Ozempic, Wegovy, Mounjaro and Zepbound have become some of the most widely discussed weight-loss medications in recent years. Once known mainly to people with diabetes and to doctors, they are now widely discussed on social media, recommended among friends and sometimes treated almost like cosmetic procedures. The phrase “weight-loss injection” can make it sound as though these drugs offer a simple way to get rid of a few extra kilograms. In reality, however, they involve complex effects on the body. What were these medications originally developed for? Why do they actually lead to weight loss? Who are they intended for, and what risks do they carry? We look at the evidence without myths or advertising promises.
First, it is important to understand the names. Today, the word “Ozempic” is sometimes used to refer to almost all medications that act on the GLP-1 hormone system. But these are different drugs, with different active ingredients, indications and dosing schedules.
Ozempic contains semaglutide and is intended for the treatment of type 2 diabetes. A different medication containing the same active ingredient, Wegovy, is used for weight management. It has a separate approved indication for the treatment of obesity and overweight under certain conditions.
Mounjaro contains a different active ingredient, tirzepatide. It acts on two hormonal systems at once, GIP and GLP-1. In the European Union, Mounjaro is used both for type 2 diabetes and for weight management in adults with obesity or overweight accompanied by weight-related health problems. In the United States, tirzepatide for weight management is marketed under the name Zepbound.
Another medication in this group is Saxenda, which contains liraglutide. In other words, “weight-loss injections” are not a single drug, but a group of medications that affect the mechanisms regulating appetite and metabolism.
So why do these medications cause weight loss?
GLP-1 is a hormone naturally produced by the body after eating. It plays a role in regulating appetite, feelings of fullness and blood sugar levels. Drugs that mimic the effects of GLP-1 act, among other things, on brain centers involved in hunger and also slow gastric emptying. As a result, people tend to feel full for longer and, in many cases, eat less.
That is why it would be inaccurate to say that these medications simply “burn fat.” Their main effect is related to changes in appetite and in the physiological mechanisms that regulate food intake. They can make it easier for a person to maintain a lower calorie intake, which in turn leads to weight loss.
This raises an important question: who actually needs these medications?
Weight-management drugs are primarily intended for people with obesity or excess weight when they also have diseases or other health problems associated with their weight.
For example, according to the European indication for Wegovy, it may be prescribed to adults with a BMI of 30 or higher, or to people with a BMI between 27 and 30 who have at least one weight-related health condition. These may include abnormal glucose metabolism, high blood pressure, abnormal blood lipid levels, cardiovascular disease and obstructive sleep apnea.
Zepbound is also approved in the United States for chronic weight management in adults with obesity or overweight who have at least one weight-related medical condition. The medication is used together with changes in diet and physical activity, rather than as a replacement for them.
This is particularly important given the popularity of these medications on social media. They are often discussed as a way to lose the “last five kilograms” or quickly return to a previous weight after the holidays. However, the clinical trials underlying their approved indications were conducted primarily among people with obesity or overweight and relevant medical indications.
For this reason, the results of these studies cannot simply be applied to a person with a normal BMI who wants to become slightly slimmer.
At the same time, there is little reason to question the effectiveness of these medications themselves.
In the large STEP 1 trial, participants with obesity or overweight who did not have diabetes received semaglutide along with recommendations for lifestyle changes. After 68 weeks, average body weight decreased by about 14.9%, compared with about 2.4% in the placebo group.
Tirzepatide also produced substantial weight loss in the SURMOUNT-1 trial. After 72 weeks, average weight loss was approximately 15% at one of the doses studied and up to 20.9% at the highest dose. In the placebo group, the reduction was about 3.1%.
These results help explain why the medications have become so widely sought after. For people with obesity, they have become an additional treatment option, particularly when changes in diet and physical activity alone are not sufficient.
But effectiveness does not mean complete safety.
The most common side effects involve the digestive system. These include nausea, vomiting, diarrhea, constipation, abdominal pain or discomfort, and a pronounced reduction in appetite. Such symptoms are often particularly noticeable when treatment begins or when the dose is increased.
More serious complications are also possible. These include pancreatitis, gallbladder problems, dehydration and associated kidney problems. Delayed gastric emptying may also be important for people with certain gastrointestinal conditions and can be relevant before surgery when anesthesia is used.
That is why describing these medications as a “harmless injection that simply takes away your appetite” is an oversimplification. They are full-fledged prescription medicines that affect several physiological processes.
The issue of the thyroid gland also deserves particular attention. In animal studies, semaglutide and tirzepatide have been associated with tumors of the thyroid C-cells. It remains unknown whether the same risk exists in humans. For this reason, these medications are contraindicated in people with a personal or family history of medullary thyroid carcinoma and in patients with multiple endocrine neoplasia syndrome type 2.
Therefore, the statement “Ozempic causes thyroid cancer” does not accurately reflect the available evidence. It is more accurate to say that there is a regulatory warning based primarily on findings from animal studies, while an equivalent risk in humans has not been established.
In recent years, a number of other alarming claims about GLP-1 medications have also emerged. For example, there were concerns that they might increase the risk of suicidal thoughts. In January 2026, the FDA said that after reviewing the available data, it had not identified an increased risk of suicidal thoughts or behavior with GLP-1 receptor agonists and requested the removal of the corresponding warning from the labeling of Saxenda, Wegovy and Zepbound.
But there is another issue that is discussed much less often: what happens after the medication is stopped?
During treatment, a person may eat less because of reduced appetite and other physiological effects of the medication. After treatment ends, these effects gradually wear off. This does not mean that every patient will necessarily regain all of the weight they lost. However, studies show that a substantial proportion of the lost weight can return.
In the follow-up to the STEP 1 trial, participants regained about two-thirds of the weight they had lost, on average, one year after stopping semaglutide. At the same time, some of the improvements in metabolic measures also moved back toward their original levels.
A similar pattern was observed in the SURMOUNT-4 trial of tirzepatide. After initially losing weight, participants who stopped treatment and switched to placebo regained a substantial portion of the weight they had lost over the following year. Those who continued treatment, by contrast, maintained their weight reduction and continued to lose weight.
This does not mean that the medications “do not work.” Rather, the studies highlight another important point: for many people, obesity is a chronic disease. The idea of taking a short course, losing weight and then never having to think about it again does not match what clinical evidence shows.
There is another problem that has emerged alongside the enormous demand for these medications: products of unknown origin, counterfeit drugs and independently compounded versions of semaglutide and tirzepatide.
The FDA has warned about the risks associated with unapproved and inadequately controlled products. The agency has also reported numerous adverse events associated with compounded versions of semaglutide and tirzepatide. At the same time, the FDA emphasizes that such reports do not always establish a direct causal relationship and may be incomplete.
The situation can be particularly risky when someone purchases a medication through social media or a questionable website and then determines the dosage themselves. In such cases, the usual risks of the medication are compounded by uncertainty about the product’s ingredients, concentration and correct dosing.
Ultimately, the reality is much more complicated than the popular formula “one injection and several kilograms gone.”
Yes, semaglutide, tirzepatide and other medications in this group can significantly reduce body weight. Yes, their effectiveness has been demonstrated in large clinical trials. And yes, for people with obesity, they can be part of comprehensive medical treatment.
But these are not cosmetic injections or a universal way to lose weight. They have contraindications and side effects, some of which can be serious. After treatment is discontinued, many patients regain at least part of the weight they lost. And purchasing medications from unknown sources creates additional risks.
The key question, therefore, is not which “weight-loss injection” is the most popular or how many kilograms an advertisement promises. What matters far more is whether a particular person has a medical indication for this type of treatment and whether it is appropriate for them given their health, other medications and potential contraindications.
That is why Ozempic, Wegovy, Mounjaro, Zepbound and similar medications should primarily be viewed as medicines for treating specific medical conditions, not as a quick way to improve one’s appearance.
