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Guide 2 of 9 · Stage: Understand

What do GLP-1 medicines do?

How semaglutide (Wegovy, Ozempic) and tirzepatide (Zepbound, Mounjaro) work in your body, what they can and can't do, the risks, and what the FDA has approved.

GLP Guidebook teamUpdated Oct 12, 202613 min read41 sourcesWritten from published sources
The short answer

GLP-1 stands for glucagon-like peptide-1, a hormone your gut makes after you eat. GLP-1 medicines copy it. They help you feel full sooner, slow your stomach and help control blood sugar. Most weight loss comes from eating less. Common side effects are nausea and diarrhea, and weight usually returns after stopping. Talk to a doctor before starting, stopping or changing one.

Key facts

What they are
Medicines that copy GLP-1, a gut hormone1
Main ones in the US
  • Semaglutide: Wegovy (injection and pill), Ozempic (injection and tablets), Rybelsus10,29,30
  • Tirzepatide: Zepbound, Mounjaro4,11
  • Orforglipron: Foundayo, a daily pill, since April 20267,34
  • Liraglutide: Saxenda, Victoza and generic versions5,6,31
FDA-approved for
  • Type 2 diabetes: Ozempic, Rybelsus, Mounjaro, Trulicity, Victoza
  • Weight management: Wegovy, Zepbound, Foundayo, Saxenda29,30,11,8,10,4,7,5
How weight loss happens
Mostly by eating less. They don't speed up the calories you burn at rest.9
Main risks
Nausea, diarrhea and vomiting are common. Rarer serious risks include pancreatitis and gallbladder problems. See the risks, in brief10
Prescription
Yes, all of them need a prescription. Compounded versions sold online are not FDA-approved.2
On this page
  1. What does GLP-1 mean?
  2. What do the medicines do in your body?
  3. Why do people lose weight on them?
  4. Do the results hold for people of different ethnic backgrounds?
  5. How is this different from metformin and other diabetes pills?
  6. Who these are for, and who they're not for
  7. Which GLP-1 medicines are FDA-approved?
  8. Is Ozempic the same as Wegovy?
  9. How long do they take to work, and how long do they last?
  10. Do they do anything besides weight and sugar?
  11. What these medicines do not do
  12. Are these medicines new?
  13. Questions people ask

What does GLP-1 mean?

GLP-1 is short for glucagon-like peptide-1. It's a hormone your gut releases after you eat, telling your body that food has arrived.

It's made mostly in the lower part of your small intestine and in your large intestine.1 It sends signals to your brain, stomach and pancreas (an organ behind your stomach that makes hormones).

Your body's own GLP-1 doesn't last long. An enzyme (a protein that breaks other things down) clears it in about 1 to 2 minutes.1 So its natural effect is small and short.

What do GLP-1 medicines do in your body?

A GLP-1 medicine is a lab-made version of GLP-1, changed so your body can't break it down quickly. Your own GLP-1 is gone in 1 to 2 minutes. Semaglutide, the medicine in Wegovy and Ozempic, keeps working for about a week.1,10

So the "food has arrived" message that your gut normally sends for a few minutes after a meal is sent all day, every day, and more strongly.

Your own GLP-1Made by your gut after you eatGone in 1 to 2 minutes
A GLP-1 medicineA weekly injection or a daily pillKeeps sending the message all week

The message reaches four parts of your body

BrainYou feel less hungryThe part of your brain that controls appetite is told you've eaten, so you want less food and feel full sooner.Helps with weight
StomachYou stay full for longerYour stomach empties more slowly, so a meal stays with you longer before you feel hungry again.Helps with weight
PancreasSugar leaves your blood fasterYour pancreas releases more insulin, the hormone that moves sugar out of your blood. It does this only when your sugar is high.Helps with blood sugar
LiverLess extra sugar is addedYour liver stores sugar and releases it when a hormone called glucagon tells it to. The medicine lowers glucagon when your sugar is high, so the liver adds less.Helps with blood sugar

Simplified. Based on a scientific review of GLP-1 and the US Wegovy label.1,10

In short: the brain and stomach effects are why people eat less and lose weight. The pancreas and liver effects are why these medicines lower blood sugar in type 2 diabetes.

Because the pancreas and liver effects only switch on when sugar is high, the medicine mostly leaves normal sugar levels alone. Sugar can still drop too low if it's taken with insulin or certain other diabetes medicines.10

Why do people lose weight on them?

Mainly because they eat less. In a small study, semaglutide did not change the calories people burned at rest.9

That study gave 30 people semaglutide 1 mg or a dummy injection (one with no medicine in it) for 12 weeks each. On semaglutide, they ate 24% less across a full day of eat-as-much-as-you-like meals.9 They also reported less hunger and fewer food cravings.9

The two biggest weight trials are called STEP 1 (semaglutide, made by Novo Nordisk) and SURMOUNT-1 (tirzepatide, made by Eli Lilly). Each one worked the same way:

  • People were split at random into two groups. One group got the medicine. The other got a dummy injection.
  • Both groups got the same diet and exercise advice, and nobody knew which injection they were on.
  • So the gap between the two groups is what the medicine itself added.14,15

Average weight lost. Pounds are for someone who starts at 200 lb (91 kg).

STEP 1Semaglutide 2.4 mg (Wegovy), weekly

1,961 adults with obesity or overweight, without diabetes. 68 weeks.14

Semaglutide14.9%about 30 lb
Dummy injection2.4%about 5 lb
SURMOUNT-1Tirzepatide 15 mg (Zepbound), weekly

2,539 adults with obesity or overweight, without diabetes. 72 weeks.15

Tirzepatide20.9%about 42 lb
Dummy injection3.1%about 6 lb

Which works better? You can't tell by comparing the two cards above, because they were different trials with different people. One trial, SURMOUNT-5, tested the two medicines against each other in the same group of people. Each person took the highest dose they could manage. It was paid for by Eli Lilly, the maker of tirzepatide.16 Tirzepatide also acts on a second gut hormone, called GIP (glucose-dependent insulinotropic polypeptide), which may help explain the difference.4

SURMOUNT-5Tirzepatide vs semaglutide, head to head

751 adults with obesity, without diabetes. 72 weeks.16

Tirzepatide20.2%about 40 lb
Semaglutide13.7%about 27 lb

People with type 2 diabetes usually lose less. STEP 2 ran the same test in people with diabetes: 9.6% lost instead of 14.9%, on the same dose.14,17

STEP 2Semaglutide 2.4 mg, people with diabetes

1,210 adults with overweight or obesity and type 2 diabetes. 68 weeks.17

Semaglutide9.6%about 19 lb
Dummy injection3.4%about 7 lb

All of these are averages. Some people lose much more, and some lose very little.14

RisksThe risks, in brief
44 in 100had nausea on Wegovy, against 16 in 100 on a dummy injection (US label)10
24 in 100vomited on Wegovy, against 6 in 100 on a dummy injection (US label)10
  • Pancreatitis (a swollen, painful pancreas) has been seen. Severe stomach pain that may spread to the back is a warning sign.10
  • Gallbladder problems, such as gallstones, are more common.10
  • Dehydration from vomiting or diarrhea can hurt the kidneys.10
  • Thyroid warning. Semaglutide caused thyroid tumors in rodents. The US label's boxed warning (its most serious kind) says people with a personal or family history of medullary thyroid cancer, or MEN 2 (a rare inherited condition that causes gland tumors), must not use it.10
  • Not in pregnancy. The Wegovy label says to stop at least 2 months before a planned pregnancy.10
  • Report side effects to the FDA's MedWatch program, online or at 1-800-FDA-1088.18
Full detail in Guide 5: Side effects and risks →
Weight usually comes back when you stop. In a STEP 1 follow-up, people had lost 17.3% on semaglutide. A year after stopping, they had regained about two-thirds of it, on average.19 More in Guide 9: Stopping

Not all the weight lost is fat. A scan study inside STEP 1 looked at 140 people. On semaglutide, fat fell by about 19% and lean mass by about 10%.20 In other words, people lost about a fifth of their fat and about a tenth of their lean mass. Lean mass means everything that isn't fat. It includes muscle, but also organs, bone and water.21 Guide 8 covers eating and exercise on these medicines.

Do the results hold for people of different ethnic backgrounds?

Trials in East Asian people show similar weight loss. None of the trials on this page was run specifically in South Asian, Black or Hispanic people.

  • Japan and South Korea (401 people, STEP 6): 13.2% of body weight lost on semaglutide 2.4 mg over 68 weeks, against 2.1% on a dummy injection.22
  • China (210 people, SURMOUNT-CN): 13.6% of body weight lost on tirzepatide 10 mg and 17.5% on 15 mg over 52 weeks, against 2.3% on a dummy injection.23
  • Japan (225 people analyzed, SURMOUNT-J): people on tirzepatide lost about 16% (10 mg) and 21% (15 mg) more of their body weight than people on a dummy injection, over 72 weeks.24

In England, South Asian people at a BMI of about 24 have the same diabetes risk as white people at 30.25 In the US, the USPSTF suggests diabetes screening for Asian Americans from a BMI of 23, and the ADA uses 27.5 in place of 30 for Asian Americans in some diabetes advice.26,12 The FDA labels for weight medicines don't lower their BMI numbers for any group.10,4 Guide 1 explains these numbers.

How is this different from metformin and other diabetes pills?

GLP-1 medicines lower blood sugar and also reduce appetite. For people with type 2 diabetes and overweight or obesity, the American Diabetes Association now prefers semaglutide or tirzepatide when a medicine for weight is added.12

Metformin mainly stops your liver releasing too much sugar and helps your body use insulin better. It doesn't make your pancreas release more insulin.28 The ADA's 2026 Standards of Care say that when choosing diabetes medicines for people with overweight or obesity, doctors should favor ones that help with weight.12

HbA1c (A1C) is a blood test showing your average sugar over the past few months. In a 30-week trial of Ozempic on its own, HbA1c started around 8.1%. It fell by 1.4 to 1.6 percentage points, against 0.1 on a dummy injection.29 That means from about 8.1% to about 6.5–6.7%.

Watch for low blood sugar if you take other diabetes medicines. The risk goes up when GLP-1s are combined with insulin or a sulfonylurea (an older type of diabetes pill that makes the pancreas release insulin).10,29 Doctors may lower the dose of these when starting a GLP-1.10

The type 2 diabetes path covers this in more detail: I have type 2 diabetes.

Who these are for, and who they're not for

In the US, the FDA approves each brand for set uses, written in its label (the official prescribing information). Whether one suits you is a decision for you and a doctor.

  • Type 2 diabetes: Ozempic (injection and tablets), Rybelsus, Mounjaro, Trulicity and Victoza.29,30,11,8
  • Weight management: Wegovy (injection and pill), Zepbound, Foundayo, Saxenda and generic liraglutide that copies Saxenda.10,4,7,5,31
  • Teenagers: Wegovy injection is approved from age 12 for young people with obesity. Saxenda is approved from 12 for those with obesity who weigh more than 132 lb (60 kg).10,5
  • Heart protection: since March 2024, Wegovy is approved to lower the risk of heart attack, stroke and heart-related death in adults with heart or blood-vessel disease and obesity or overweight.32,10
  • Sleep apnea: since December 2024, Zepbound is approved for moderate to severe obstructive sleep apnea in adults with obesity.33,4
  • Fatty liver with scarring (MASH): Wegovy injection was approved for adults with this in August 2025, under accelerated approval (the FDA can approve early on promising results while a longer trial confirms the benefit).10,35

The label rules for weight are a BMI of 30 or more, or 27 to under 30 with at least one weight-related condition such as high blood pressure.4,10 Insurance plans often add their own, stricter rules before they pay. Guide 6 explains coverage. These are points to start a conversation with a doctor, not a sign that you qualify.

Not tested in type 1 diabetes. The US Wegovy label says its use in people with type 1 diabetes has not been studied.10

Tirzepatide and the birth control pill. Tirzepatide (Zepbound, Mounjaro) can make the pill work less well. Its label says to switch to a non-pill method, or add a barrier method such as condoms, for 4 weeks after starting and for 4 weeks after each dose increase. The Foundayo label gives the same advice for 30 days.4,7

They're not for losing a few pounds before a vacation or event. The labels approve them only for people who meet the BMI rules above, alongside diet and activity changes.10,4 They're long-term treatments, and weight tends to come back after stopping.19,36 If that's you, read I want to lose a few pounds.

Who shouldn't take them, and what to ask, is in Guide 4: Should I consider a GLP-1?

Which GLP-1 medicines are FDA-approved?

Every GLP-1 medicine has two kinds of name: the name of the medicine itself, and the brand names it's sold under. Knowing the difference stops a lot of confusion.

The medicineWhat actually works in your bodyThe same chemical, whoever makes it. Doctors and labels use this name.
e.g. Acetaminophen
The brandThe name on the boxChosen by the company selling it. One medicine can have many brands.
e.g. Tylenol

GLP-1 medicines work the same way. Semaglutide is the medicine, and Wegovy, Ozempic and Rybelsus are brands of it. Two brands of the same medicine can still differ in dose, in whether they're an injection or a pill, and in what they're approved for.

A generic is a copy made by another company once the patent ends, and approved by the FDA. In the US, liraglutide is the only one of these medicines with generic versions: one copying Victoza (for diabetes) since December 2024, and one copying Saxenda (for weight) since August 2025.6,31 There is no FDA-approved generic semaglutide or tirzepatide. "Compounded" versions are different: they are mixed by pharmacies and are not FDA-approved.2

The GLP-1 medicines approved in the US, and their brands

MedicineSemaglutide →Weekly injection, or a daily pill
  • Wegovy injectionNovo Nordisk · weekly pen, including Wegovy HD 7.2 mgFor weight, heart protection and MASH10
  • Wegovy pillNovo Nordisk · daily tablet, approved December 2025For weight and heart protection10,37
  • OzempicNovo Nordisk · weekly injection, and tablets since 2026For type 2 diabetes, heart and kidney protection in diabetes29,30
  • RybelsusNovo Nordisk · daily tabletFor type 2 diabetes30
MedicineTirzepatide →Weekly injection
  • ZepboundEli Lilly · weekly pen, vial or KwikPenFor weight and sleep apnea4
  • MounjaroEli Lilly · weekly injectionFor type 2 diabetes11
MedicineOrforglipronDaily pill
  • FoundayoEli Lilly · daily tablet, approved April 2026. Taken with or without food.For weight management7,34
MedicineDulaglutide →Weekly injection
  • TrulicityEli Lilly · weekly injectionFor type 2 diabetes8
MedicineLiraglutide →Daily injection
  • SaxendaNovo Nordisk · daily injectionFor weight management5
  • VictozaNovo Nordisk · daily injectionFor type 2 diabetes6
  • Generic liraglutideFDA-approved copies of Victoza (diabetes) and Saxenda (weight).6,31

Names only, not a recommendation. Doses, costs and how to get them are in Guide 6.

US note: compounded versions and fakes. Compounded semaglutide and tirzepatide are not FDA-approved: the FDA doesn't review their safety, effectiveness or quality before they're sold.2 Fake Ozempic pens have also been found in the US supply chain; the FDA warned about them in 2023 and again in April and December 2025.3 Guide 6 covers safe ways to get them.

Is Ozempic the same as Wegovy?

Same medicine, different brand. Both are semaglutide, made by Novo Nordisk. They are approved for different uses and go up to different doses.

  • Ozempic is approved for type 2 diabetes, including heart and kidney protection in people with diabetes. Its highest dose is 2 mg a week.29
  • Wegovy is approved for weight management, heart protection and MASH. Its usual full dose is 2.4 mg a week, and 7.2 mg (Wegovy HD) is allowed for some adults.10

Using a medicine for something its label doesn't cover is called "off-label". The FDA says doctors may generally prescribe an approved medicine for an unapproved use when they judge it medically appropriate for their patient.41 Wegovy is the semaglutide brand approved for weight.10

Zepbound and Mounjaro are another pair: both are tirzepatide, made by Eli Lilly. Zepbound is for weight and sleep apnea, Mounjaro for type 2 diabetes.4,11

How long do they take to work, and how long do they last?

You start on a low dose and go up slowly, usually every 4 weeks. Weight loss builds over many months.10,14

In trials, stomach side effects happened mostly while the dose was still going up.15,16

Your prescriber sets your dose. This is the US label schedule for Wegovy injections for weight, shown so you know what to expect.10
  1. Weeks 1–40.25 mg once a week
  2. Weeks 5–80.5 mg
  3. Weeks 9–121 mg
  4. Weeks 13–161.7 mg
  5. Week 17 on2.4 mg, the recommended dose (1.7 mg is also allowed)
  6. If needed7.2 mg (Wegovy HD), for adults who have managed 2.4 mg for at least 4 weeks

The Wegovy pill follows the same idea: 1.5 mg a day for 30 days, then up each month to 25 mg.10 In STEP 1, average weight kept falling until about week 60, then leveled off.14 Unlike Saxenda's label, which says to stop if you haven't lost 4% after 16 weeks, the Wegovy and Zepbound labels set no fixed cut-off; your prescriber weighs your response.5,10,4

Why once a week, and how long it stays in your body

Your body clears semaglutide slowly. About half of each dose is gone after a week, so one injection a week keeps enough in your blood.1,10

The same slow clearing means the medicine doesn't leave your body the day you stop taking it. After your last dose, it takes about 5 weeks to leave your blood.29

This matters most if you're planning a pregnancy. The Wegovy label says to stop at least 2 months before a planned pregnancy, because of how long semaglutide stays in the body.10

Do they do anything besides weight and sugar?

In one large trial, semaglutide lowered the combined risk of heart attack, stroke and heart-related death. Most of that came from fewer heart attacks.

The SELECT trial followed 17,604 people for about 3 years.38 All were 45 or older, had overweight or obesity, and no diabetes. All already had heart or blood-vessel disease: a past heart attack, a past stroke, or narrowed leg arteries.10

6.5 in 100had a heart attack, stroke or heart-related death on semaglutide38
8.0 in 100did on a dummy injection38

That's about 1 fewer event for every 67 people treated. Heart attacks fell from 3.8 to 2.8 in 100. Strokes went from 2.0 to 1.8 in 100, a difference too small to be sure of.10

The FDA approved Wegovy for this in March 2024.32 It applies to people who already have that disease, not to everyone. Medicare can't cover medicines used for weight loss alone, but this is a separate approved use, which Medicare drug plans can cover.39

Tirzepatide (Zepbound) is approved for obstructive sleep apnea in adults with obesity, and Wegovy for fatty liver with scarring (MASH).33,10

What these medicines do not do

  • They're not a cure. On average, people regained two-thirds of the lost weight within a year of stopping.19
  • They don't burn fat. They work by cutting how much you eat, not by speeding up your body.9
  • They're not a short course. WHO treats obesity as a long-term condition needing lifelong care.36
  • They don't replace food changes. Every FDA weight label approves them alongside a reduced-calorie diet and more physical activity.10,4,7
  • They're not for cosmetic weight loss. The labels cover people with obesity, or overweight with a weight-related condition.10,4

Are these medicines new?

No. The first GLP-1 medicine was approved in the US in 2005. What's new is how strong and long-lasting they've become, and that some now come as pills.40,37

The first one, exenatide, came from a substance found in the saliva of a lizard, the Gila monster.1 Later versions were changed to last longer in the body. Half of each semaglutide dose is cleared in about a week.1,10

  1. 2005Exenatide (Byetta) approved in the US40
  2. Jun 2021Wegovy approved for weight management13
  3. Nov 2023Zepbound (tirzepatide) approved for weight management27
  4. Mar 2024Wegovy approved for heart protection32
  5. Dec 2024Zepbound approved for sleep apnea; first generic liraglutide33,6
  6. Aug 2025Wegovy approved for MASH; first generic for weight (liraglutide)35,31
  7. Dec 2025Wegovy pill approved37
  8. Mar 2026Wegovy HD 7.2 mg approved10
  9. Apr 2026Foundayo (orforglipron) approved34

Questions people ask

What does GLP-1 stand for?

Glucagon-like peptide-1. It's a hormone your gut makes after you eat.1

Can I get Ozempic for weight loss?

Ozempic is FDA-approved for type 2 diabetes, not weight loss.29 A doctor may prescribe it off-label (for a use the label doesn't cover).41 Wegovy is the same medicine, semaglutide, approved for weight management.10 See Ozempic vs Wegovy.

Is a GLP-1 medicine the same as insulin?

No. Insulin lowers blood sugar directly. GLP-1 medicines help your body release its own insulin when sugar is high, and they also reduce appetite.1

Why is it an injection once a week?

Half of each semaglutide dose is cleared in about a week, which is why it's taken once a week.1,10 Your own GLP-1 lasts 1 to 2 minutes.1 The Wegovy pill and Foundayo are daily pills instead.10,7

Is compounded semaglutide the same as Wegovy or Ozempic?

No. Compounded drugs are not FDA-approved, and the FDA does not review their safety, effectiveness or quality. They are also not the same as generics.2 See Guide 6.

Do I need a prescription?

Yes. All of these medicines need a prescription from a licensed prescriber, such as a doctor, nurse practitioner or physician assistant, depending on your state's rules.10 Guide 6 explains the routes, including insurance and Medicare.

Before you move on

  • GLP-1 medicines mostly work by making you eat less.
  • Wegovy and Ozempic are both semaglutide; Zepbound and Mounjaro are both tirzepatide.
  • Stomach side effects are common, and there are rarer serious risks.
  • They're long-term. Weight usually returns after stopping.
  • They need a prescription. Compounded versions are not FDA-approved.
Side effects and risksShould I consider a GLP-1?I have type 2 diabetes
Was this guide easy to understand?

Sources

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  4. Eli Lilly. Zepbound (tirzepatide) US prescribing information, revised August 2026 (indications, dose schedule, oral contraceptives). uspl.lilly.com
  5. Novo Nordisk. Saxenda (liraglutide) US prescribing information, revised February 2026 (ages 12 and up weighing more than 60 kg; stop if under 4% loss at 16 weeks). novo-pi.com
  6. FDA. FDA approves first generic of once-daily GLP-1 injection to lower blood sugar in patients with type 2 diabetes (generic liraglutide, Victoza reference). December 23, 2024. fda.gov
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  33. FDA. FDA approves first medication for obstructive sleep apnea. December 20, 2024. fda.gov
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  35. HCPLive. FDA approves semaglutide (Wegovy) injection 2.4 mg for noncirrhotic MASH. August 15, 2025. Trade press. hcplive.com
  36. World Health Organization. WHO issues global guideline on the use of GLP-1 medicines in treating obesity. December 1, 2025. WHO
  37. Novo Nordisk. Wegovy pill approved in the US as first oral GLP-1 for weight management. December 22, 2025. Company announcement. placera.se
  38. Lincoff AM et al. Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT). NEJM, 2023;389:2221–32. doi:10.1056/NEJMoa2307563
  39. KFF. Nearly four million Medicare beneficiaries met the eligibility criteria in 2023 for the Medicare GLP-1 Bridge (the Bridge "does not change the current statutory prohibition on Medicare coverage of drugs used for weight loss"; people with type 2 diabetes, sleep apnea or MASH can get GLP-1 coverage under Part D). June 29, 2026. kff.org
  40. US Food and Drug Administration. Byetta (exenatide) NDA 021773, approved April 28, 2005. 2005 label (FDA)
  41. FDA. Understanding unapproved use of approved drugs "off label" ("healthcare providers generally may prescribe the drug for an unapproved use when they judge that it is medically appropriate for their patient"). Content current as of February 5, 2018. fda.gov

Last checked Oct 12, 2026 · Next review due Jan 2027 · Report an error

How this guide was made. Written by the GLP Guidebook team from the sources above: published trials, FDA labels, FDA and ADA guidance and named reports. It's general information, not medical advice. Talk to your doctor or another qualified prescriber before starting, stopping or changing any medicine. In an emergency, call 911. Report suspected side effects to the FDA's MedWatch program (1-800-FDA-1088). How we write