Metformin is an old diabetes pill that doctors also use off-label for weight, prediabetes and PCOS
Metformin is a tablet approved by the FDA to treat type 2 diabetes. Physicians also prescribe it off-label for prediabetes, insulin resistance, polycystic ovary syndrome (PCOS) and, less often, weight. It is not a GLP-1 medicine, and its effect on weight is small. This page covers what the trials show, the real warnings, and when a doctor may consider it.
Information current as of November 10, 2026. Educational only. Confirm everything here with your own clinician before starting, stopping or changing any medication.
At a glance
Drug class
A biguanide. It lowers the glucose your liver releases and makes muscle and fat respond better to insulin. It is not a GLP-1 and not an injection.
How it is taken
A tablet by mouth with food, as immediate-release (usually twice a day) or extended-release (usually once with the evening meal). Doses are raised slowly to limit stomach upset.
Approval status
FDA-approved for type 2 diabetes in adults. Use for weight, prediabetes and PCOS is off-label, which means a physician decides it fits you.
Who it is for
People with type 2 diabetes. A physician may also consider it for prediabetes or PCOS with insulin resistance, or alongside other care.
Main cautions
Boxed warning for lactic acidosis. Not for severe kidney disease. Can lower vitamin B12. Often causes diarrhea and nausea at the start.
What it is
What metformin does in the body
Metformin has been used for diabetes for decades. It works mainly by cutting how much glucose the liver puts into the blood. It also helps tissues use insulin more effectively. It does not push the pancreas to make more insulin, so on its own it rarely causes low blood sugar.
The label says it is an adjunct to diet and exercise to improve blood sugar control in adults with type 2 diabetes. That is the approved use. Everything else on this page is off-label. Off-label is legal and common in medicine, but it means the FDA has not reviewed metformin for that purpose, and the evidence may be thinner.
Metformin is a different kind of drug from semaglutide or tirzepatide. Those mimic gut hormones and lower appetite strongly. Metformin does not act on appetite in the same way. If you want to compare the medicines Summit does prescribe, start with semaglutide and tirzepatide.
Who it is for
Who it is for, and who should not take it
People with type 2 diabetes are the people the label was written for. Beyond that, a physician may consider metformin in a few situations after reviewing your history and labs.
Prediabetes
Elevated fasting glucose or A1c, especially with a higher BMI, a family history, or a past pregnancy with gestational diabetes.
PCOS with insulin resistance
Used off-label to support cycle regularity and insulin sensitivity. Results vary, and it does not replace a full PCOS evaluation.
Weight alongside other care
Off-label and modest. Sometimes considered when a GLP-1 is not appropriate, not tolerated, or has been stopped.
Metformin is not for people with severe kidney impairment (eGFR below 30), people with acute or chronic metabolic acidosis, or anyone allergic to it. The label says not to start it when eGFR is between 30 and 45. It is also a poor fit for heavy alcohol use and for significant liver disease.
Honest caveat on weight
Metformin is not a weight-loss drug in the FDA sense. The average effect in studies is small, and some people lose nothing. A physician should not offer it as a replacement for a medicine that works much more strongly unless there is a reason.
How it is taken
Dosing and what the label says
For type 2 diabetes, the extended-release label starts at 500 mg once daily with the evening meal. The dose goes up by 500 mg each week as tolerated, to a maximum of 2,000 mg once daily. Immediate-release tablets are usually split across meals. Extended-release tablets must be swallowed whole.
Slow titration matters because the main side effects are in the gut. In the extended-release label’s diabetes trials, diarrhea affected 10 percent of people on metformin versus 3 percent on placebo, and nausea or vomiting 7 percent versus 2 percent. Diarrhea led to stopping in 0.6 percent. For many people the stomach effects fade after a few weeks.
The label does not give a dose for weight loss or PCOS, because those are not approved uses. Any dose used for them is a physician’s judgment.
The evidence
What the trials actually show
Preventing diabetes: the Diabetes Prevention Program
The Diabetes Prevention Program randomized 3,234 adults with elevated blood sugar to placebo, metformin 850 mg twice daily, or an intensive lifestyle program. Mean age was 51 and mean BMI was 34. Over an average of 2.8 years, diabetes developed in 11.0, 7.8 and 4.8 cases per 100 person-years in the placebo, metformin and lifestyle groups. Lifestyle change cut the risk by 58 percent and metformin by 31 percent versus placebo. Lifestyle was significantly more effective than metformin.
The follow-up study tracked 2,776 of those people for about 15 years. Diabetes incidence was 27 percent lower with lifestyle and 18 percent lower with metformin than with placebo, and the gaps narrowed over time. By year 15, cumulative diabetes was 55 percent, 56 percent and 62 percent in the lifestyle, metformin and placebo groups. Metformin delayed diabetes in many people. It did not prevent it in most.
Weight
A 2024 systematic review pooled 49 randomized trials of metformin in adults without diabetes who had overweight or obesity. Compared with placebo, BMI fell by an average of 0.56 points. There was no significant difference between metformin and lifestyle modification. For scale, a semaglutide 2.4 mg trial of 1,961 adults without diabetes reported a mean loss of 17.3 percent of body weight over 68 weeks. Those are different populations and designs and are not directly comparable, but the gap in size is large.
PCOS
Metformin is widely used in PCOS, mostly for insulin resistance and cycle regularity. It is off-label there too, and results differ from person to person. For how GLP-1 medicines compare in PCOS, and why timing around pregnancy matters, read our fertility and PCOS guide and semaglutide for women with PCOS.
Safety
Warnings, side effects and monitoring
Boxed warning: lactic acidosis
- Lactic acidosis is rare but can be fatal. The label says it can start subtly, with malaise, muscle aches, trouble breathing, sleepiness and abdominal pain.
- Risk goes up with kidney impairment, liver impairment, excessive alcohol intake, and some other drugs. Get urgent care if you have those symptoms, especially during a vomiting illness or dehydration.
- Kidney function. Your physician checks eGFR before you start and periodically after. The label says to stop if eGFR falls below 30, and to reassess the benefit and risk if it falls below 45.
- Contrast dye. Tell the imaging team you take metformin. The label says to stop it at or before iodinated contrast in people with eGFR 30 to 60, liver disease, alcoholism or heart failure, or with intra-arterial contrast, then recheck eGFR 48 hours later.
- Vitamin B12. In the Diabetes Prevention Program follow-up, low B12 (203 pg/mL or less) was found in 4.3 percent of metformin users versus 2.3 percent on placebo at 5 years, and each added year of use raised the odds of deficiency (odds ratio 1.13 per year). The label advises checking blood counts yearly and B12 every 2 to 3 years.
- Stomach effects. Diarrhea, nausea, gas and a metallic taste are the common ones.
- Interactions. Topiramate, zonisamide and acetazolamide can raise lactic acidosis risk. Cimetidine, dolutegravir, ranolazine and vandetanib can raise metformin levels. With insulin or a sulfonylurea, blood sugar can drop too low.
Because both can upset the stomach, metformin and a GLP-1 can stack nausea and diarrhea. A physician usually changes one thing at a time so the cause is clear.
If you stop
What happens if you stop metformin
Stopping metformin is simple. It does not cause withdrawal. Whatever blood sugar or weight effect it was giving usually fades over weeks to months. The Diabetes Prevention Program results show why: the benefit was measured while people were taking it.
If you are coming off a GLP-1, weight regain is the larger concern. After semaglutide 2.4 mg and lifestyle support were stopped in a 327-person extension study, people regained about two-thirds of the weight they had lost within a year. Metformin has not been shown to prevent that on its own. See how to stop Ozempic without regaining the weight and what regain looks like.
Summit’s role
Summit’s role with metformin
Metformin is not one of the medicines Summit’s programs are built around, and I cannot confirm from Summit’s documents that Summit prescribes it. Dr. Miranda reviews the medicines you already take, including metformin, and decides whether any add-on fits you after your history and labs.
Summit is telehealth only. It does not dispense brand-name products. If you already take metformin from your own doctor, tell us, and do not stop it on your own. Labs for kidney function and B12 are part of how that decision gets made.
Common questions
Questions people ask
Is metformin approved for weight loss?
No. The FDA approved metformin for type 2 diabetes. Doctors prescribe it off-label for weight, prediabetes and PCOS, and the average weight effect in trials is small.
Is metformin a GLP-1 medicine?
No. Metformin is a biguanide that lowers liver glucose output and improves insulin sensitivity. GLP-1 medicines such as semaglutide and tirzepatide are injections that act on gut-hormone receptors and lower appetite.
Does metformin cause vitamin B12 deficiency?
It can lower B12, and the risk rises with years of use. In a Diabetes Prevention Program follow-up, low B12 was more common with metformin at 5 years. The label advises checking B12 every 2 to 3 years.
Why is contrast dye a problem with metformin?
Contrast can injure the kidneys, and poor kidney function raises the risk of lactic acidosis. The label says metformin may need to be stopped at or before the scan in some people, with kidney function rechecked 48 hours afterward.
Can I take metformin if I am trying to get pregnant?
That is a question for your physician or OB team. Metformin is sometimes used in PCOS, but pregnancy planning changes which medicines are appropriate, and GLP-1 medicines must be stopped before conception.
Sources
Where this comes from
All retrieved November 10, 2026.
- Knowler WC et al. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med 2002. https://europepmc.org/article/MED/11832527
- Diabetes Prevention Program Research Group. Long-term effects of lifestyle intervention or metformin on diabetes development and microvascular complications over 15-year follow-up: the DPPOS. Lancet Diabetes Endocrinol 2015. https://europepmc.org/article/MED/26377054
- Aroda VR et al. Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab 2016. https://europepmc.org/article/MED/26900641
- Haber R et al. The impact of metformin on weight and metabolic parameters in patients with obesity: a systematic review and meta-analysis of randomized controlled trials. Diabetes Obes Metab 2024. https://europepmc.org/article/MED/38468148
- Wilding JPH et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: the STEP 1 trial extension. Diabetes Obes Metab 2022. https://europepmc.org/article/MED/35441470
- Metformin hydrochloride extended-release tablets, prescribing information. DailyMed, U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=356b02d0-d239-4441-bb33-c4c54166fcd7
Confirm with your provider. This page is general education, not medical advice, and it does not diagnose or treat anything. Every medication carries risks and is not right for everyone. Medication is prescribed only after a physician evaluation determines it is appropriate for you. Individual results vary and no outcome is guaranteed. Information current as of November 10, 2026.
Next step
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