How do GLP-1 medications actually work?
Written and medically reviewed by Zahraa Sater, M.D., M.P.H.
Reviewed August 12, 2026 · Next review due August 12, 2027
Short answer
GLP-1 medications copy a gut hormone your body already releases when you eat. Semaglutide and tirzepatide slow how fast the stomach empties, increase insulin release only when glucose is high, and change appetite signalling in the brain so hunger and food preoccupation fall. Tirzepatide acts on a second receptor, GIP, as well. Weight loss follows from eating less, not from burning more.
GLP-1 medications work by imitating hormones your gut releases within minutes of a meal. Semaglutide, sold as Wegovy and Ozempic, acts on the GLP-1 receptor. Tirzepatide, sold as Zepbound and Mounjaro, acts on both the GLP-1 and the GIP receptor. Both slow stomach emptying, raise insulin secretion when blood glucose is high, lower glucagon, and act on appetite pathways in the brain. You feel full earlier and stay full longer.
What GLP-1 medications do in the gut
Natural GLP-1 survives in the bloodstream for a couple of minutes before the enzyme DPP-4 breaks it down. Semaglutide and tirzepatide are engineered to resist that enzyme, which is why one injection lasts a week. Slower gastric emptying is the mechanism behind early fullness, and it is also the mechanism behind most of the nausea. The Zepbound label carries a warning about pulmonary aspiration during anaesthesia for the same reason: the stomach may still hold food when it is expected to be empty.
What GLP-1 medications do in the brain
GLP-1 receptors sit in the hypothalamus and brainstem, the regions that set how much food feels like enough. This is the part patients describe as the quieting of food noise: the constant background negotiation about the next meal drops away. That effect is pharmacological, not motivational, which is why it appears within weeks of starting and reverses within weeks of stopping.
Why tirzepatide adds a second receptor
GIP is the other main incretin hormone. Tirzepatide activates GIP and GLP-1 receptors together, and in the head to head SURMOUNT-5 trial that combination produced more weight loss than semaglutide at 72 weeks, 20.2 percent versus 13.7 percent. The extra receptor also appears to improve tolerability at comparable weight loss: gastrointestinal side effects led 2.7 percent of the tirzepatide group to stop, compared with 5.6 percent on semaglutide.
| Medication | Receptors | Route |
|---|---|---|
| Semaglutide (Wegovy, Ozempic) | GLP-1 | Weekly injection |
| Semaglutide tablets (oral Wegovy) | GLP-1 | Daily tablet, empty stomach |
| Tirzepatide (Zepbound, Mounjaro) | GLP-1 and GIP | Weekly injection |
| Orforglipron (Foundayo) | GLP-1, non-peptide | Daily tablet, no food restriction |
Understanding the mechanism tells you what the medication cannot do. It does not raise your metabolic rate, it does not build muscle, and it does not change what happens after you stop. Ask your prescriber which receptor your medication targets, what the escalation schedule is, and what the plan is for protein and resistance training while the appetite signal is suppressed.
The clinical detail
Semaglutide is a 94 percent homologous human GLP-1 analogue with an albumin-binding fatty diacid side chain and an Aib substitution at position 8 that confers DPP-4 resistance, giving a half life of about one week. Tirzepatide is a 39 amino acid synthetic peptide with dual GIP and GLP-1 receptor agonism and a comparable weekly half life. Approved maintenance doses are semaglutide 2.4 mg weekly (7.2 mg weekly for Wegovy HD, approved March 2026), tirzepatide 5, 10 or 15 mg weekly, and oral semaglutide 25 mg daily. Both injectables carry a boxed warning for rodent thyroid C-cell tumours and are contraindicated with personal or family history of medullary thyroid carcinoma or MEN 2.
Written and medically reviewed by Zahraa Sater, M.D., M.P.H.
Reviewed August 12, 2026 · Next review due August 12, 2027
Sources
- ZEPBOUND (tirzepatide) injection, prescribing information, revised January 2026. U.S. Food and Drug Administration
- WEGOVY (semaglutide) injection, prescribing information. U.S. Food and Drug Administration
- SURMOUNT-5: Greater Loss of Weight With Tirzepatide Than Semaglutide. American College of Cardiology. doi:10.1056/NEJMoa2416394
- Prescription Medications to Treat Overweight and Obesity. NIDDK, National Institutes of Health
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