A study published in the British Medical Journal shows that overweight individuals lose about 20% of their total body weight when using these injections. However, after stopping their use, they gain an average of 0.8 kg per month.
Thus, after a year and a half, they return to their original weight from which they started.
Susan Jebb, a researcher from the University of Oxford, emphasizes that those using these medications need to be aware of the risks of rapid weight gain after therapy ends.
She also adds that these findings are based on clinical trials rather than real-world data. Therefore, more research is needed to understand the long-term effects of new weight loss injections.
As part of the analysis of 37 studies involving over nine thousand participants, scientists compared the effectiveness of weight loss injections with traditional methods such as diets and pills.
Only eight of these studies focused on new GLP-1 medications like Wegovy and Mounjaro, and the maximum observation period was one year after treatment cessation, making the results approximate.
Researchers note that people who lose weight through dieting lose less weight than those using injections. However, after completing the diet, weight returns more slowly—about 0.1 kg per month, although this depends on individual characteristics.
Risk of Relapse
In the UK, the National Health Service (NHS) prescribes injections only to overweight patients who have medical indications but does not recommend them to those who simply want to lose a little weight.
Doctors must also prescribe lifestyle changes, including a balanced diet and regular physical activity, to help patients maintain their achieved weight.
Many specialists believe that such treatment should be viewed as lifelong, considering the risk of relapse.
Those who have stopped the injections describe the condition as "turning on a switch that triggers a strong feeling of hunger." One woman shared: "I felt something click in my head and say, 'Eat everything, you deserve it, you haven't eaten for so long.'"
Dr. Adam Collins, a nutrition specialist from the University of Surrey, explains that the mechanisms of action of these medications may be the reason for the rapid weight regain after stopping their use.
These medications mimic the natural hormone GLP-1, which is responsible for regulating hunger.
"Long-term maintenance of high GLP-1 levels may lead to reduced natural production of this hormone and decreased sensitivity to its effects," he says. "When taking the medications, there are no issues, but as soon as you stop using them, appetite control is lost, and the risk of overeating significantly increases."
Collins emphasizes that coping with this withdrawal is a real challenge.
"The situation is exacerbated if a person relies solely on GLP-1 to suppress appetite and does not take steps to change their habits and diet, which could help them in the long term," he adds.
According to the latest data, last year about 1.6 million adults in the UK used weight loss injections, primarily through private prescriptions rather than through the NHS program.
A study by Cancer Research UK, based on a nationwide survey conducted in the first quarter of 2025, showed that another 3.3 million people would like to try "slimming injections," meaning that one in ten adults has either already used them or plans to try them.
At the same time, women sought these injections twice as often as men, and their use was particularly common among people aged 40 to 50.
Chronic Nature of Obesity
Professor Navid Sattar from the University of Glasgow notes that weight loss injections may have additional health benefits as they promote rapid weight loss.
"Losing weight even for two to three years through short-term use of these injections may help slow down damage to joints, heart, or kidneys. More extensive and long-term studies will be needed to explore this issue," he says.
"It is important to remember that long-term use of these medications over three to four years may help people maintain their weight at significantly lower levels, which is not achieved through lifestyle changes, as most people tend to regain lost weight," Sattar adds.
Doctors and weight management specialists cannot simply prescribe Mounjaro and Wegovy medications, even if a patient has already received them through a private prescription. These medications are prescribed only to those who genuinely need them for medical reasons and meet certain criteria, such as having health problems due to excess weight.
Currently, there are no restrictions on the duration of prescriptions for Mounjaro within the NHS system in the UK, while Wegovy can only be prescribed for a maximum of two years.
A representative from Eli Lilly, the manufacturer of Mounjaro, emphasized that treatment using weight loss medications should be accompanied by healthy eating, physical activity, and medical supervision.
"When treatment is stopped, weight may return, reflecting the biological nature of the disease rather than a lack of effort," she added.
"These results highlight the chronic nature of obesity and indicate the need for continued treatment to maintain improvements in weight and overall health, as is done with other chronic conditions such as diabetes or hypertension," stated the pharmaceutical company Novo Nordisk, which produces Wegovy.
