What Is GLP-1? How the Hormone and GLP-1 Drugs Work

GLP-1 went from an obscure hormone to one of the most talked-about words in health in just a few years, thanks to medications like Ozempic, Wegovy, Mounjaro and Zepbound. But what actually is GLP-1, and how do these drugs cause so much weight loss?

This guide explains what the GLP-1 hormone does in your body, how GLP-1 medications work, the most common side effects, and what is worth tracking if you take one.

This article is for general information and is not medical advice. GLP-1 medications must be prescribed and monitored by a licensed clinician.

What is GLP-1?

GLP-1 (glucagon-like peptide-1) is a hormone made by specialized cells in your gut, called L-cells, when you eat. It is one of the "incretin" hormones that help your body handle a meal. GLP-1:

• boosts insulin release from the pancreas when blood sugar is high (and only then, which is why it rarely causes low blood sugar on its own)
• lowers glucagon, a hormone that raises blood sugar
• slows gastric emptying, so food leaves your stomach more slowly
• signals fullness to your brain, reducing hunger and how much you eat

Your own GLP-1 is broken down within minutes. Its effects are short-lived, and that is where the medications come in.

What are GLP-1 medications?

GLP-1 receptor agonists are medications that activate the same receptors as natural GLP-1, but last far longer: many are injected just once a week.

Active ingredient	Brand names	How it works	Mainly used for
Semaglutide	Ozempic, Wegovy, Rybelsus (tablet)	GLP-1 receptor agonist	Ozempic and Rybelsus: type 2 diabetes. Wegovy: weight management
Tirzepatide	Mounjaro, Zepbound	GLP-1 and GIP receptor agonist	Mounjaro: type 2 diabetes. Zepbound: weight management
Liraglutide	Victoza, Saxenda	GLP-1 receptor agonist (daily injection)	Victoza: type 2 diabetes. Saxenda: weight management

Approved uses and brand names vary by country. Newer drugs, such as retatrutide, which also targets the glucagon receptor, are still investigational.

Why the medications last days instead of minutes

The difference between your own GLP-1 and the medications comes down to one number: how long they last in the body.

	Half-life	How often it is taken
Natural GLP-1	About 2 minutes	Released with each meal
Liraglutide (Saxenda, Victoza)	About 13 hours	Daily injection
Tirzepatide (Zepbound, Mounjaro)	About 5 days	Weekly injection
Semaglutide (Wegovy, Ozempic)	About 7 days	Weekly injection

An enzyme called DPP-4 breaks natural GLP-1 down almost immediately. The medications are built to resist it and to bind to a protein in the blood, which is how a single weekly shot keeps working all week.

How GLP-1 medications cause weight loss

Because they act on the same pathways as natural GLP-1, but around the clock:

• You feel full sooner and hungry less often. This is the biggest driver: people simply eat less.
• Food stays in your stomach longer, which adds to fullness.
• Food "noise" often quiets down. Many people report fewer cravings and less preoccupation with food.

In clinical trials, adults taking semaglutide 2.4 mg lost an average of about 15% of their body weight over 68 weeks, and those on the highest dose of tirzepatide about 21% over 72 weeks. See GLP-1 weight loss results by month for the full breakdown.

Common side effects

The most common side effects are gastrointestinal:

• nausea
• diarrhea
• vomiting
• constipation

In the STEP 1 trial, nausea and diarrhea were the most common adverse events with semaglutide, and they were typically mild to moderate and temporary, often easing as the body adjusts. This is one reason doses start low and increase gradually.

There are also rare but serious risks, and a 2023 study in JAMA found GLP-1 drugs used for weight loss were linked to a higher risk of some serious gastrointestinal problems, such as pancreatitis, bowel obstruction and gastroparesis, than another weight-loss medication. This is why GLP-1 medications should only be used under medical supervision, and why you should report severe or persistent symptoms to your doctor.

What happens when you stop taking a GLP-1?

For most people, the weight comes back if the medication stops and nothing else changes. Two trials measured this directly:

• Semaglutide (STEP 1 extension): one year after stopping, participants had regained about two-thirds of the weight they had lost (Wilding et al., 2022).
• Tirzepatide (SURMOUNT-4): after 36 weeks on tirzepatide, people switched to a placebo regained about 14% of their body weight over the next year, while those who stayed on it lost a further 5.5% (Aronne et al., 2024).

That does not make the medication pointless, but it changes how to think about it: as a long-term treatment, like blood pressure medication, rather than a short course. It is also why the habits you build while taking it (enough protein, strength training, regular movement and tracking) matter so much. They are what you keep when the medication stops.

Who can get GLP-1 medications for weight loss?

In the US, Wegovy and Zepbound are approved for adults with a BMI of 30 or more, or a BMI of 27 or more plus at least one weight-related condition, such as high blood pressure, type 2 diabetes or high cholesterol, alongside a reduced-calorie diet and more physical activity. Rules and brand names differ by country, and only a clinician can decide whether it is right for you.

Watch out for "GLP-1 supplements"

Products sold as GLP-1 supplements, GLP-1 patches or "natural Ozempic" do not contain GLP-1 medications, and there is no good evidence they produce anything like the same effect. Compounded or online products outside of a legitimate prescription can also carry real safety risks.

What to track if you take a GLP-1

GLP-1 medications change how much you eat very quickly, which makes a few things especially worth tracking:

1. Your weight trend, not single weigh-ins. Weekly averages show real progress through the noise.
2. Body measurements and progress photos. They show whether you are losing fat, not just weight. Our free body measurement tracker is a printable sheet to start with.
3. Protein intake. With a small appetite, protein often falls, which raises the risk of losing muscle. See why GLP-1s cause muscle loss.
4. Strength. If your strength holds up, you are likely keeping muscle.
5. Side effects and dose changes, so you and your clinician can see how each step affected you.

Curious what the trials suggest for your own starting weight? Our GLP-1 weight loss calculator plots the average trial curves for you.

Bodly brings these together in one app: a smoothed weight trend, tape measurements, side-by-side progress photos and AI photo meal logging to keep an eye on protein. With an Apple Watch, it also tracks your activity, sleep and recovery.

Bottom line

GLP-1 is a gut hormone that raises insulin when blood sugar is high, lowers glucagon, slows digestion and makes you feel full. GLP-1 medications such as semaglutide and tirzepatide mimic it for much longer, which reduces appetite and can lead to substantial weight loss.

They are powerful prescription drugs with common gastrointestinal side effects and some rare serious risks, so they belong under medical supervision. If you take one, track your weight trend, measurements, protein and strength, not just the scale.

Keep reading

• GLP-1 weight loss results by month: what the trials show, week by week.
• Why GLP-1s cause muscle loss: how to lose fat without losing strength.
• Metabolic adaptation: why weight loss slows down over time.

Sources

• Wilding JPH, et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: the STEP 1 trial extension. Diabetes Obes Metab (2022).
• Aronne LJ, et al. Continued treatment with tirzepatide for maintenance of weight reduction in adults with obesity: the SURMOUNT-4 randomized clinical trial. JAMA (2024).
• Müller TD, et al. Glucagon-like peptide 1 (GLP-1). Molecular Metabolism (2019).
• Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). New England Journal of Medicine (2021).
• Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). New England Journal of Medicine (2022).
• Sodhi M, et al. Risk of gastrointestinal adverse events associated with glucagon-like peptide-1 receptor agonists for weight loss. JAMA (2023).