Metformin vs. GLP-1s for Weight Loss: What the Trials Actually Show
For weight loss, GLP-1 medications are in a different league from metformin. In its largest long-term trial, metformin produced about 2 kg (roughly 2%) of weight loss, while semaglutide and tirzepatide produced 15% to 21% in their pivotal trials. Metformin's case rests on price, safety history, and durability, not on the size of the effect.
Key takeaways
- Metformin is FDA-approved only for type 2 diabetes; using it for weight loss is off-label. Wegovy, Zepbound, and Foundayo carry weight-management approvals.
- In the Diabetes Prevention Program, metformin users lost 2.1 kg on average over 2.8 years versus 0.1 kg on placebo; the loss held at about 2% over 10 years of follow-up.
- In STEP 1 and SURMOUNT-1, semaglutide and tirzepatide produced 14.9% and up to 20.9% average weight loss.
- Metformin costs under $10 a month with discount cards. Self-pay GLP-1s run roughly $149 to $449 a month as of October 2026.
The decision in one sentence
If your goal is meaningful weight loss (10% or more), the trial evidence points to a GLP-1. If your goal is a small, cheap, low-risk nudge, or you cannot get a GLP-1 covered or tolerated, metformin is a reasonable conversation to have with your prescriber. The rest of this article shows the numbers behind that split.
Head-to-head comparison
| Factor | Metformin | Semaglutide (Wegovy) | Tirzepatide (Zepbound) |
|---|---|---|---|
| FDA-approved for weight loss? | No (type 2 diabetes only) | Yes | Yes |
| Key trial weight loss | 2.1 kg vs. 0.1 kg placebo over ~2.8 years (DPP) | 14.9% vs. 2.4% placebo at 68 weeks (STEP 1) | 20.9% (15 mg) vs. 3.1% placebo at 72 weeks (SURMOUNT-1) |
| Durability | About 2.0% vs. 0.2% placebo held over 10 years | Weight tends to return after stopping | 14% regain within a year of switching to placebo (SURMOUNT-4) |
| How it's taken | Pill, once or twice daily | Weekly injection or daily pill | Weekly injection |
| Self-pay cost (Oct 2026) | About $7 to $10 with GoodRx | $349 injection; $149 to $299 pill | $299 to $449 with Lilly's savings card |
| Most common side effects | Diarrhea (53% vs. 12% placebo), nausea/vomiting (26% vs. 8%) | Nausea (44%), diarrhea (30%), vomiting (24%) | Mostly GI, concentrated during dose increases |
| Boxed warning | Lactic acidosis (rare) | Thyroid C-cell tumors in rodents | Same class warning |
Source: ACC DPP summary, DPPOS, Diabetes Care 2012, ACC STEP 1, ACC SURMOUNT-1, Consultant360 (STEP 2), JAMA (SURMOUNT-4), metformin and Wegovy labels, GoodRx and manufacturer pricing.
What the metformin trials actually show
The Diabetes Prevention Program
The best long-term metformin data come from the Diabetes Prevention Program (DPP), which enrolled 3,234 adults with prediabetes and an average BMI of 34. People taking metformin 850 mg twice daily lost an average of 2.1 kg, compared with 5.6 kg in the intensive lifestyle group and 0.1 kg on placebo over about 2.8 years, according to the American College of Cardiology's trial summary.
Ten years later
The follow-up study is where metformin looks better. Over 10 years, weight loss stayed significantly greater in the metformin group than the placebo group, about 2.0% versus 0.2%, and was tied to how consistently people took the drug (Diabetes Care, 2012). The authors concluded the weight loss was "durable for at least 10 years of treatment."
Other evidence
A 2023 review in the Cleveland Clinic Journal of Medicine pulled together the wider literature. A meta-analysis found BMI fell by about 1.3 units in people with obesity. In a meta-analysis of people with weight gain from antipsychotic medications, metformin was associated with about 3.3 kg of weight loss. The review notes that weight loss usually starts after about 4 weeks and mostly happens in the first 6 to 12 months.
What the GLP-1 trials show
In STEP 1, adults with obesity or overweight (without diabetes) on semaglutide 2.4 mg lost 14.9% of body weight at 68 weeks versus 2.4% on placebo, and 86.4% lost at least 5% (ACC). In SURMOUNT-1, tirzepatide produced 15%, 19.5%, and 20.9% weight loss at the 5, 10, and 15 mg doses (ACC).
People with type 2 diabetes usually lose less on any weight drug. In STEP 2, semaglutide 2.4 mg produced 9.6% weight loss versus 3.4% on placebo (Consultant360). That is still several times what metformin achieved in the DPP.
Thinking past metformin?
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The durability question cuts both ways
Metformin's small effect is unusually stable. GLP-1 weight loss is much larger, but it depends on staying on the drug. In SURMOUNT-4, people who lost weight on tirzepatide for 36 weeks and were then switched to placebo regained 14% of body weight over the next year, while those who continued lost another 5.5% (JAMA, 2024).
Can metformin hold onto GLP-1 weight loss after stopping? The evidence is thin. One small study of 25 women with PCOS who stopped semaglutide but stayed on metformin found they regained about a third of the lost weight over two years. It had no control group, so it cannot show metformin caused the better-than-expected result (Frontiers in Endocrinology, 2024).
Side effects and safety
Both drugs mainly cause GI symptoms, but the patterns differ.
- Metformin: In the label's main U.S. trial, diarrhea affected 53% of users versus 12% on placebo, and nausea or vomiting 26% versus 8% (metformin prescribing information). The CCJM review notes taking it after meals or using extended-release forms reduces these. About 7% of users developed low vitamin B12 in 29-week trials, and the label carries a boxed warning about lactic acidosis, a rare but serious risk mostly tied to kidney impairment and other risk factors.
- GLP-1s: On the Wegovy label, nausea affected 44% of users (16% on placebo), diarrhea 30%, vomiting 24%, and constipation 24%. Warnings include pancreatitis, gallbladder disease, and a boxed warning about thyroid C-cell tumors seen in rodents.
Cost and coverage
Metformin is one of the cheapest prescription drugs available. GoodRx lists a lowest price of about $7.32 against an average retail price of about $26 (GoodRx). Because it is a long-established generic, it is far easier to afford without insurance than any GLP-1.
GLP-1 self-pay prices have fallen but remain far higher. Wegovy injections are $349 a month and the Wegovy pill $149 to $299 by dose (Fierce Pharma). Zepbound runs $299 to $449 with Lilly's self-pay card (Lilly). Eligible Medicare beneficiaries can get certain GLP-1s for $50 a month through the Medicare GLP-1 Bridge, which began July 1, 2026.
If you already have type 2 diabetes
This is less of an either/or. Metformin remains a common diabetes medication, and the two are often prescribed together. For weight management specifically in type 2 diabetes, the American Diabetes Association's 2026 Standards of Care name GLP-1 and dual GLP-1/GIP drugs as the preferred agents, as summarized by Rosalind Franklin University's HelixTalk. Ask your prescriber whether adding or switching makes sense for your glucose and weight goals.
Who each option tends to fit
Metformin may make sense if
- You have prediabetes or insulin resistance and want a cheap, well-studied first step.
- You are gaining weight on antipsychotic medication, where trials show a measurable benefit.
- You have no GLP-1 coverage and cannot sustain $150 to $450 a month.
- A few percent of body weight is enough to meet your goal.
A GLP-1 may make sense if
- You need 10% to 20% weight loss to improve a condition like sleep apnea, joint pain, or cardiovascular risk.
- You already tried metformin and the scale barely moved.
- You qualify for insurance coverage or the Medicare Bridge.
- You are prepared to treat it as a long-term medication, given the regain seen after stopping.
The verdict
On weight loss alone, GLP-1s win decisively: roughly 10% to 21% average loss in their main trials versus about 2% for metformin. Metformin's strengths are real but different. It costs a few dollars a month, has decades of safety data, and its small effect has lasted at least 10 years in people who keep taking it. Choose metformin when cost, access, or a modest goal drives the decision. Choose a GLP-1 when you need substantial weight loss and can sustain the medication. If you have type 2 diabetes, the answer may be both, and that is a question for your prescriber.
See what a GLP-1 would cost you
Check providers' current self-pay pricing side by side.
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Frequently asked questions
How much weight do people lose on metformin?
In the Diabetes Prevention Program, people on metformin lost an average of 2.1 kg over about 2.8 years, versus 0.1 kg on placebo. Over 10 years of follow-up, weight loss held at about 2.0% versus 0.2% on placebo.
Is metformin FDA-approved for weight loss?
No. Metformin is approved for type 2 diabetes, so using it for weight loss is off-label. Wegovy, Zepbound, and Foundayo carry FDA approvals for weight management.
Is a GLP-1 better than metformin for weight loss?
For the amount of weight lost, yes. Semaglutide produced 14.9% average weight loss in STEP 1 and tirzepatide up to 20.9% in SURMOUNT-1, compared with roughly 2% for metformin in its largest long-term trial.
Can you take metformin and a GLP-1 together?
They are often prescribed together in people with type 2 diabetes. Whether combining them makes sense for you is a decision for your prescriber.
Does metformin prevent weight regain after stopping a GLP-1?
The evidence is limited. One small uncontrolled study of 25 women with PCOS who stayed on metformin after stopping semaglutide found they regained about a third of the lost weight over two years, but without a control group it cannot show metformin caused that result.
Sources
- Diabetes Prevention Program trial summary, American College of Cardiology
- Long-term safety, tolerability, and weight loss associated with metformin in the DPPOS, Diabetes Care (2012)
- Should I consider metformin therapy for weight loss in patients with obesity but without diabetes? Cleveland Clinic Journal of Medicine (2023)
- STEP 1 trial summary, American College of Cardiology
- SURMOUNT-1 trial summary, American College of Cardiology
- Semaglutide improves weight loss in patients with diabetes (STEP 2), Consultant360
- Continued tirzepatide for maintenance of weight reduction (SURMOUNT-4), JAMA (2024)
- Jensterle et al. Weight after semaglutide withdrawal with continued metformin in PCOS, Frontiers in Endocrinology (2024)
- Metformin prescribing information, Drugs.com
- Wegovy prescribing information, FDA (2025)
- Metformin prices, GoodRx
- Wegovy pill launches at $149 per month, Fierce Pharma
- Zepbound coverage and savings, Eli Lilly
- Medicare GLP-1 Bridge: GLP-1 drugs for $50 a month, Medicare.gov
- HelixTalk 197: Updates from the ADA 2026 Standards of Care, Rosalind Franklin University
Medical disclaimer: This article is general information, not medical advice, and does not replace a licensed clinician who knows your history. Approvals, labels, coverage and prices change; check the linked sources and talk with your prescriber or pharmacist before making decisions about any medication.