Updated: October 2026
In short: A weight-loss injection is a prescription-only medicine containing semaglutide (Wegovy) or tirzepatide (Mounjaro), injected under the skin once a week, that reduces hunger and appetite. In large studies, participants lost on average about 15 to 21 percent of their body weight, accompanied by changes to diet and exercise. The most common side effects are nausea, vomiting, diarrhoea and constipation. Whether a weight-loss injection is right for you is decided by your doctor. As of: October 2026.
Hardly any topic is searched as much as the weight-loss injection. This article gives a sober overview of what the products do according to approval documents and studies, what they don't, and what you should watch out for. How to eat alongside it is covered in the article Nutrition on Weight-Loss Injections. You'll find general basics on losing weight in our guide Losing Weight Healthily.
What is a weight-loss injection?
Weight-loss injection usually refers to medicines from the group of GLP-1 receptor agonists. In Germany, the ones that matter mainly for weight management are Wegovy (active ingredient semaglutide) and Mounjaro (active ingredient tirzepatide). According to the European Medicines Agency (EMA), both are injected once a week under the skin of the abdomen, thigh or upper arm and are available on prescription only.
Ozempic also contains semaglutide, but is approved for adults with inadequately controlled type 2 diabetes. Weight loss is not an approved indication there, but was observed as an accompanying effect. The article explains the differences between the three products Wegovy, Ozempic and Mounjaro Compared.
According to the prescribing information, Wegovy and Mounjaro are approved as an addition to a reduced-calorie diet and more physical activity. So the injection doesn't replace this foundation; it is meant to support it.
How does the weight-loss injection work?
Semaglutide is a GLP-1 receptor agonist. The EMA describes that the active ingredient regulates appetite by increasing the feeling of fullness while reducing food intake, hunger and cravings. According to the EMA, tirzepatide acts via both the GLP-1 and the GIP receptor, increases insulin release and lowers appetite.
In addition, according to the prescribing information, there is delayed stomach emptying. With tirzepatide, this effect decreases over time. For you in everyday life, that means: you feel full sooner, think less about food and usually eat smaller amounts. That's exactly why nutrition matters. When portions get small, every meal has to do more.
How much weight do you lose with the weight-loss injection?
In the approval study STEP 1 with 1,961 adults without diabetes, the average weight change after 68 weeks was minus 14.9 percent on semaglutide 2.4 mg, compared with minus 2.4 percent on placebo. In the SURMOUNT-1 study with 2,539 adults, weight loss after 72 weeks ranged from minus 15.0 percent (tirzepatide 5 mg) to minus 20.9 percent (15 mg), compared with minus 3.1 percent on placebo.
| Study | Active ingredient | Duration | Average weight loss | Placebo |
|---|---|---|---|---|
| STEP 1 (NEJM 2021) | Semaglutide 2.4 mg | 68 weeks | 14.9% | 2.4% |
| SURMOUNT-1 (NEJM 2022) | Tirzepatide 5 to 15 mg | 72 weeks | 15.0% to 20.9% | 3.1% |
With a starting weight of 100 kilograms, an average of 15 percent works out at 15 kilograms. That's an average: some people lose significantly more, others less. Both studies ran in addition to lifestyle counselling, so the figures show the combination of medication and changes.
The Hamburg consumer advice centre (Verbraucherzentrale Hamburg) points out that it's unclear whether people with only slight overweight can lose weight this way, because the studies examined severe overweight.
Who is allowed to get a weight-loss injection?
According to the prescribing information for Wegovy and Mounjaro, use is intended for adults with a body mass index (BMI) of at least 30, or from 27 with at least one weight-related accompanying condition such as high blood pressure, sleep apnoea, prediabetes or type 2 diabetes. Wegovy is also approved for adolescents from 12 years with obesity and a body weight over 60 kilograms.
Semaglutide must not be used during pregnancy and breastfeeding. Contraception is recommended for women of childbearing age. Anyone who wants to become pregnant must, according to the prescribing information, stop Wegovy at least two months beforehand; for Mounjaro, the prescribing information gives at least one month. Whether you're suitable is clarified by your doctor based on your pre-existing conditions and medication.
A prescription is mandatory. Costs and health insurance status are explained in the article Wegovy Costs.
What side effects can the weight-loss injection have?
Complaints most often occur in the digestive tract: nausea, vomiting, diarrhoea, constipation and abdominal pain. According to the prescribing information, they arise mainly while the dose is being increased. The NHS also lists tiredness, reactions at the injection site and hair loss.
| Note | What the sources say |
|---|---|
| Fluid loss | Vomiting and diarrhoea can lead to dehydration, in rare cases with worsening kidney function. Drink enough. |
| Pancreas | Acute pancreatitis has been observed. With severe, persistent abdominal pain, get it checked by a doctor immediately. |
| Gallstones | Gallstones and gallbladder inflammation have been reported (cholelithiasis in 1.6% of those treated according to the Wegovy prescribing information). |
| Anaesthesia | Before procedures with general anaesthesia or deep sedation, inform doctors about the injection (risk of residual stomach contents). |
| Diabetes and eyes | In type 2 diabetes, diabetic retinopathy can worsen, so eye checks are important. |
According to the Hamburg consumer advice centre, around half of those treated were affected by nausea, diarrhoea or constipation. The NHS also advises not stopping suddenly, but talking to your doctor beforehand. Always mention symptoms instead of stopping or changing the injection on your own.
How are Ozempic, Wegovy and Mounjaro used?
The following information comes from the prescribing information and is not personal instruction. The dose is always set by your doctor.
Wegovy: It is injected once a week, independent of meals, under the skin of the abdomen, thigh or upper arm. Treatment starts at 0.25 mg and is increased step by step to reduce digestive complaints: four weeks each of 0.25 mg, 0.5 mg, 1 mg and 1.7 mg, then the maintenance dose of 2.4 mg. So dose escalation takes 16 weeks. If stronger symptoms occur, the doctor can delay the increase.
Mounjaro: Start with 2.5 mg once a week, after four weeks 5 mg, then if needed in 2.5 mg steps every at least four weeks. Recommended maintenance doses are 5, 10 or 15 mg.
Ozempic: According to the EMA, treatment in diabetes starts with 0.25 mg per week and is raised to 0.5 mg after four weeks. The prescribing information sets out further steps.
If you've forgotten a dose, check the package leaflet or ask at the pharmacy. You shouldn't take on dosing information from forums or online sellers.
What goes with the weight-loss injection?
Both approvals list the injection as an addition to a reduced-calorie diet and more physical activity. In practice, three things are important: enough protein and fibre, sufficient fluids and strength training, so that not only fat but also muscle stays a topic. Details and an example day are in the article Nutrition on Weight-Loss Injections.
A joint recommendation from American professional societies from 2025 gives as a guideline regular strength training on at least three days a week and at least 150 minutes of moderate endurance activity per week. These are suggestions for adults on GLP-1 therapy, not a must for starting. It's better to start small and build up slowly, ideally with your doctor's backing.
And what happens when treatment ends? In the extension of the STEP 1 trial, participants regained about two thirds of the weight they had lost. More on this in the article Stopping Weight-Loss Injections.
Pairs well with frank.
If your appetite is small and you still want to eat sensibly, soups can be an option for small, light meals. You'll find them under Soups from frank.. Soups are a food, not a replacement for medical treatment.
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Frequently asked questions about weight-loss injections
What is the weight-loss injection?
A prescription-only medicine containing semaglutide (Wegovy) or tirzepatide (Mounjaro), injected under the skin once a week, that increases the feeling of fullness.
Is Ozempic a weight-loss injection?
According to the EMA, Ozempic is approved for type 2 diabetes. Weight loss is not an approved indication; prescribing it for weight loss would be use outside the approval (off-label).
How much weight do you lose with Wegovy or Mounjaro?
In the approval studies, on average about 15 percent (Wegovy, STEP 1, 68 weeks) or 15 to 21 percent (Mounjaro, SURMOUNT-1, 72 weeks) respectively, each together with nutrition and exercise counselling.
What are the most common side effects?
Nausea, vomiting, diarrhoea, constipation and abdominal pain, especially during the dose-escalation phase. Pancreatitis and gallbladder problems are rarely possible.
Who must not use the weight-loss injection?
Wegovy must not be used during pregnancy and breastfeeding. Further restrictions are in the prescribing information, for example in severe kidney or liver impairment.
Can I buy the weight-loss injection without a prescription?
No. Wegovy, Mounjaro and Ozempic are prescription-only. Offers online without a prescription are a warning sign, because the Hamburg consumer advice centre warns of counterfeits.
What happens if I stop the injection?
In the extension of the STEP 1 trial, participants regained about two thirds of the weight they had lost. You make the decision to stop together with your doctor.
Note: This article is for general information and does not replace medical advice, diagnosis or treatment. You make decisions about medication, dosage or stopping together with your doctor. If you have symptoms or pre-existing conditions, are pregnant or take medication, please speak to them first.
