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GLP-1 vs GLP-2 vs GLP-3: what the names mean, and which ones you can actually buy

GLP-1 drugs such as semaglutide act on one receptor. "GLP-2" is used online for tirzepatide (Zepbound, Mounjaro), which acts on two, GIP and GLP-1, and "GLP-3" for retatrutide, which adds the glucagon receptor. Neither nickname is the real name: GLP-2 is a separate gut hormone whose drug, Gattex, treats short bowel syndrome. Retatrutide produced 28.3% mean weight loss at 80 weeks in its phase 3 trial but is not FDA-approved; Lilly plans to file in Q1 2027. Tirzepatide is the strongest option you can buy today.

Julian CaraulaniFounder, PriceTracker GLP-1Last checked Oct 2

8 min read · published Oct 2

Key takeaways

  • 1GLP-1: one receptor. Semaglutide (Wegovy, Ozempic) and orforglipron (Foundayo).
  • 2"GLP-2" online usually means tirzepatide: GIP plus GLP-1. The real GLP-2 drug, Gattex, is for short bowel syndrome.
  • 3"GLP-3" usually means retatrutide: GIP, GLP-1 and glucagon. Investigational, not FDA-approved; filing planned Q1 2027.
  • 4FDA: retatrutide "cannot be used in compounding under federal law". Anything sold as retatrutide today is unapproved.
  • 5Of what you can legally buy, tirzepatide beat semaglutide head to head.

The numbers count receptors, and the names are mostly nicknames. Here is what each one refers to, and which ones a pharmacy can legally fill.

The three, side by side

Name you will seeWhat it usually meansReceptorsStatus
GLP-1Semaglutide (Wegovy, Ozempic, Rybelsus), orforglipron (Foundayo)GLP-1FDA-approved18
"GLP-2"Tirzepatide (Zepbound, Mounjaro)GIP and GLP-1FDA-approved1
"GLP-3"RetatrutideGIP, GLP-1 and glucagonInvestigational; filing planned Q1 20272
GLP-2, properlyTeduglutide (Gattex)GLP-2Approved for short bowel syndrome, not weight67

GLP-1: one receptor

Semaglutide is, in the words of the SURPASS-2 paper, "a selective GLP-1 receptor agonist".1 It is the molecule in Wegovy, Ozempic and the semaglutide pills. Foundayo is also a GLP-1 receptor agonist, but a small-molecule pill rather than a peptide.8

"GLP-2": usually tirzepatide, which is not a GLP-2

When a page or a video calls something a "GLP-2", it nearly always means tirzepatide, counting its two targets. The trial literature names it precisely: "a dual glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 (GLP-1) receptor agonist".1 It acts on GIP and GLP-1. It does not act on GLP-2.

GLP-2 is a different hormone. The FDA label for Gattex describes it as "a peptide secreted by L-cells of the distal intestine",6 and Gattex, a GLP-2 analog, is approved for short bowel syndrome in people who depend on intravenous nutrition.7 It is not a weight-loss drug, and nobody should go looking for it as one.

Tirzepatide is also the strongest weight-loss drug you can buy today on the evidence. In SURMOUNT-5, the one head-to-head trial, tirzepatide produced a mean body-weight change of -20.2% at 72 weeks against -13.7% for semaglutide.9

"GLP-3": retatrutide, and it is not for sale legally

Retatrutide adds a third target. Lilly calls it a "triple hormone receptor agonist" that "activates the body's receptors for glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon."2 No source here, including its maker, calls it a GLP-3.

The results are the largest yet reported for a weight drug:

TrialWhoLengthMean weight change, top dose
Phase 2 (NEJM 2023)Adults with obesity48 weeks-24.2% at 12 mg vs -2.1% placebo4
TRIUMPH-1 (phase 3)Obesity, no diabetes80 weeks-28.3% at 12 mg3
TRIUMPH-2 (phase 3)Obesity with type 2 diabetes80 weeks-20.8%2
TRIUMPH-3 (phase 3)Severe obesity and heart disease80 weeks-22.6%2

Side effects rose with the dose. In TRIUMPH-1, nausea was reported by 28.6%, 38.4% and 42.4% on the 4, 9 and 12 mg doses against 14.8% on placebo, and 11.3% of people on 12 mg stopped because of side effects.3

Worth knowing

Retatrutide is not approved. Lilly plans to submit it to the FDA in the first quarter of 2027.2 The FDA's position on what is being sold now is blunt: "Retatrutide and cagrilintide cannot be used in compounding under federal law. Additionally, these are not components of FDA-approved drugs and have not been found safe and effective for any condition."5 It has sent warning letters to sellers offering it online.11 A vial labelled retatrutide or "GLP-3" today is an unapproved drug of unknown contents.

What you can buy instead, and what it costs

The legal options today are the GLP-1s and tirzepatide. Lilly sells the Zepbound KwikPen or vial direct from $299 a month.10 Compounded tirzepatide through telehealth programs is usually cheaper and is not FDA-approved. Every program we track, ranked by what you pay each month:

For the molecule-against-molecule comparison, see tirzepatide vs semaglutide. For the pills, every GLP-1 pill compared.

Sources11
  1. SURPASS-2, Frías et al., N Engl J Med 2021;385:503-515 (PubMed abstract) read Oct 2“Tirzepatide is a dual glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 (GLP-1) receptor agonist ... as compared with semaglutide, a selective GLP-1 receptor agonist” ↩
  2. Eli Lilly, retatrutide TRIUMPH-2 and TRIUMPH-3 results (press release, 23 July 2026) read Oct 2“Retatrutide is an investigational, once-weekly, triple hormone receptor agonist. Retatrutide is a single molecule that activates the body's receptors for glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon. ... Lilly plans to submit a Biologics License Application (BLA) for retatrutide to FDA in Q1 2027 ... In TRIUMPH-2, adults with obesity or overweight and type 2 diabetes ... lost up to an average of 49.6 lbs (20.8%) at 80 weeks ... In TRIUMPH-3, adults with severe obesity and established cardiovascular disease, with or without type 2 diabetes, lost up to an average of 55.8 lbs (22.6%) at 80 weeks” ↩
  3. AJMC, retatrutide phase 3 TRIUMPH-1 topline results (21 May 2026) read Oct 2“participants on the highest dose of retatrutide (12 mg) achieved an average weight loss of 70.3 lb (28.3%) from a baseline average of 248.5 lb ... Participants receiving the 9-mg dose lost an average of 64.4 lb (25.9%). At the lowest dose of 4 mg ... patients experienced an average reduction of 47.2 lb (19.0%) at 80 weeks. ... Nausea: reported by 28.6%, 38.4%, and 42.4% of retatrutide participants, respectively, compared with 14.8% for placebo ... Discontinuation rates rose to 6.9% in the 9-mg group and 11.3% in the 12-mg group.” ↩
  4. Retatrutide phase 2 trial, Jastreboff et al., N Engl J Med 2023;389:514-526 (PubMed abstract) read Oct 2“Retatrutide (LY3437943) is an agonist of the glucose-dependent insulinotropic polypeptide, glucagon-like peptide 1, and glucagon receptors. ... At 48 weeks, the least-squares mean percentage change in the retatrutide groups was -8.7% in the 1-mg group, -17.1% in the combined 4-mg group, -22.8% in the combined 8-mg group, and -24.2% in the 12-mg group, as compared with -2.1% in the placebo group.” ↩
  5. FDA, concerns with unapproved GLP-1 drugs used for weight loss read Oct 2“Retatrutide and cagrilintide cannot be used in compounding under federal law. Additionally, these are not components of FDA-approved drugs and have not been found safe and effective for any condition. The agency has warned: Telehealth companies for marketing unapproved drugs such as retatrutide, including instances of direct marketing to consumers.” ↩
  6. GATTEX (teduglutide) prescribing information, FDA read Oct 2“glucagon-like peptide-2 (GLP-2), a peptide secreted by L-cells of the distal intestine. ... glucagon-like peptide-2 receptors located in intestinal subpopulations of enteroendocrine cells, subepithelial myofibroblasts and enteric neurons” ↩
  7. GATTEX, patient site (indication) read Oct 2“For people 1 year of age and older with SHORT BOWEL SYNDROME (SBS) who are dependent on parenteral support (PS)” ↩
  8. Eli Lilly, Foundayo FDA approval announcement read Sep 2“U.S. Food and Drug Administration (FDA) approved Foundayo™ (orforglipron) for adults with obesity, or overweight with weight-related medical problems. ... a once-daily small molecule (non-peptide) oral glucagon-like peptide-1 receptor agonist” ↩
  9. American College of Cardiology on SURMOUNT-5, tirzepatide vs semaglutide head to head read Sep 2“at 72 weeks, the least-squares mean percent change in body weight was −20.2% for tirzepatide users vs. −13.7% for semaglutide users (p<0.001)” ↩
  10. Eli Lilly, Zepbound coverage and savings read Oct 2“Self-pay, starting at $299 ... The $299 per month is for the Zepbound KwikPen or vial, not Zepbound single-dose Pen.” ↩
  11. FDA warning letter to GLP-1 Solution, 9 September 2025 read Oct 2“your website offers various compounded drug products, including retatrutide, semaglutide and tirzepatide. ... Drug products compounded using retatrutide are not eligible for the exemptions provided by section 503A, because retatrutide is not the subject of an applicable USP or NF monograph, is not a component of an FDA-approved human drug, and does not appear on the 503A bulks list.” ↩

Common questions

What is the difference between GLP-1, GLP-2 and GLP-3?

Online, the numbers usually count receptors. GLP-1 drugs such as semaglutide act on the GLP-1 receptor. "GLP-2" is used for tirzepatide, which acts on GIP and GLP-1. "GLP-3" is used for retatrutide, which acts on GIP, GLP-1 and glucagon. Properly, GLP-2 is a separate gut hormone; its drug, Gattex, treats short bowel syndrome.

Is tirzepatide a GLP-2?

No. Tirzepatide is a dual GIP and GLP-1 receptor agonist. It is sometimes called a "GLP-2" because it has two targets, but it does not act on the GLP-2 receptor. It is sold as Zepbound for weight and Mounjaro for type 2 diabetes.

Is retatrutide FDA approved?

No. Retatrutide is investigational. Lilly reported phase 3 results in 2026 and plans to submit it to the FDA in the first quarter of 2027. The FDA says retatrutide cannot be used in compounding under federal law and has not been found safe and effective for any condition.

How much weight do people lose on retatrutide?

In the phase 3 TRIUMPH-1 trial, people without diabetes on the 12 mg dose lost an average of 28.3% of body weight at 80 weeks. In TRIUMPH-2, with type 2 diabetes, the top result was 20.8%; in TRIUMPH-3, with severe obesity and heart disease, 22.6%.

Can I buy retatrutide online?

Not legally as a medicine. It is not approved, the FDA says it cannot be used in compounding, and the agency has warned sellers marketing it online. Anything sold as retatrutide or "GLP-3" today is an unapproved product.

What is the strongest weight-loss drug I can get now?

Of approved drugs, tirzepatide (Zepbound) has the strongest head-to-head result: -20.2% mean body weight at 72 weeks against -13.7% for semaglutide in SURMOUNT-5. Lilly sells the Zepbound KwikPen or vial from $299 a month self-pay.

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