Weight loss injections (Ozempic, Wegovy): what to know

Weight loss injections such as Ozempic, Wegovy, or Mounjaro are everywhere now: on social media, in the tabloids, and in the pub. They are prescription drugs. In people with obesity, and with a disease that excess fat makes worse, they work, and they have a documented health benefit. They also have rules, side effects, and one thing that gets said too little: what happens to muscle, bone, and weight once the drug stops. Here is what the trials say, and what that means for someone who is losing fat.

How Ozempic, Wegovy, and Mounjaro work

Ozempic and Wegovy contain the same active ingredient, semaglutide. Mounjaro contains tirzepatide. All three mimic gut hormones that the body releases after a meal and that tell the brain "that is enough." The result: you are less hungry, you feel full sooner, and between meals nothing pulls you toward the fridge. On television last year, one patient described the drug as quieting the constant thinking about food.

These drugs do not burn fat directly. They work because, with hunger turned down, you eat less, and a calorie deficit appears. Intake versus expenditure still holds. The drug just helps you hold a lower intake without a daily fight against hunger. You could see that in the placebo groups too: people who got a placebo, plus the same support with food and movement, also lost weight, just much less.

In the EU, Wegovy and Mounjaro are authorized for the treatment of obesity: from a BMI of 30, or from a BMI of 27 with a disease that weight makes worse (diabetes, high blood pressure, sleep apnea, and others). The label says, in so many words, that they are an add-on to a reduced-calorie diet and to more physical activity. Ozempic is authorized for type 2 diabetes. Someone who takes it only for weight is using it outside that authorization.

Who the drug is for, and who it is not for

This is the most important distinction in the whole topic, for me. In a person with marked obesity and diabetes, high blood pressure, or heart disease, the drug is treatment, not cosmetics. The SELECT trial followed more than 17,000 people with heart disease for more than three years: on semaglutide they had about one fifth fewer heart attacks, strokes, and deaths from heart disease. In people with prediabetes, diabetes developed over three years in 1 in 100 on tirzepatide, and in 13 in 100 on placebo. In some people a doctor can then reduce other drugs, for blood pressure or cholesterol, for example. A doctor decides all of that.

For a person with a few extra kilos, no disease, and no doctor, this is not the drug. The risks are the same. A documented health benefit is not. Food, lifting, and time are the better road there.

And never from the internet. On 28 August 2026 the Czech medicines regulator (SUKL) warned about counterfeit Ozempic that showed up in Poland. It did not reach Czech pharmacies. The drug belongs in a pharmacy, on a prescription. A parcel from an ad can contain anything.

What the trials show

The numbers are high, and it is honest to say so. The STEP 1 trial (Wilding and colleagues, 2021) gave semaglutide or placebo to 1,961 adults with obesity, or with overweight plus a related disease, and without diabetes. Both groups also had support with food and movement. Over 68 weeks, weight fell by 14.9% on the drug and 2.4% on placebo, on average. At least 5% was lost by 86 in 100 on the drug, and by 32 in 100 on placebo.

SURMOUNT-1 (Jastreboff and colleagues, 2022) tested tirzepatide in 2,539 people. Over 72 weeks the average loss was 15 to 21% depending on the dose, and 3% on placebo. At the highest dose, 57 in 100 lost a fifth of their weight or more.

That is a lot, for a drug. These are also trials in patients with obesity, with a doctor, and with a routine alongside the drug. You cannot carry that across to someone who wants to drop the last 4 kg (about 9 lb) before summer.

Muscle leaves with the fat, and some bone does too

When you lose weight quickly, you do not lose only fat. In trials of these drugs, fat-free mass (muscle, water, organs) is about a quarter to a third of the loss: about 25% with tirzepatide, about 35% with semaglutide (a summary of 20 studies, Eisa and colleagues, 2026). Muscle itself is about half of that, so with semaglutide it is about a fifth of the kilos lost. Ordinary dieting without lifting lands in a similar place. With a diet plus lifting, the share of fat-free mass fell to 17.5%.

Why this matters: muscle holds your expenditure, your strength, and the way you move. Someone who drops 20 kg (about 44 lb), and 4 kg of that is muscle (about 9 lb), is lighter, and weaker. On the drug it is easy to eat so little that you never reach your protein, even when you want to.

Bone: any larger weight loss takes something from bone as well. A one-year study of semaglutide (a lower dose, 64 people with low bone density, mostly without obesity) measured lower bone density at the hip and the spine than on placebo. The authors put that down to the weight loss itself. A large summary of 60 studies did not find an increase in fractures with these drugs, so there is no point scaremongering. What does have a point: in the Danish S-LiTE study, people who exercised under supervision alongside the drug (liraglutide) kept their bone density, while on the drug alone it fell more than with exercise alone. They also lost the most weight of any group.

That is why lifting and protein at every meal belong with the drug from week one. How to protect muscle in a deficit is in the article on muscle while you lose fat.

What happens after you stop

This gets said too little. In the extension of STEP 1 (Wilding and colleagues, 2022), 327 people stopped both the drug and the support after 68 weeks. The semaglutide group had lost 17.3%. A year after stopping, they had regained 11.6 percentage points. The authors write that about two thirds of the loss came back within a year.

SURMOUNT-4 (Aronne and colleagues, 2024): after 36 weeks of tirzepatide, the average loss was 20.9%. People who then got placebo regained 14% of their weight over a year. People who stayed on treatment lost another 5.5%. Everyone kept getting advice on food and movement. A summary of 37 studies (West and colleagues, 2026) puts it this way: after newer drugs are stopped, about 0.8 kg a month comes back (about 1.8 lb), and the authors found no evidence that more intensive support during treatment slowed the regain.

This is not weak willpower. Hunger comes back, and the body defends the weight it was used to. I wrote about that in the article on the yo-yo effect.

What doctors do with it: for some people, instead of stopping, they choose a lower maintenance dose. The SURMOUNT-MAINTAIN trial (Horn and colleagues, 2026) measured that. People who moved to a lower dose after a year on the full dose of tirzepatide kept most of the result. People who moved to placebo lost a larger part. Whether, when, and how to stop is a doctor's decision. I prepare a person for both possibilities.

Side effects

I am not a doctor, and this is not medical advice. The numbers from the trials are public, though. The most common problems are digestive. In STEP 1, 74 in 100 had them on the drug, and 48 in 100 on placebo. Mostly nausea, diarrhea, constipation, or heartburn, mainly at the start and when the dose goes up, and mostly mild and temporary. Digestive problems led 4.5% of people on semaglutide to stop.

Rarer complications exist, and a doctor handles them. That is also why the dose goes up gradually, under supervision. In the few people I work with who use these drugs, I have seen both: some felt awful on the drug, some noticed nothing. If problems do not settle, that belongs with a doctor. For some people the dose gets adjusted, or the drug gets changed.

If you take other medicines, you are pregnant, you have a history of an eating disorder, or you are dealing with any illness, the decision about the drug belongs only to your doctor. I will not make a doctor's numbers or a doctor's decision for you, and nobody online should.

Time on the drug is the best time to build habits

The drug turns hunger down. For me, as a coach, that is mostly an opportunity. The stretch when nothing pulls you toward food is the best time to learn how many calories you actually eat, to put protein in every meal, to add fiber, and to build a way of eating you can still live with a year from now.

Now, honestly, what is measured and what is not. Exercise is measured. In S-LiTE, people who exercised under supervision regained 6 kg less (about 13 lb) in the year after the program ended than people who had only the drug, and the combination of drug plus exercise came out 5 kg better (about 11 lb) than the drug alone. Studies have not yet shown that food advice during treatment, on its own, prevents weight regain after stopping. In SURMOUNT-4, people regained even with that advice. Habits built on the drug are insurance, not a guarantee. I will add one thing. Someone who eats very little of everything on the drug, and learns nothing new, has nothing to stand on after they stop. The drug bought them time. If they do not use it, they start again where they were.

If you want to start with the part no injection will do for you, the free Fat Loss Kit is a 7-day place to begin.

What to do in practice, whether you take the drug or not

Whether a doctor has you on the drug, or you are losing fat without it, the boring part that works is still the same. This is what holds the result:

• A modest calorie deficit, not a starvation diet. On the drug, some people overshoot on their own: they eat 800 kcal and nothing appeals. A slower pace protects muscle more.
• Protein at every meal. Professional groups, for people on these drugs, cite roughly 1.2 to 1.6 g per kilogram. If you have a lot of weight to lose, calculate it from your target weight. More in the article on protein.
• Strength training 2 to 3 times a week, so the body has a reason to keep muscle and bone. A chair squat, a machine press, and a row are enough. If you have high blood pressure or heart disease, ask your doctor first.
• Fiber: vegetables, legumes, whole grains. On the drug, this is often the first thing to disappear from the plate, and regular digestion with it.
• Steps every day, and 7 to 9 hours of sleep.
• A flexible approach to food: no forbidden foods. You watch macros and the whole picture. More in flexible dieting.

Counting calories, building a way of eating, and keeping it without the drama is a skill. The calorie calculator is a starting number. A simple workout plan covers the lifting. Coaching is where I set the rest with you. The skill works with an injection and without one.

Frequently asked questions

Will I lose weight on Ozempic without a diet or exercise?

For a while, yes. The drug quiets hunger and you eat less. In the trials, about two thirds of the lost weight came back within a year of stopping, and without lifting and protein you lose muscle along with the fat. A doctor decides whether the drug is started.

Are weight loss injections safe?

They are prescription drugs with side effects and reasons not to use them. Digestive problems are the most common. For people with obesity and a related disease they have a documented benefit. For other people they are a poor fit. I am not a doctor, and your doctor has to make that call, especially if you take medicines, you are pregnant, or you are dealing with an illness.

What is the difference between Ozempic, Wegovy, and Mounjaro?

Ozempic and Wegovy contain semaglutide. Mounjaro contains tirzepatide. Ozempic is authorized for type 2 diabetes. Wegovy and Mounjaro are authorized for the treatment of obesity. A doctor decides the specific drug and the dose.

What should I do if I take the drug, or I just stopped, so I do not lose the result?

Protein in every meal, strength training two to three times a week, daily steps, and learning to eat in a way you can keep without the drug. Talk to your doctor about lowering the dose or stopping. For some people they choose a lower maintenance dose instead of stopping.

You might also like

• Yo-yo dieting: why the weight comes back
• Calorie deficit: how much to eat
• How to keep muscle while you lose fat
• Flexible dieting, without forbidden foods

Sources, so you can check:

1) Wilding et al. (2021), N Engl J Med. STEP 1: semaglutide vs placebo, 68 weeks. PMID 33567185

2) Jastreboff et al. (2022), N Engl J Med. SURMOUNT-1: tirzepatide vs placebo, 72 weeks. PMID 35658024

3) Wilding et al. (2022), Diabetes Obes Metab. STEP 1 extension: weight regain one year after stopping semaglutide. PMID 35441470

4) Aronne et al. (2024), JAMA. SURMOUNT-4: continuing vs stopping tirzepatide. PMID 38078870

5) Horn et al. (2026), Lancet. SURMOUNT-MAINTAIN: a lower maintenance dose of tirzepatide vs stopping. PMID 42119587

6) West et al. (2026), BMJ. Meta-analysis of 37 studies: weight regain after stopping weight loss drugs. PMID 41500720

7) Eisa et al. (2026), Diabetes Obes Metab. Meta-analysis of 20 studies: the share of fat-free mass in the loss, on the drugs and on a diet. PMID 41877354

8) Look et al. (2025), Diabetes Obes Metab. SURMOUNT-1 DXA substudy: what the loss on tirzepatide was made of. PMID 39996356

9) Mozaffarian et al. (2025), Obesity. Joint statement of four American professional societies on nutrition and activity during GLP-1 treatment. PMID 40445127

10) Hansen et al. (2024), eClinicalMedicine. One-year study: semaglutide and bone density in people with low bone density. PMID 38737002

11) Beaudart et al. (2026), Drugs. Meta-analysis of 60 studies: GLP-1 drugs, muscle, bone, and fractures. PMID 42730869

12) Lundgren et al. (2021), N Engl J Med. S-LiTE: exercise, liraglutide, or both, after a diet. PMID 33951361

13) Jensen et al. (2024), eClinicalMedicine. S-LiTE one year after treatment and exercise ended. PMID 38544798

14) Jensen et al. (2024), JAMA Netw Open. S-LiTE: bone density after exercise, liraglutide, or both. PMID 38916894

15) Lincoff et al. (2023), N Engl J Med. SELECT: semaglutide and heart events in people with heart disease. PMID 37952131

16) Jastreboff et al. (2025), N Engl J Med. SURMOUNT-1 at 3 years: tirzepatide and new diabetes in prediabetes. PMID 39536238

17) Kosiborod et al. (2023), Diabetes Obes Metab. STEP 1 and STEP 4: blood pressure and cholesterol drugs during semaglutide. PMID 36200477

18) STOB / the Czech Society for the Study of Obesity: protein and strength training for people on weight loss drugs. obesitas.cz

19) SUKL (28 August 2026): warning about counterfeit Ozempic. sukl.gov.cz

20) EMA summaries of product characteristics for Wegovy and Mounjaro (authorization, indications). EPAR Wegovy, EPAR Mounjaro

Martin Barna

Online nutrition and fitness coach. Since 2013 I have helped 600+ people lose fat and keep the result. Science, not crash diets.

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This article is for information only. It is not medical advice, and it is not a diagnosis. These are prescription drugs. Do not start, stop, or change a medicine because of an article. If you have a health condition, talk to your doctor.