GLP-1 Peptides Explained: Semaglutide, Tirzepatide, and the Incretin Revolution

What GLP-1 is

After you eat, cells in your small intestine release a hormone called GLP-1, short for glucagon-like peptide-1. It does four things at once. It tells the pancreas to release insulin, but only when blood sugar is high, which is why it rarely causes low blood sugar on its own. It turns down glucagon, the hormone that raises blood sugar. It slows the stomach, so food sits longer and you feel full. And it acts on the brain’s hunger centers directly.

Natural GLP-1 lasts about two minutes before an enzyme called DPP-4 breaks it down. That is why the drugs built from it had to be re-engineered: a hormone that disappears in two minutes cannot be a once-a-week injection.

Most peptides on this site have animal data and user reports behind them. GLP-1 drugs are the exception. They went through the full clinical pipeline, they are FDA approved, and millions of people take them. When you read a claim about semaglutide, it usually traces back to a trial with thousands of people in it. That is not true of almost anything else here.

The drugs

Semaglutide (Ozempic, Wegovy, Rybelsus)

Semaglutide, made by Novo Nordisk, is GLP-1 with a fatty acid chain attached. The chain lets it stick to albumin, a blood protein, which stretches its life from two minutes to about a week. Ozempic is the diabetes dose, Wegovy is the higher weight-loss dose, and Rybelsus is a daily pill for diabetes that works only because it is packaged with an absorption helper.

The trial people quote most is STEP 1: 1,961 adults with obesity, 68 weeks, and an average weight loss of 14.9% on semaglutide against 2.4% on placebo (Wilding et al., 2021). The SELECT trial then followed 17,604 overweight adults with heart disease for over three years and found 20% fewer heart attacks, strokes, and cardiovascular deaths, the first time a weight-loss drug had shown that (Lincoff et al., 2023).

Tirzepatide (Mounjaro, Zepbound)

Tirzepatide, made by Eli Lilly, activates the GLP-1 receptor and a second gut-hormone receptor called GIP. Mounjaro is the diabetes brand, Zepbound the weight-loss brand, both weekly injections.

In SURMOUNT-1, 2,539 adults with obesity lost an average of 20.9% of their body weight over 72 weeks on the top dose, against 3.1% on placebo (Jastreboff et al., 2022). In a head-to-head diabetes trial, every tirzepatide dose beat semaglutide 1 mg on both blood sugar and weight (Frías et al., 2021), and a later head-to-head at full weight-loss doses (SURMOUNT-5) again favored tirzepatide.

Liraglutide (Victoza, Saxenda)

The older GLP-1 drug, also from Novo Nordisk. It is a daily injection and produces about half the weight loss of semaglutide. It proved the idea and has mostly been replaced by the weekly drugs.

AOD-9604 is not one of these

AOD-9604, a fragment of growth hormone, gets sold in the same breath as GLP-1 drugs because it is marketed for fat loss. It works through a different mechanism, its human trials were disappointing, and it is not in the same evidence category. See the fat loss guide.

How the two main drugs compare

	Semaglutide (Wegovy)	Tirzepatide (Zepbound)
Works on	GLP-1 receptor	GLP-1 and GIP receptors
Average weight loss in the main trial	14.9% at 68 weeks	20.9% at 72 weeks
Dosing	Weekly injection, up to 2.4 mg	Weekly injection, up to 15 mg
Heart protection proven	Yes (SELECT)	Trial still running
Stomach side effects	About a third of people	About a third of people
Years on the market	More	Fewer

Tirzepatide wins on weight lost. Semaglutide has the heart-outcomes trial that tirzepatide is still waiting on. Individual response varies more than the averages suggest; some people do better on one than the other for no reason anyone can predict.

The GIP part of tirzepatide surprised researchers. Mice without a GIP receptor resist obesity, so you might expect activating that receptor to add fat, not remove it. Tirzepatide does the opposite. The working theory is that flooding the receptor with a drug behaves differently from the small, natural pulses of GIP, and that GIP helps fat tissue handle insulin and adds a second route into the brain’s appetite system (Drucker, 2018).

What happens when you stop

Most of the weight comes back. When STEP 1 participants stopped semaglutide, they regained about two-thirds of what they had lost within a year. SURMOUNT-4 showed the same pattern for tirzepatide. Doctors now describe obesity the way they describe high blood pressure: a condition you manage as long as you have it, not one you cure in a course. Research into lower maintenance doses and taking breaks is underway but has not settled anything.

The muscle question

Losing a lot of weight by any method means losing some muscle along with the fat. In GLP-1 trials, roughly 25 to 40% of the weight lost was lean tissue, which includes muscle. Whether lifting weights and eating enough protein protects against this fully is being studied and is not proven. It matters most for older adults, who have less muscle to spare.

Side effects

Nausea, vomiting, diarrhea, and constipation are the common ones. They are worst while the dose is going up and usually fade. Starting low and raising the dose slowly is standard practice because it cuts these down.

The rare but serious ones: pancreatitis; gallbladder problems, which come with fast weight loss from any cause; a boxed warning about thyroid tumors, based on rat studies and never confirmed in people; and, in people with diabetes, a temporary worsening of eye disease when blood sugar drops fast. See the peptide safety guide for what to monitor.

Compounded and “research” versions

Shortages and list prices of $1,000 to $1,500 a month drove people to compounding pharmacies and to vendors selling semaglutide and tirzepatide as research peptides. The FDA has warned about both. Compounded products are not tested the way the branded drugs are, some have used a different salt form of the molecule, and research-vendor vials come with no guarantee of what is inside. A bargain version of a drug with excellent trial data is not the same drug. Our guide on reading peptide claims covers how to weigh this.

Why these are treated differently from other peptides here

Semaglutide and tirzepatide sit in a different tier from BPC-157 or ipamorelin, and the site reflects that on purpose. SELECT alone enrolled more people than every research peptide on this site combined has ever had in a published human trial. The drugs are made under pharmaceutical manufacturing rules, prescribed by doctors, and covered by insurance. Millions of prescriptions have produced safety data that no research peptide has.

A claim about semaglutide is backed by randomized trials in thousands of people. A claim about BPC-157 is backed by rat studies. Both are evidence. They are not the same weight of evidence, and knowing the difference is the most useful skill in this whole subject.

Frequently Asked Questions

Which is better, semaglutide or tirzepatide?

Tirzepatide produces more weight loss on average. Semaglutide has proven heart protection. Insurance coverage, side effects, and how your body happens to respond decide it in practice. Both work.

Are GLP-1 drugs peptides?

Yes. Semaglutide and tirzepatide are lab-made versions of a natural 30-amino-acid gut hormone. They are among the few peptides on this site that made it all the way through clinical development.

Do you have to take them forever?

The evidence so far says most people regain most of the weight after stopping, so the honest answer is that they work while you take them. Some people hold part of the loss. Strategies for stepping down are being studied.

Can you take them if you do not have diabetes?

Yes. Wegovy and Zepbound are approved for weight management in adults with a BMI of 30 or more, or 27 or more with a weight-related condition, with or without diabetes.

How do they compare to other fat-loss peptides?

Nothing else comes close. AOD-9604 has weak human data. Tesamorelin reduces belly fat but is only approved for a specific HIV-related condition. See the fat loss guide.

What about the pill?

Rybelsus is oral semaglutide for diabetes. Very little of it is absorbed, so it is a large pill taken on an empty stomach with a small sip of water. A higher-dose pill for weight loss is in development.

References

1. Wilding JPH, et al. (2021). Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. PubMed
2. Jastreboff AM, et al. (2022). Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. PubMed
3. Lincoff AM, et al. (2023). Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes. N Engl J Med. PubMed
4. Frías JP, et al. (2021). Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes. N Engl J Med. PubMed
5. Drucker DJ. (2018). Mechanisms of Action and Therapeutic Application of Glucagon-like Peptide-1. Cell Metab. PubMed

See also

• Peptide
  AOD-9604: Research, Mechanism, and What the Evidence Actually Shows

  AOD-9604 is the tail end of the growth hormone molecule, kept for its fat-burning signal and stripped of everything else. Mice lost half their fat on it; a 300-person human trial missed its primary goal.

• Peptide
  Semaglutide: What the Clinical Evidence Shows About This GLP-1 Agonist

  Semaglutide is a lab-made, week-long version of the gut hormone GLP-1, sold as Ozempic, Wegovy, and Rybelsus. It has the largest trials of any peptide on this site: about 15% weight loss over 68 weeks and 20% fewer major heart events.

• Peptide
  Tesamorelin: The FDA-Approved GHRH Analog With Clinical Evidence for Visceral Fat Reduction

  Tesamorelin (Egrifta) is a lab-made version of GHRH, the signal that tells the pituitary to release growth hormone, and the only one with current FDA approval. In Phase III trials it cut deep belly fat by about 18% in people with HIV.