Weight-loss injections: what you should know before you start
"Weight-loss injections" is the popular name for a class of prescription medicines. What they actually are, how they act, how much weight is lost in the clinical trials, which side effects are common, who is not a candidate and which questions are worth asking at the consultation.
English translation of content medically reviewed in Romanian by Dr. Claudiu Ruscu, intensive care physician (anaesthesia & intensive care)· Romanian text reviewed 20 September 2026
They are not available without a prescription and are not chosen from an article. This text explains what they involve, so that you arrive at the consultation with questions, not expectations.
What exactly are they?
They are medicines that mimic the action of hormones produced naturally in the gut after a meal. A GLP-1 agonist acts on the receptor for the hormone GLP-1. Tirzepatide acts on two receptors — GLP-1 and GIP — which is why it is described as a dual agonist.
The effects that matter for weight are three, and they overlap:
- the stomach empties more slowly, so the feeling of fullness lasts longer after a meal;
- the fullness signals change, so the portion that used to satisfy you becomes too large;
- insulin secretion is stimulated depending on blood glucose, which is why the same class is also used in type 2 diabetes.
All of them are prescription medicines. The authorised indications, contraindications and special warnings are published by the European Medicines Agency for each product — Mounjaro, Ozempic and Wegovy.
How much weight is lost in the trials?
| Trial | Substance | Duration | Average weight loss | Placebo |
|---|---|---|---|---|
| SURMOUNT-1 | tirzepatide 15 mg | 72 weeks | 20.9% | 3.1% |
| SURMOUNT-1 | tirzepatide 5 mg | 72 weeks | 15.0% | 3.1% |
| STEP-1 | semaglutide 2.4 mg | 68 weeks | 14.9% | 2.4% |
The sources are SURMOUNT-1, New England Journal of Medicine 2022 and STEP-1, New England Journal of Medicine 2021.
Two clarifications that articles on the internet almost always skip:
- The figures are group averages at the end of the trial, not guaranteed results and not a monthly rate. Some participants lost much more, others much less.
- In both trials, participants also received advice on diet and physical activity. The figures describe the combination, not the medicine in isolation.
If your question is how much weight is lost per month, the honest answer is that the trials do not report anything of the kind, because the pace is not constant: it is faster at first and slows until it nears a plateau. We have gone into this in the article on why I am not losing weight on Mounjaro.
What side effects occur?
The most common are digestive: nausea, diarrhoea, vomiting, constipation, abdominal pain. They appear mainly at the start and with each dose increase, and usually ease over time.
Rarer, but important, are acute pancreatitis, gallstones and, in people who also take insulin or sulphonylureas, hypoglycaemia. The details — what is common, what passes on its own and what calls for immediate contact with the doctor — are in the article on the side effects of weight-loss injections.
A practical consequence that is discussed less often: when food intake drops suddenly, fluid intake drops too, because part of it came from food. What can be done at home, and when that is no longer enough, we have written about in the article on hydration and food intake.
Who is not a candidate?
The approved product documents list absolute contraindications — among them hypersensitivity to the active substance and, for some products, a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2. Pregnancy and breastfeeding are situations in which these medicines are not given.
There are also situations that call for caution and individual assessment: a history of pancreatitis, advanced kidney disease, diabetic retinopathy, severe gastrointestinal conditions. The complete list and the reasons behind each item are in the article on contraindications and situations in which the treatment is not indicated.
None of this is self-assessed. These are criteria applied by the prescribing doctor, with your history in front of them.
What do they cost and where are they obtained?
The cost differs from product to product and from one form to another, and some of them are considered for reimbursement (by the Romanian national health insurance system) only for certain indications. We do not publish pharmacy prices and we do not discuss where they are sold: they are prescription medicines, and a medical clinic that promotes them to the general public breaches Law 95/2006 (the Romanian healthcare law).
What is up to us and what we can say: the prices of Synerva's services are published in full on the prices page. We do not prescribe or dispense prescription medicines.
What do I ask at the consultation?
A good consultation does not start from "I want to lose weight". It starts from:
Question 5 is the one most often left out and the one with the biggest consequences. We have covered it separately, with the data from the trials, in how long Mounjaro is taken and what happens when you stop.
In short
- "Weight-loss injections" are prescription medicines from the class of GLP-1 agonists, or GLP-1 and GIP agonists, given subcutaneously, usually weekly.
- In the authorisation trials, the average weight loss was about 14.9% at 68 weeks for semaglutide and up to 20.9% at 72 weeks for tirzepatide.
- The figures are group averages obtained together with advice on diet and physical activity, not the isolated effect of the medicine.
- The common side effects are digestive and appear mainly at the start; acute pancreatitis and gallstones are rare, but call for immediate contact with the doctor.
- Eligibility is established solely by the prescribing doctor, on the basis of your history and current medication, not by self-assessment.
This article is for information only and does not replace a medical consultation. Injectable weight-loss treatment is given only on the indication and under the supervision of your treating doctor. Synerva does not prescribe or dispense prescription medicines.
Frequently asked questions
- What are weight-loss injections?
- They are prescription medicines from the class of GLP-1 receptor agonists, or GLP-1 and GIP receptor agonists, given subcutaneously, usually once a week. They act by slowing the emptying of the stomach and changing the fullness signals.
- How much weight is lost with weight-loss injections?
- In the authorisation trials, the average weight loss was about 14.9% of starting weight at 68 weeks for semaglutide and up to 20.9% at 72 weeks for tirzepatide, compared with 2-3% in the placebo groups. These are group averages, not guaranteed results.
- Do weight-loss injections have side effects?
- The most common are digestive — nausea, diarrhoea, vomiting, constipation, abdominal pain — especially at the start. Rarer, but important, are acute pancreatitis, gallstones and hypoglycaemia in people who also take insulin or sulphonylureas.
- Can they be taken without a prescription?
- No. They are prescription medicines. Eligibility, the choice of product and the administration schedule are set solely by the prescribing doctor, on the basis of your history and current medication.
- How much do weight-loss injections cost?
- We do not publish pharmacy prices and we do not discuss sources of supply: they are prescription medicines, and promoting them to the general public is prohibited by Law 95/2006 (the Romanian healthcare law). The cost is discussed with the prescribing doctor and with the pharmacy.
- What happens if I stop the treatment?
- Withdrawal studies show that a significant part of the weight lost is regained after stopping. The details, with the figures from the trials, are in the article on how long the treatment is taken and what happens when you stop.
More in Guides
- Side effects of weight-loss injections: what is common and when to call the doctor
Most side effects are digestive, appear at the start and ease. A few, however, are not something to wait out. What the approved documents report by frequency, what each category means in practice and which signs call for immediate contact with the doctor.
- Mounjaro: opinions, realistic expectations and what the leaflet says
"Mounjaro opinions" is one of the most searched phrases in this area, and what you find is almost entirely individual experiences. We have set what is discussed on forums side by side with what the approved product document says — including where the two contradict each other.
- How long Mounjaro is taken and what happens when you stop
The question is usually asked too late, and the answer changes the way you look at the whole treatment. What withdrawal studies show about the weight regained, how quickly and why it happens — with the published figures, not impressions.
- Contraindications and situations in which injectable treatment is not indicated
Not everyone who wants to lose weight is a candidate. What the approved documents list as absolute contraindications, what falls under special warnings, the practical difference between the two and why neither can be self-assessed.
References
The studies below support the statements on mechanism, pharmacokinetics and safety on this page. They do not support a guaranteed therapeutic outcome — each one is followed by its limitation.
[1] Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022.
randomised trial · sample: 2539 · PMID 35658024
Limitation: All participants also received diet and physical-activity counselling, so the figures describe the combination, not the substance alone. People with type 2 diabetes were excluded. Follow-up stopped at 72 weeks, with no information on what happens after stopping. Funded by the manufacturer.
[2] Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021.
randomised trial · sample: 1961 · PMID 33567185
Limitation: People with type 2 diabetes were excluded, so the result does not transfer to that population — in them, the weight loss seen in other studies is smaller. Both arms received lifestyle counselling. Funded by the manufacturer, with authors employed by it.
[3] European Medicines Agency (EMA) Mounjaro — product information (summary of product characteristics). European Medicines Agency. 2026.
marketing authorisation document · EMEA/H/C/005620
Limitation: The summary of product characteristics describes the frequencies observed in the registration trials, not the incidence in everyday use, and does not establish causality for reactions reported after marketing. It is a living document: the valid version is the one on the EMA page, not a summary quoted elsewhere.
[4] European Medicines Agency (EMA) Ozempic — product information (summary of product characteristics). European Medicines Agency. 2026.
marketing authorisation document · EMEA/H/C/004174
Limitation: The summary of product characteristics describes the frequencies observed in the registration trials, not the incidence in everyday use, and does not establish causality for reactions reported after marketing. It is a living document: the valid version is the one on the EMA page, not a summary quoted elsewhere.
[5] European Medicines Agency (EMA) Wegovy — product information (summary of product characteristics). European Medicines Agency. 2026.
marketing authorisation document · EMEA/H/C/005422
Limitation: The summary of product characteristics describes the frequencies observed in the registration trials, not the incidence in everyday use, and does not establish causality for reactions reported after marketing. It is a living document: the valid version is the one on the EMA page, not a summary quoted elsewhere.
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