Few medicines have changed a conversation as fast as Ozempic, Wegovy and Mounjaro. A few years ago, a drug that took 15–20% off someone’s body weight sounded like a fantasy. Now these weekly injections are prescribed to millions of people, discussed on every social feed, and sold, sometimes illegally, online. So what do they actually do, how well do they work, what are the side effects, and what happens when you stop?
This article goes through the large randomized trials behind these drugs, in plain language. It’s part of our complete guide to how to lose weight, based on the evidence, which covers diet, exercise, sleep and medication in one place.
This article is for general information and isn’t medical advice. These are prescription medicines. Whether one is right for you depends on your health, your other medicines and your history, so talk to your doctor before starting, stopping or changing one.
In this article:
- Ozempic, Wegovy, Mounjaro, Zepbound: What’s the Difference?
- How GLP-1 Drugs Cause Weight Loss
- How Much Weight People Lose in the Trials
- Benefits Beyond the Scale
- Side Effects and Risks
- What Happens When You Stop
- Compounded and Online Versions
- If You Are Considering One
- Frequently Asked Questions
- Key Takeaways
Ozempic, Wegovy, Mounjaro, Zepbound: What’s the Difference?
The brand names are confusing, but there are really only two medicines behind them:
- Semaglutide is made by Novo Nordisk. It’s sold as Ozempic for type 2 diabetes and as Wegovy, at a higher maximum dose (2.4 mg weekly), for weight management.
- Tirzepatide is made by Eli Lilly. It’s sold as Mounjaro for type 2 diabetes (approved by the US FDA in May 2022) and as Zepbound for weight management (approved in November 2023).
So when people say they’re “on Ozempic for weight loss”, they often mean semaglutide in some form. The weight-management versions are licensed for adults with a BMI of 30 or more, or 27 or more with a weight-related condition such as high blood pressure or type 2 diabetes. They’re meant to be used alongside a reduced-calorie diet and more physical activity, not instead of them.
How GLP-1 Drugs Cause Weight Loss
GLP-1 is a hormone your gut releases after you eat. It tells your pancreas to release insulin, slows the rate your stomach empties, and acts on the brain to reduce appetite. Semaglutide mimics GLP-1 but lasts far longer, which is why it can be injected once a week. Tirzepatide acts on GLP-1 receptors and also on the receptor for a second gut hormone, GIP.
The result is that people feel full sooner and think about food less, so they eat less. There’s no hidden fat-burning trick. These drugs work through the same energy balance as every other approach, which is covered in our main weight-loss guide. They just make eating less much easier for many people.
How Much Weight People Lose in the Trials
Semaglutide: the STEP 1 trial
In STEP 1 (Wilding and colleagues, New England Journal of Medicine, 2021), 1,961 adults with obesity or overweight but without diabetes were randomly assigned to weekly semaglutide 2.4 mg or placebo, both with lifestyle support. After 68 weeks, the semaglutide group had lost an average of 14.9% of their body weight, compared with 2.4% on placebo.
Tirzepatide: the SURMOUNT-1 trial
In SURMOUNT-1 (Jastreboff and colleagues, NEJM, 2022), 2,539 adults without diabetes were given one of three tirzepatide doses or placebo for 72 weeks. People on the highest dose (15 mg) lost an average of 20.9% of their body weight, compared with 3.1% on placebo.
Head to head: SURMOUNT-5
The two drugs were compared directly in SURMOUNT-5 (Aronne and colleagues, NEJM, 2025). The trial randomly assigned 751 adults with obesity but without diabetes to the highest tolerated dose of either tirzepatide or semaglutide for 72 weeks. Average weight loss was 20.2% with tirzepatide and 13.7% with semaglutide. The trial was funded by Eli Lilly, which makes tirzepatide.
Remember that these are averages. Some people in every trial lost far more, and some lost very little. And for comparison, a well-run diet and exercise program typically produces losses in the 5–10% range, which is still enough to bring real health benefits.
Benefits Beyond the Scale
The biggest question about any weight-loss drug is whether it actually helps people live longer and healthier, not just weigh less. For semaglutide, the clearest answer so far comes from the SELECT trial (Lincoff and colleagues, NEJM, 2023).
SELECT enrolled 17,604 adults who had overweight or obesity and existing cardiovascular disease, but not diabetes. Over an average follow-up of about 40 months, a heart attack, stroke or cardiovascular death occurred in 6.5% of the semaglutide group and 8.0% of the placebo group, a 20% relative reduction in risk. People on semaglutide lost about 9.4% of their weight on average.
Two caveats: the trial only included people who already had heart disease, so it doesn’t tell us about people without it, and it was funded by Novo Nordisk. For more on reducing heart risk in other ways, see our guide to heart health.
Side Effects and Risks
Digestive side effects are common
In STEP 1, 74.2% of people on semaglutide reported a gastrointestinal side effect, compared with 47.9% on placebo. Nausea alone affected 44.2% on semaglutide versus 9.8% on placebo. Diarrhoea, vomiting and constipation were also common. These effects were usually mild to moderate, tended to happen while the dose was being increased, and eased over time. Even so, 4.5% of the semaglutide group stopped treatment because of digestive side effects, compared with 0.8% on placebo.
Tirzepatide’s pattern was similar. In SURMOUNT-1, nausea affected roughly a quarter to a third of people depending on dose, and between 4.3% and 7.1% of people on tirzepatide stopped because of side effects, compared with 2.6% on placebo.
Muscle loss
When you lose weight by any method, some of what you lose is lean tissue, including muscle, not just fat. A body-composition substudy of STEP 1 scanned about 140 participants. Their lean mass fell along with their fat, but fat fell by more, so lean mass made up a larger share of their body afterwards. Published analyses disagree on exactly what fraction of the weight lost was lean tissue, so treat any single headline percentage with caution.
The practical point is the same one that applies to any diet: eat enough protein and do some strength training while losing weight, to protect as much muscle as you can.
Rarer but serious risks
The official US prescribing information for these drugs lists several warnings, including:
- Thyroid tumours: semaglutide and tirzepatide caused thyroid C-cell tumours in rodents. It isn’t known whether they do this in humans, but they carry a boxed warning and shouldn’t be used by people with a personal or family history of medullary thyroid cancer or MEN 2 syndrome.
- Pancreatitis: inflammation of the pancreas has been reported. Severe, persistent stomach pain, sometimes spreading to the back, needs urgent medical attention.
- Gallbladder problems, including gallstones, which are also linked to rapid weight loss in general.
- Pregnancy: these drugs aren’t for use in pregnancy. The Wegovy label advises stopping at least two months before a planned pregnancy because the drug stays in the body for a long time.
Your doctor will also want to know about any other diabetes medicines you take, as the combination can raise the risk of low blood sugar.
What Happens When You Stop
This is the part that gets the least attention, and it matters most for anyone deciding whether to start.
After STEP 1 ended, researchers followed 327 participants for a year after everyone stopped their injections (Wilding and colleagues, Diabetes, Obesity and Metabolism, 2022). People who had been on semaglutide regained about two-thirds of the weight they had lost. Their improvements in blood pressure, blood sugar and cholesterol mostly drifted back towards where they started.
SURMOUNT-4 (Aronne and colleagues, JAMA, 2024) tested this more directly for tirzepatide. All participants took tirzepatide for 36 weeks, then were randomly assigned to keep taking it or switch to placebo for another year. Those who continued lost a further 5.5% of their weight. Those switched to placebo regained 14.0%. By the end, 89.5% of the people who stayed on tirzepatide had kept off at least 80% of their earlier weight loss, compared with 16.6% of those on placebo.
The researchers’ conclusion in both cases was that obesity behaves like a chronic condition, similar to high blood pressure. The drugs control it while you take them, but they don’t cure it. That doesn’t make them a bad option, but it does mean many people should expect to stay on them long term, or to put serious work into the habits that help people keep weight off. Our article on whether fitness trackers help with weight loss covers some of those, including regular self-monitoring.
Compounded and Online Versions
Shortages and high prices have pushed many people towards “compounded” semaglutide and tirzepatide, made up by pharmacies rather than the manufacturer, or towards products sold online. The US FDA has repeatedly warned about these. Its concerns include:
- Dosing errors: some patients have accidentally injected several times their intended dose when drawing up compounded drug from multi-dose vials, partly because of confusion between milligrams, millilitres and syringe “units”.
- Unknown contents: compounded and online products aren’t FDA-approved, so their quality, purity and strength haven’t been checked in the same way. Some websites sell products labelled “for research purposes” or “not for human consumption”.
- Fake pharmacies and labels, including labels that use a real pharmacy’s name falsely.
The FDA’s advice is to get these medicines only with a prescription, from a licensed pharmacy, and to ask your prescriber or pharmacist to show you exactly how to measure each dose.
If You Are Considering One
- Start with a doctor’s assessment. They’ll check whether you meet the criteria, review your medical and family history, and look at your other medicines.
- Plan for the long term. Ask what happens if you stop, how the cost will work for you over years, and what the plan is if side effects are a problem.
- Protect your muscle. With a much smaller appetite, it’s easy to eat too little protein. Strength training two or more times a week helps.
- Keep building habits. The trials all combined the drugs with diet and activity support. Habits you build while appetite is easier to manage are the ones that may help if you ever come off.
- Track trends, not daily numbers. If you use a smart scale, look at weekly averages. Our guide to how accurate smart scales are explains why body-fat readings in particular can mislead.
And if a medicine isn’t right or available for you, other approaches still work. Changing when you eat is one option some people find easier; see what the trials found on intermittent fasting for weight loss.
Frequently Asked Questions
How much weight can you lose on Ozempic or Wegovy?
In the STEP 1 trial, people on semaglutide 2.4 mg weekly (the Wegovy dose) lost an average of 14.9% of their body weight over 68 weeks, compared with 2.4% on placebo. Results vary widely between individuals. Ozempic is licensed for type 2 diabetes and is used at lower maximum doses.
Is Mounjaro better than Ozempic for weight loss?
In the head-to-head SURMOUNT-5 trial, tirzepatide (the drug in Mounjaro and Zepbound) produced average weight loss of 20.2% over 72 weeks, compared with 13.7% for semaglutide (the drug in Ozempic and Wegovy). The trial was funded by tirzepatide’s maker. Which drug suits a given person also depends on side effects, other conditions and availability.
Do you regain weight after stopping Ozempic or Wegovy?
Most people do. A year after stopping semaglutide in the STEP 1 extension, people had regained about two-thirds of the weight they had lost. In SURMOUNT-4, people switched from tirzepatide to placebo regained 14% of their weight within a year.
What are the most common side effects of GLP-1 weight-loss drugs?
Digestive problems: nausea, diarrhoea, vomiting and constipation. In STEP 1, 74.2% of people on semaglutide reported a digestive side effect, compared with 47.9% on placebo. These were usually mild to moderate and eased over time, but about 4.5% stopped treatment because of them.
Do GLP-1 drugs cause muscle loss?
Some of the weight lost is lean tissue, as with any weight loss. Body-composition data from STEP 1 showed lean mass fell, though fat fell more. Eating enough protein and doing strength training can help limit muscle loss.
Key Takeaways
- Ozempic and Wegovy contain semaglutide. Mounjaro and Zepbound contain tirzepatide. They reduce appetite and help people eat less.
- In large trials, semaglutide led to about 15% average weight loss and the highest dose of tirzepatide about 21%, far more than placebo.
- In people with existing heart disease, semaglutide cut the risk of heart attack, stroke or cardiovascular death by 20% in the SELECT trial.
- Digestive side effects are very common, and rare risks include pancreatitis and gallbladder problems.
- Most people regain much of the weight after stopping, so these drugs work more like ongoing treatment than a one-off fix.
- Avoid compounded or online products from unverified sources. The FDA has documented dosing errors and fake products.
For the bigger picture on diets, exercise, sleep and keeping weight off, see our full guide to how to lose weight.
