The hormone behind the drugs
GLP-1, short for glucagon-like peptide-1, is an incretin hormone your gut releases after you eat. It tells the pancreas to release insulin when blood sugar is high, slows the stomach and signals fullness to the brain.
Natural GLP-1 breaks down in minutes. Drug makers changed the molecule so it resists breakdown, which is why semaglutide and tirzepatide can be taken once a week.
Three effects that drive weight loss
- Less hunger. The drugs act on appetite centers in the brain, so you feel satisfied sooner and think about food less.
- Slower stomach emptying. Food stays in the stomach longer, which extends fullness. It also explains nausea and burping.
- Better blood sugar control. Insulin release goes up and glucagon goes down when glucose is high, which is why these drugs started as diabetes treatments.
Explainer · 0:23
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- After you eat, your gut releases a hormone called GLP-1. GLP-1 medicines copy it, but last much longer in the body.
- It slows how fast your stomach empties, so you feel full sooner and stay full for longer.
- It also acts on appetite centers in the brain, which quiets hunger and constant thoughts about food.
- When blood sugar is high, it helps the pancreas release insulin. Tirzepatide also acts on a second gut hormone, GIP.
Single versus dual action
| Ingredient | Acts on | Brand names |
|---|---|---|
| Semaglutide | GLP-1 | Wegovy, Ozempic, Rybelsus |
| Tirzepatide | GLP-1 and GIP | Zepbound, Mounjaro |
| Liraglutide | GLP-1, daily | Saxenda, Victoza |
| Dulaglutide | GLP-1 | Trulicity |
Tirzepatide also activates the GIP receptor. In separate trials it produced larger average weight loss than semaglutide, and a head-to-head trial reported similar results. Compare them in tirzepatide vs semaglutide.
Why the dose starts low
Every label starts with a low dose and raises it every few weeks, a process called titration. This gives your gut time to adjust and reduces nausea and vomiting.
The starting dose is not meant to cause much weight loss. See your schedule in the dose schedule tool.
How long until it works
Many people notice less hunger within the first weeks. Weight loss builds more slowly, because the dose climbs in steps and the maintenance dose takes months to reach.
| Drug | Label dose steps | Earliest week at highest dose |
|---|---|---|
| Wegovy | 0.25, 0.5, 1, 1.7, 2.4 mg weekly, each step 4 weeks | Week 17 |
| Zepbound | Starts at 2.5 mg weekly, rising by 2.5 mg after at least 4 weeks per step, to 5, 10 or 15 mg | Week 21 for 15 mg |
In STEP 1 and SURMOUNT-1, average weight kept falling for more than a year before leveling off. Your prescriber may keep you at a lower dose if it works for you or if side effects are hard to tolerate.
What the drugs don't do
- They don't burn fat directly; weight loss comes mainly from eating less
- They don't replace protein, sleep or activity
- Their effect fades after stopping; see stopping a GLP-1
Estimate your own expected range with the weight-loss projection.
When to call your doctor
- Severe stomach pain, which may signal pancreatitis or gallbladder problems
- A lump or swelling in the neck, trouble swallowing or hoarseness
- Signs of an allergic reaction such as swelling of the face or trouble breathing
Run the numbers
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Common questions
What does GLP-1 stand for?
Glucagon-like peptide-1, a hormone your gut releases after eating that helps control blood sugar and appetite.
Is tirzepatide a GLP-1?
Tirzepatide acts on both GLP-1 and GIP receptors, so it's often called a dual agonist. It's grouped with GLP-1 drugs in everyday use.
Do GLP-1 drugs speed up metabolism?
Not meaningfully. Most of the weight loss comes from eating less because of reduced appetite and slower stomach emptying.
Sources
Clinician review pending. Drafted from primary sources and waiting for sign-off by a registered dietitian before launch. Every number is checked against its source. Drug facts are re-checked quarterly, prices monthly and whenever a label changes. How we review · Report an error