How GLP-1 drugs actually work, and what that explains

Almost every practical question about these medicines — why the dose climbs so slowly, why the nausea arrives in the same week as each increase, why the weight comes back, why a service that never asks what else you take is not doing its job — has the same answer, and it is written in the prescribing information. This page quotes it rather than paraphrasing it.

What the molecule binds to

GLP-1 is a hormone the gut releases after eating. The drugs in this class are built to look like it. Novo Nordisk’s label for Wegovy puts the resemblance in numbers: “Semaglutide is a GLP-1 analogue with 94% sequence homology to human GLP-1,” which “acts as a GLP-1 receptor agonist that selectively binds to and activates the GLP-1 receptor, the target for native GLP-1.”

The reason that changes appetite rather than just blood sugar is anatomical. The same label states that “GLP-1 is a physiological regulator of appetite and caloric intake, and the GLP-1 receptor is present in several areas of the brain involved in appetite regulation,” and that in animal studies semaglutide “distributed to and activated neurons in brain regions involved in regulation of food intake.” This is a drug that acts on the brain by imitating a gut hormone.

Tirzepatide adds a second receptor. Lilly’s label describes it as “a GIP receptor and GLP-1 receptor agonist” that “selectively binds to and activates both the GIP and GLP-1 receptors,” and is careful about how strong the claim for the second one is: “Nonclinical studies suggest the addition of GIP may further contribute to the regulation of food intake.” Suggest, may, nonclinical. That is the manufacturer’s own hedging on the mechanism behind the larger average result, and it is worth noticing how much more confident the marketing is than the label.

Why the dose climbs so slowly

Both drugs slow the stomach down. The Wegovy label says flatly that “semaglutide delays gastric emptying.” Lilly’s adds the detail that makes titration make sense: “Tirzepatide delays gastric emptying. The delay is largest after the first dose and this effect diminishes over time.”

That is the whole rationale for the ladder. Each label instructs prescribers to follow the escalation schedule specifically “to reduce the risk of gastrointestinal adverse reactions”, and each offers the same remedy when a step is not tolerated — wait longer at the dose you are on rather than push through. The nausea is not a sign the drug is working, and it is not something to be endured on principle. It is the predicted consequence of moving up a step, and the label’s own instruction is to slow down. The full ladder for every product is set out in Every dose step, and what to do when you miss one.

Fat, and the part that is not fat

Both labels use the same construction, and the construction matters. Wegovy: “Semaglutide lowers body weight with greater fat mass loss than lean mass loss.” Zepbound: “Tirzepatide lowers body weight with greater fat mass loss than lean mass loss.”

Greater than is not instead of. Neither sentence says lean mass is preserved; both say more of the loss is fat than is not. That is the pharmacological basis for every piece of advice about protein intake and resistance training on these drugs, and it is why the loss of lean mass sits on this site’s side-effect list as expected without countermeasures rather than as a rare event. It is also, bluntly, a subject most subscription services do not raise, because raising it means recommending work that the subscription does not sell.

Why appetite returns when the drug stops

The label describes the appetite effect as a drug effect: semaglutide “decreases calorie intake”, and “the effects are likely mediated by affecting appetite.” Nothing in either label describes a change that persists once the receptor is no longer being occupied. So the honest reading of the mechanism is that stopping the drug removes the mechanism — which is what the withdrawal and extension data show, and what makes the monthly price closer to a standing cost than to the price of a course. That evidence is set out in What happens when you stop, and its consequences for anything advertised as a programme price in What GLP-1s actually cost in 2026.

What the manufacturers say they do not know

The candid lines in these documents are the useful ones. On the cardiovascular indication, Novo writes: “The exact mechanism of semaglutide in CV risk reduction in adults has not been established.” On the liver indication: “the precise mechanism of action of semaglutide is not fully understood and may involve multiple pathways mediated by weight loss and other factors,” and “the relationship between the pathophysiology of MASH in animal models and humans has not been fully established.”

Those benefits are real — they are approved indications supported by outcome trials, and they are covered in What else these drugs are approved to do. But the drug is not understood well enough for anyone to tell you which of its effects is doing the work. Treat any marketing page that explains the mechanism more confidently than the label does as marketing.