Obesity affected an estimated 2.2 billion people worldwide by 2020. That number is expected to increase to 3.3 billion by 2035.

Obesity is associated with an increased risk of cardiovascular disease, certain cancers and diabetes.

Anti-obesity medications like the new GLP-1 drugs have become an important addition to bariatric surgery, lifestyle modifications like eating more fruits and vegetables and exercising more, and behavioral therapy for the treatment of obesity.

People who completed 36 weeks of treatment with tirzepatide, a dual GLP-1/GIP RA drug, regained almost half the weight they had lost within a year after they switched to a placebo.

Six medications have been approved by the U.S. Food and Drug Administration to treat obesity: orlistat, naltrexone-bupropion, liraglutide, semaglutide, tirzepatide and phentermine-topiramate.

Discontinuing any weight-loss method can result in regaining at least some of the weight people have lost, an outcome known as weight rebound. Studies of rebound have focused on bariatric surgery and lifestyle interventions such as diet and exercise, A recent study looked at the potential for weight rebound after discontinuing the new weight-loss drugs.

Researchers at Peking University analyzed data from 11 studies done worldwide and found that, overall, discontinuing anti-obesity medication was significantly associated with weight rebound. “Weight re-gain is common with various weight loss strategies, and it is necessary to establish an effective long-term strategy for weight loss in clinical practice,” the researchers said.

The studies included more than 1,570 participants in treatment groups that took anti-obesity medications and almost 900 in control groups. These studies tracked the amount of weight participants lost while taking the medication and the amount of weight rebound. Participants took the weight-loss medication for at least four weeks and were followed for at least four weeks after they stopped.

The study controlled for factors including the presence of diabetes, lifestyle changes and the type of anti-obesity medication used in each study.

The amount of weight the participants regained depended on how long it had been since they discontinued the medication. Four weeks after discontinuing anti-obesity medication, those in the treatment group had lost 32 kilograms, or more than 70 pounds. At week eight, they had regained about three pounds, however; at week 12, almost four pounds; and at week 20, more than five pounds. Weight rebound started to level off, or plateau, starting at 26 weeks after discontinuing medication and through one year of follow-up.

The amount of weight participants regained depended on the type of anti-obesity medication they took. Glucagon-like peptide-1 receptor agonists (GLP-1 RAs) and dual GLP-1 and glucose-dependent insulinotropic polypeptide (GIP) RAs were associated with a significant weight rebound of almost four pounds. In one study, for example, participants who completed 36 weeks of treatment with tirzepatide, a dual GLP-1/GIP RA, regained almost half the weight they had lost within a year after they switched to a placebo.

Four weeks after discontinuing anti-obesity medication, those in the treatment group had lost 32 kilograms, or more than 70 pounds. At week eight, however, they had regained about three pounds. At week 12, they had regained almost four pounds.

Weight rebound also depended on whether or not participants stuck with lifestyle changes after discontinuing anti-obesity medication. Participants who continued with lifestyle changes after discontinuing medication regained about four pounds, while those who discontinued lifestyle changes regained a little more than two pounds.

Studies with a longer follow up period are needed to find out how different factors contribute to weight rebound following treatment with anti-obesity medications, the researchers said

The study was published in BMC Medicine.