GLP-1 receptor agonists are a class of medications that mimic glucagon-like peptide-1, a hormone released naturally by the gut after eating. GLP-1 signals to the brain that the body has consumed food, reduces appetite, slows the rate at which the stomach empties, and stimulates insulin secretion from the pancreas. The pharmaceutical versions of this hormone, injected subcutaneously, produce these effects in a sustained and amplified form that results in significant caloric restriction without the constant conscious effort of dieting.
The Three Drugs Currently in Clinical Use for Weight Loss
Semaglutide is a weekly injection developed by Novo Nordisk. It was originally approved for type 2 diabetes under the brand name Ozempic and subsequently approved for weight loss at a higher dose under the brand name Wegovy. Clinical trials in adults without diabetes found an average body weight reduction of 14.9 to 17.4 percent over 68 weeks at the 2.4mg weekly dose, published in the New England Journal of Medicine in 2021 by Wilding and colleagues.
Liraglutide is a daily injection, also from Novo Nordisk, sold as Saxenda for weight loss and Victoza for diabetes. It was the first GLP-1 drug approved specifically for obesity management. Clinical trials in the SCALE programme found an average weight loss of 8 percent of body weight over 56 weeks, meaningfully less than semaglutide. Its requirement for daily rather than weekly injection makes adherence more demanding.
Tirzepatide is a weekly injection from Eli Lilly sold as Mounjaro. It is the newest and most effective of the three. Unlike semaglutide and liraglutide, which act on GLP-1 receptors only, tirzepatide is a dual agonist that also activates GIP receptors, a second gut hormone involved in insulin secretion and fat storage. The SURMOUNT-1 trial, published in the New England Journal of Medicine in 2022, found average weight reductions of 20.9 percent at the 15mg dose over 72 weeks.
What These Drugs Do Not Do
None of the three drugs selectively removes body fat. The caloric deficit they produce results in weight loss from all tissues, including skeletal muscle. This distinction matters considerably for health and longevity, and is covered in detail in the companion articles on muscle loss. The drugs are also not permanent: weight typically returns after stopping, at a rate and extent that depends on the duration of treatment and whether lifestyle changes were embedded during it.
- Semaglutide: weekly injection, 0.5mg to 2.4mg dose, brand names Ozempic and Wegovy
- Liraglutide: daily injection, up to 3mg dose, brand names Victoza and Saxenda
- Tirzepatide: weekly injection, 5mg to 15mg dose, brand name Mounjaro; dual GIP and GLP-1 agonist
- All three require a prescription and medical supervision
- None selectively removes fat: muscle loss accompanies all three without appropriate exercise and nutrition
- Average weight loss: liraglutide 8%, semaglutide 15 to 17%, tirzepatide 20 to 22%