Weight-Loss Injection (Ozempic, Wegovy, Mounjaro): What It Can and Cannot Do

Maybe you know this conversation. A friend tells you she has been injecting for a few weeks. The hunger is gone, she says, and so are the constant thoughts about food. And inside you, two feelings mix: hope and a quiet unease.

Hope, because you know the struggle with food. The planning, the going without, the slipping back. Many people describe it as a noise in the head that never quite stops.

Unease, because you sense that it cannot be that simple. What does this do to my body? And what happens when I stop? Both feelings are justified. They are exactly what this article is about.

This article draws the big picture. The mechanism of action, side effects, muscle loss and stopping each have their own in-depth articles, which I link for you along the way. One important point up front: semaglutide and tirzepatide are prescription medicines. This text explains connections. It is not a recommendation for or against a treatment and does not replace medical advice.

Ozempic, Wegovy, Mounjaro: what is behind the names?

Three names you probably know from the headlines. They sound like three different medicines. But behind them are only two active substances.

Ozempic and Wegovy both contain semaglutide. Ozempic was introduced as a medicine for type 2 diabetes, Wegovy for weight management. The difference lies in the approval and the dose range, not in the molecule. Mounjaro contains tirzepatide. It docks at two sites: at the receptor for GLP-1 and, in addition, at the receptor for GIP, a second gut hormone. Mounjaro is used for type 2 diabetes and for weight management.

Imagine GLP-1 as a messenger. After every meal, your gut sends it off with a simple message to the brain: enough has arrived. Your body's own GLP-1 is broken down again within a few minutes. The medicines are modified so that the messenger stays on the move for days. A whisper becomes a loudspeaker.

The message reaches the hypothalamus and the brainstem, the control centres for hunger and satiety. That is the level of the nervous system. At the same time, GLP-1 can boost insulin release, but only when blood sugar rises. That is the hormonal level. This is why not only hunger can fall, but also the rise in blood sugar after a meal.

One detail from the same study is often overlooked. Delayed gastric emptying, often cited as the main mechanism, could not be detected after 20 weeks in this indirect measurement. So the effect appears to sit mainly where hunger arises and is evaluated: in the head.

And now you know why so many people on the injection talk about a quiet head. You will find the mechanism in detail in the article How does the weight-loss injection work?, and the particular features of tirzepatide in Mounjaro and tirzepatide.

What can the weight-loss injection do? A look at the large studies

If you have fought your own weight for years, you have probably grown suspicious. Too many promises, too many powders, too many before and after photos. That is why a sober look at the numbers is worthwhile here.

Documented by large randomised trials: the effects are substantial. In STEP 1, 1,961 adults without diabetes received semaglutide or placebo for 68 weeks. Both groups also received lifestyle counselling. On semaglutide, weight fell by a mean of 14.9 percent, on placebo by 2.4 percent (Wilding 2021).

In SURMOUNT-1, with 2,539 participants, weight on tirzepatide fell by 15.0 to 20.9 percent after 72 weeks, depending on the dose level. On placebo, it was 3.1 percent (Jastreboff 2022). In both trials, risk factors related to the heart, blood vessels and blood sugar also improved.

These figures apply to the groups studied: adults with marked overweight, in SELECT also with heart disease. There, 16.6 percent on semaglutide stopped taking it because of side effects, compared with 8.2 percent on placebo. These are averages, and individual courses vary widely. And one detail is easily missed: in STEP 1, the injection never ran on its own. Both groups received lifestyle counselling.

And now you know why the medical community takes the injection seriously. It can do a lot. The interesting question is what it cannot do.

Where the injection reaches its limit: appetite is not metabolism

Imagine your weight as a draughty house. The injection is like a very good fan heater. The room warms up quickly, and you feel it straight away. But the leaky windows are still there.

In metabolism, the leaky windows are often called insulin resistance. This means that your cells respond more weakly to insulin, and your body has to release more and more of it. This development has many contributors: too little muscle that can take up sugar. Too little movement. Too little sleep. Constant stress with raised cortisol. And visceral fat in the abdomen, which is considered inflammatory tissue. More on this in the articles Insulin resistance and weight loss and Silent inflammation and weight.

On the injection, many metabolic values can improve, mainly because weight falls. It does not automatically change the drivers in your everyday life. That becomes clear as soon as you look more closely.

Liraglutide is an older relative of semaglutide that is injected daily. Whether the result transfers one to one to the newer medicines is an open question. The direction is still remarkable.

This is a cross-sectional snapshot. It shows an association, not proof of cause and effect. Biologically, though, it fits well with what we know about muscle: working muscle can even take up sugar partly independently of insulin.

Sleep is one of the windows in the house, too. In a randomised trial, young adults with overweight and habitually short sleep extended their night's rest by about 1.2 hours. Afterwards, in everyday life, they took in around 270 kilocalories less per day than the comparison group (Tasali 2022). Without a prescribed diet, simply through more sleep.

A second perspective comes from anthroposophic medicine. There, metabolism is not only chemistry, it also includes the will, meaning what we do. The injection can turn down the volume of hunger. The doing is still up to you. That is not a reproach. It is an invitation.

And now you know why two people with the same injection and the same weight loss can end up in very different places.

Muscle: the quiet price of rapid weight loss

Your trousers get looser, the number on the scales gets smaller. And still some people say: I feel weaker. The stairs are harder than they should be.

Every larger weight loss takes not only fat with it, but also fat-free mass. That includes muscle, but also water, organs and connective tissue. This applies to diets just as much as to medicines.

A meta-analysis of 20 randomised trials with 15,782 participants paints a similar picture (Eisa 2026). On semaglutide, lean mass made up a mean of 35.2 percent of the loss, on tirzepatide 25.4 percent. With lifestyle change alone, it was 26.2 percent. With lifestyle plus strength training, the share fell to 17.5 percent.

And now you know why strength training and protein belong with the injection the way a seatbelt belongs in a car. You can go deeper in the articles Weight-loss injections and muscle loss and Muscle loss with age.

Side effects: factual, not dramatic

Side effects are either met with silence or with shouting. Both are frightening. Let's look at the data instead.

Side effects most often affect the gastrointestinal tract. In STEP 1, nausea and diarrhoea were the most common adverse events, mostly temporary and mild to moderate. Because of gastrointestinal complaints, 4.5 percent on semaglutide stopped treatment, compared with 0.8 percent on placebo. In SURMOUNT-1, the complaints occurred mainly during the dose escalation phase. Depending on the dose level, 4.3 to 7.1 percent stopped there because of side effects, compared with 2.6 percent on placebo.

Less common, but to be taken seriously, are gallstones and inflammation of the pancreas, which the prescribing information lists as risks. In the Copenhagen study, gallstones occurred more often on liraglutide alone than in combination with exercise (Lundgren 2021). Severe, persistent abdominal pain should therefore always be checked by a doctor.

One point that often gets lost: if you eat much less, you usually also take in fewer nutrients. Four US professional societies explicitly name nutrient deficiencies as well as muscle and bone loss as challenges of the treatment (Mozaffarian 2025). On the injection, protein, vegetables and nutrient-dense foods therefore become more important rather than less.

And now you know why good medical support is more than a prescription. You will find all the details in the article Side effects of the weight-loss injection.

What happens after stopping?

This story mirrors what large studies show. Documented by randomised trials: in the STEP 1 extension, 327 participants were followed for another year. On semaglutide, they had lost a mean of 17.3 percent. One year after the end of the medicine and the lifestyle programme, around two thirds of that had returned (Wilding 2022). Most of the improved heart and metabolic values moved back towards baseline.

With tirzepatide, the picture is similar. In SURMOUNT-4, 670 people had lost a mean of 20.9 percent after 36 weeks. Those who then switched to placebo regained 14.0 percent within 52 weeks. Those who continued treatment lost a further 5.5 percent (Aronne 2024).

Another analysis from Cambridge traced the course over time. One year after stopping GLP-1 medicines, a mean of 60 percent of the loss had returned. Projected forward, the curve levels off at about 75 percent (Budini 2026). So part of the success can remain, but it shrinks considerably.

Stopping is not the exception, it is everyday reality. In a large US analysis of 125,474 people, 64.8 percent of those without type 2 diabetes stopped treatment within one year (Rodriguez 2025). There, stopping was linked, among other things, to gastrointestinal side effects.

Why the body reacts like this is well described. After diet-induced weight loss, the hunger hormone ghrelin stayed raised and satiety signals such as leptin stayed lowered, in one study still a year later (Sumithran 2011). The body seems to defend its old weight the way a thermostat holds its set temperature.

And now you know why the end of a treatment should be planned not at the end, but at the beginning. More on this in the article Stopping weight-loss injections: avoiding the yo-yo effect.

The injection as a bridge: what an exit strategy can look like

Maybe you are now asking yourself: is it worth it at all, then? Or will I have to inject forever? Both questions are fair. And for neither of them is there an answer that fits everyone.

For some people, long-term treatment can make medical sense, for example with a high cardiovascular risk. That weighing up belongs in a medical consultation, not in a blog. For many others, the injection can be a bridge: a time in which hunger is quieter and there is room to build foundations.

The same applies here: liraglutide is not semaglutide or tirzepatide. Still, transferring the finding is mechanistically plausible, because muscle, fitness and habits stay when the medicine goes. Four US professional societies for lifestyle medicine, nutrition and obesity stress in a joint advisory that physical activity, strength training, sleep and stress should be considered from the start. They explicitly name preserving muscle and bone through strength training and suitable nutrition (Mozaffarian 2025).

In my clinical experience, people who use their time on the injection to build strength and routines less often experience the end of treatment as a crash. That is an observation from practice, not a study.

Three levers for your exit strategy

And now you know why the injection does not replace the work on your metabolism, but at best gives it room. How all the building blocks fit together is shown in the overview Understanding weight as a whole.

Frequently asked questions about the weight-loss injection

Connections to other topics