How GLP-1 and Dual GIP/GLP-1 Drugs Work in the Body
Semaglutide activates the GLP-1 receptor; tirzepatide activates both GLP-1 and GIP receptors. Both slow gastric emptying and reduce appetite; tirzepatide's dual action may explain its larger average weight loss. Semaglutide activates the GLP-1 receptor while tirzepatide activates both GIP and GLP-1 receptors, and this dual action is thought to drive tirzepatide's greater average weight loss in head-to-head evidence.
- GLP-1 and GIP are natural incretin hormones released after eating.
- Semaglutide mimics GLP-1; tirzepatide mimics both GLP-1 and GIP.
- Both slow gastric emptying, increase satiety, and improve glucose control.
- Dual agonism is the leading explanation for tirzepatide's larger average effect.
The incretin system
After you eat, your gut releases incretin hormones that signal the pancreas to release insulin and the brain to register fullness. Two of these are GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). GLP-1 receptor agonists like semaglutide are synthetic versions that activate this system more powerfully and for longer than the natural hormone.
This is why the drug class is described as incretin-based: it amplifies a signaling system the body already uses to regulate appetite and blood sugar.
How semaglutide works
Semaglutide binds the GLP-1 receptor, producing several effects: it slows gastric emptying so food stays in the stomach longer, it acts on brain centers that regulate appetite to increase satiety and reduce hunger, and it enhances glucose-dependent insulin release. Together these reduce calorie intake and improve blood sugar.
The slowed gastric emptying is also why nausea is the most common side effect, and why the drug is titrated slowly.
| Feature | Semaglutide | Tirzepatide |
|---|---|---|
| Receptors | GLP-1 | GLP-1 + GIP |
| Gastric emptying | Slowed | Slowed |
| Appetite | Reduced | Reduced (often more) |
| Avg weight loss | Strong | Greater |
How tirzepatide adds GIP
Tirzepatide activates the GLP-1 receptor and, additionally, the GIP receptor. GIP is another incretin hormone, and adding its activity appears to enhance the metabolic and appetite-suppressing effects for many people. This dual mechanism is the leading explanation for tirzepatide's larger average weight loss in trials like SURMOUNT-5.
The exact contribution of GIP is still an active research area, but the clinical signal — greater average weight loss with the dual agonist — is consistent across studies.
| Effect | Result |
|---|---|
| Slowed gastric emptying | Longer fullness, less intake |
| Appetite signaling | Reduced hunger |
| Insulin release | Better glucose control |
What the mechanism means for you
Understanding the mechanism clarifies a few practical points. The appetite reduction is central and physiological, not willpower; the slow titration exists because the same mechanism that suppresses appetite also causes GI effects; and the dual mechanism of tirzepatide is why it tends to produce more weight loss, not marketing.
It also explains why stopping the drug tends to bring appetite back: you are removing an amplified satiety signal, and the body returns to its prior set point. This is why these drugs are framed as long-term therapies.
Frequently asked questions
What is the difference between semaglutide and tirzepatide mechanism?
Semaglutide activates one incretin receptor (GLP-1); tirzepatide activates two (GLP-1 and GIP). The dual action may explain tirzepatide's larger average weight loss.
Why do these drugs reduce appetite?
They activate GLP-1 (and for tirzepatide, GIP) receptors, slowing gastric emptying and acting on brain appetite centers to increase fullness and reduce hunger.
Why does appetite return after stopping?
The drugs amplify a natural satiety signal. Removing them lets appetite return toward its prior level, which is why they are used long-term.
Sources
- Clinical trials via NEJM.
- Incretin physiology and GLP-1/GIP pharmacology literature.
- FDA — human drug compounding and GLP-1 status.