People who stop taking weight loss injections regain all the weight they lost in under two years, and at a much faster rate than those following other weight loss plans.
The research, led by the University of Oxford and published in the BMJ, analysed 37 previous studies involving 9,341 participants using weight loss medications known as GLP-1 agonists.
These drugs, originally developed to treat diabetes, work by mimicking a hormone that helps people feel full. The average treatment period across the studies was 39 weeks, with a follow-up period of 32 weeks.
The analysis found that people who stopped taking the medication regained weight at an average rate of 0.4kg per month. Most participants returned to their original weight within about 1.7 years of stopping treatment. On average, users lost 8.3kg during treatment but regained 4.8kg within the first year after stopping.
Researchers found that weight regain occurred almost four times faster than in people who lost weight through behavioural programmes such as diet and exercise plans, regardless of how much weight had been lost.
Dr Sam West from the University of Oxford said the rapid regain was not a failure of the medicines but reflected obesity being a chronic condition. He warned against short-term use of the drugs without long-term weight management strategies.
The study also found that improvements in blood pressure and cholesterol linked to the drugs reversed within about 1.4 years after stopping treatment.
Experts said the findings highlight the need for long-term support alongside medication. Weight loss drugs such as Wegovy can be prescribed on the NHS for up to two years, while Mounjaro has no set prescription limit.