GLP-1 Help

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How GLP-1 drugs actually work

The short answer is that these drugs copy a gut hormone and the brain responds by wanting less food. The longer answer separates what has been measured in people from what has only been shown in mice.

By Dr. Chiranjeevi Peetla · Updated November 2026 · 6 min read · 9 sources, each linked

A copy of a gut hormone

GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after you eat. It tells the pancreas to release insulin when blood sugar is high, and it acts on parts of the brain that regulate appetite. The natural hormone is broken down within minutes. Semaglutide (Wegovy, Ozempic) is a modified copy that lasts about a week, which is why it is a weekly injection.[1] The Wegovy label puts the weight effect plainly: the drug "lowers body weight through decreased calorie intake".[1]

Tirzepatide (Zepbound, Mounjaro) copies two hormones at once: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). Its label says it "decreases food intake and modulates fat utilization", and that GIP receptors are also found in brain regions that control appetite.[2]

What changes: you eat less

This is the best-measured part. In a 20-week randomized study, people on semaglutide 2.4 mg ate 1,736 kJ at a free-choice lunch versus 2,676 kJ on placebo, about 35% less, and reported less hunger, fewer cravings and less desire for savoury and sweet food.[3] Across feeding studies of liraglutide, semaglutide and tirzepatide, energy intake fell by roughly 16 to 39% compared with placebo.[4] In a 60-week trial the reduction in total intake was 30% at week 20, and still 24% at weeks 40 and 60, so the effect does not simply wear off.[5]

Free-choice lunch at week 20 · semaglutide 2.4 mg vs placebo, energy eaten

65% of placebo intake35% less

1,736 kJ vs 2,676 kJFriedrichsen et al. 2021

On semaglutide 2.4 mg, people ate about 65% of what the placebo group ate at the same meal, 20 weeks in.[3]

The stomach slows, a little

These drugs also slow how fast the stomach empties. Measured with a tracer meal scan (scintigraphy), the time for half a meal to leave the stomach was about 36 minutes longer than placebo on average, 138 versus 95 minutes.[6] The effect is strongest at the start and fades with continued use; the tirzepatide label says the delay is "largest after the first dose and diminishes over time".[2] In the semaglutide lunch study, a simpler absorption test found almost no delay at 20 weeks, yet people still ate a third less.[3] So the stomach is part of the story, but the smaller part.

The brain part

GLP-1 receptors sit in the hypothalamus and in the brainstem, including the area postrema, the region that triggers nausea. In mice, researchers have shown that the brainstem circuits that make an animal eat less and the circuits that make it feel sick are neighbours but can be separated.[7][8] That is animal work. In people the evidence is indirect: appetite questionnaires in the semaglutide trial showed fewer and weaker cravings and less desire for savoury and sweet food, which matches the "food noise goes quiet" experience many users describe.[3]

Why tirzepatide is different

Tirzepatide adds GIP activity. In the only head-to-head trial, 72 weeks at the highest tolerated doses, people on tirzepatide lost 20.2% of body weight and people on semaglutide lost 13.7%.[9] Why the second hormone adds weight loss is still debated; the label simply states what the drug does, not why the combination works better.[2] See the comparison page.

Heard vs evidence

"It just makes your stomach too full to eat."
The stomach slows by about half an hour on average, and the slowing fades. The bigger, lasting effect is in appetite: a third less eaten at a free lunch with almost no stomach delay measured.[3][6]
"It melts fat directly."
The Wegovy label attributes weight loss to lower calorie intake. The tirzepatide label mentions fat utilisation, but the measured driver in people is eating less.[1][2][4]
"The effect wears off after a few months."
Intake was still 24% lower than placebo at week 60. What does change is that weight levels off, which is covered in why the scale stops moving.[5]

The short version

Short on the way. A 35-second captioned version of this page is in production for the Fewer Bites channel. It will be embedded here when it is live.

Questions to bring to your prescriber

Print these or screenshot them. They are conversation starters, not a checklist of things to do.

  • Which of the two drugs are we talking about for me, and why that one?
  • I take insulin or a sulfonylurea: does the way this drug works change my low-blood-sugar risk?
  • I take other pills that need to be absorbed quickly: does slower stomach emptying matter for them?

Education, not advice. This page explains what trials, labels and guidelines report. It does not tell anyone to start, change or stop a medicine. Your prescriber knows your history; this page does not.

Sources

Every number on this page traces to one of these. "Primary read" means the full paper or label was read; "abstract only" means only the abstract was available; "secondary" means the figure was taken from a named review.

  1. Novo Nordisk / FDA. Wegovy (semaglutide) injection and tablets, prescribing information, revised 06/2026. label
  2. Eli Lilly / FDA. Zepbound (tirzepatide) prescribing information, revised 08/2026. label
  3. Friedrichsen M et al. Effect of semaglutide 2.4 mg on gastric emptying, appetite and energy intake in adults with obesity. Diabetes Obes Metab 2021;23:754–62. primary read · RCT · industry-funded
  4. Christensen S et al. Dietary intake by patients taking GLP-1 and dual GIP/GLP-1 receptor agonists: a narrative review. Obesity Pillars 2024;11:100121. secondary · review · industry-affiliated authors
  5. Randomized 60-week trial of semaglutide 2.4 mg on energy intake and appetite (NCT05548647). secondary · figures taken from the Fewer Bites dossier
  6. Hiramoto B et al. Quantified metrics of gastric emptying delay by GLP-1 receptor agonists: systematic review and meta-analysis. Am J Gastroenterol 2024;119:1126–40. primary read · meta-analysis
  7. Huang KP et al. Dissociable hindbrain GLP1R circuits for satiety and aversion. Nature 2024;632:585–93. primary read · animal
  8. Park 2025. Review of hindbrain GLP-1 receptor circuits for satiety and nausea (Fewer Bites nausea dossier, section 1). secondary · review
  9. Aronne LJ et al. Tirzepatide as compared with semaglutide for the treatment of obesity (SURMOUNT-5). N Engl J Med 2025. abstract only · RCT · industry-funded

Forums

What is "food noise", and why does it go quiet?

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Side effects

Why nausea happens

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Starting

How long it takes to work

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Eating

Full after three bites

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