygen states such as acute heart failure, excessive alcohol, liver impairment, mitochondrial diseases and certain drugs such as topiramate.
• Who should not take it: people with severe kidney impairment, metabolic acidosis including diabetic ketoacidosis, or an allergy to metformin. Kidney function is checked before starting and at least yearly.
• Digestive effects: in a 29-week trial, diarrhea affected 53% on metformin vs 12% on placebo and nausea or vomiting 26% vs 8%; diarrhea led 6% to stop. Rates were lower with the extended-release form.
• Vitamin B12: about 7% of people with normal levels fell below normal in 29-week trials. The label advises checking B12 every 2 to 3 years.
• Low blood sugar: the risk rises when metformin is combined with insulin or drugs that make the pancreas release insulin.
• Scans and surgery: metformin is paused for some iodinated contrast scans and while food and fluids are restricted around procedures.

Frequently Asked Questions

Peptide Labs does not sell these drugs. This article is general information, not medical advice. Read the Medication Guide or patient information that comes with a prescription, and ask a pharmacist or prescriber how this information applies to you.

Last reviewed September 2026.

Related guides: Ozempic Alternatives: FDA-Approved Options, OTC Facts and What to Avoid, Berberine Benefits: What the Research Shows and What It Does Not, What Is GLP-1? The Hormone, How GLP-1 Drugs Work and Every FDA-Approved Option, B12 Benefits: What Vitamin B12 Does, Who Runs Low and Which Form.