GLP-1 for weight loss: what the studies actually show

GLP-1 is a natural hormone released by the gut after eating that regulates appetite, digestion and blood sugar. GLP-1 drugs mimic this hormone. They appear to reduce appetite via the brain, slow gastric emptying so you feel full for longer, and improve blood sugar regulation. These mechanisms explain why they affect weight, but a mechanism on its own does not prove an outcome. Only human studies show that.

Semaglutide is sold in products such as Ozempic and Wegovy. Tirzepatide, in the products Mounjaro and Zepbound, is not only a GLP-1 — it also activates a second hormone receptor called GIP. Liraglutide is an older substance in the same family. They differ in active substance, approval, dose and trial data. Lumping them all together is not accurate.

In obesity medicine, a loss of at least 5 percent of body weight is considered meaningful, while 10 to 20 percent is associated with greater metabolic benefits. In the STEP trial for semaglutide, with almost 2,000 adults, the average loss was around 15 percent at 68 weeks. In the SURMOUNT-1 trial for tirzepatide, with more than 2,500 participants, the average loss reached around 20 percent at 72 weeks. Liraglutide showed smaller numbers. These are among the strongest results for pharmacological weight management. However, averages do not predict an individual outcome, and in real life the numbers are often smaller than in controlled trials.

Some data show benefits beyond weight itself, mainly in people at elevated risk. In the large SELECT trial, in adults with excess weight and pre-existing cardiovascular disease but without diabetes, semaglutide was associated with fewer serious cardiovascular events compared with placebo. This kind of benefit concerns specific clinical groups under medical supervision and does not automatically transfer to someone who simply wants to lose a few kilos for aesthetic reasons.

A critical point that is often left out. Stopping GLP-1s frequently leads to regaining part of the weight. In a follow-up to the STEP trial, within a year of stopping semaglutide participants regained a large share of the weight they had lost, and the metabolic benefits moved backwards. In a corresponding trial for tirzepatide, those who stopped regained weight while those who continued maintained it. This does not mean the drug is addictive. It means that for many people obesity behaves like a chronic condition that needs long-term management.

The most common side effects are gastrointestinal: nausea, diarrhoea, vomiting, constipation and abdominal pain. The official information for approved products also carries warnings about pancreatitis, gallbladder disease and other rarer events. Even rare ophthalmological signals have been investigated. Many of the rare or long-term questions remain under monitoring, which is why these drugs are given on prescription and under supervision rather than freely.

This is the most important distinction for the public. An approved GLP-1 medicine is manufactured under strict quality control, has known purity and is given under medical supervision. Products sold online as research substances or as improvised preparations do not carry those guarantees. Authorities have warned that some forms of semaglutide outside approved channels differ from the active substance of the approved medicines, and that dosing errors have been reported. The same substance name does not mean the same product.

GLP-1s are among the best-documented drugs for weight, with strong human data and clear benefits in appropriate clinical groups. At the same time, weight often returns after stopping, the side effects are real, and approved medicines are not the same as unregulated market products. At allpeptides.eu we present this data in a neutral, educational way. For any decision concerning your health or your weight, the right person to talk to is a specialist doctor, not an advert.