Mechanism-first reference for the incretin agonist class — semaglutide, tirzepatide, retatrutide, and beyond. Clinical data, synthesis complexity, and the biology behind the headline numbers.
Research reference only. Not medical advice, dosing guidance, or an offer for sale.
GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide) are gut-derived hormones released within minutes of nutrient ingestion. Together they account for roughly 50–70% of postprandial insulin release — the “incretin effect.” Both are rapidly degraded by DPP-4 (dipeptidyl peptidase-4), with half-lives under two minutes in their native form.
The defining pharmaceutical chemistry challenge for this class has been converting these transient meal signals into durable, once-weekly pharmacological agents. The solutions — C-18 fatty-diacid acylation on a mini-PEG linker (semaglutide), GIP/GLP-1 chimeric sequence design (tirzepatide), and triple receptor agonism with glucagon co-activation (retatrutide) — represent successive generations of incretin engineering.
Beyond glycemic control, incretin receptors in the hypothalamus and brainstem govern satiety and energy balance. This central mechanism is why the class produces substantial, dose-dependent body-weight reduction even in non-diabetic subjects — a finding that reshaped obesity pharmacology entirely.
Mono-agonism
Semaglutide, liraglutide
GLP-1R only. Benchmark for glucose control, satiety, and weight reduction.
Dual-agonism
Tirzepatide
GLP-1R + GIP-R. Complementary insulinotropic and adipose-tissue pathways — superior weight endpoints vs mono in head-to-head data.
Triple-agonism
Retatrutide
GLP-1R + GIP-R + GcgR. Adds thermogenic glucagon activity — the steepest weight-loss signal reported in any class to date.
| Compound | Targets | Class | FDA | Peak weight↓ | t½ |
|---|---|---|---|---|---|
| SemaglutideOzempic · Wegovy · Rybelsus | GLP-1R | Mono-agonist | Approved | ~15% | ~168 h |
| TirzepatideMounjaro · Zepbound | GIP/GLP-1R | Dual-agonist | Approved | ~22% | ~120 h |
| RetatrutideLY3437943 | GIP/GLP-1R/GcgR | Triple-agonist | Inv. | ~28% | ~168 h |
| CagrilintideCagriSema (combo) | Amylin / CGRP-R | Amylin analog | Inv. | ~15–16% | ~168 h |
| CagriSemaCagrilintide + semaglutide | Amylin + GLP-1R | Fixed-dose combination | Inv. | ~20% | ~168 h |
| SurvodutideBI 456906 | GcgR/GLP-1R | Dual-agonist | Inv. | MASH-led | ~168 h |
| MazdutideIBI362 · LY3305677 | GcgR/GLP-1R | Dual-agonist | Inv. | ~15% | ~168 h |
| LiraglutideVictoza · Saxenda | GLP-1R | Mono-agonist | Approved | ~6% | ~13 h |
Weight-reduction figures are primary endpoint means from pivotal trials; individual responses vary. Not a basis for clinical decisions.
The same trial reports more than one figure, and the gap between them is not a rounding error
Every percentage in the table above is a mean from a specific trial under a specific estimand — the pre-declared rule for what counts as an outcome when participants stop taking the drug, miss doses, or start something else. Modern obesity trials report at least two, and the difference between them is usually three to four percentage points.
Retatrutide’s Phase 3 TRIUMPH-1 trial is the clearest worked example. Across 2,339 adults with obesity or overweight without diabetes, randomised to 4 mg, 9 mg, 12 mg, or placebo over 80 weeks, the two estimands report the same study differently.
| TRIUMPH-1, 80 weeks | Efficacy estimand | Treatment-regimen estimand |
|---|---|---|
| Retatrutide 12 mg | −28.3% | −25.0% |
| Retatrutide 9 mg | −25.9% | −23.7% |
| Retatrutide 4 mg | −19.0% | −17.6% |
| Placebo | −2.2% | −3.9% |
The efficacy estimand answers what happens to people who stay on the drug as assigned. The treatment-regimen estimand keeps everyone who was randomised, whatever they went on to do. The first number is the pharmacology; the second is closer to what a population actually experiences. Neither is dishonest, and a headline that quotes one without saying which is.
This is why cross-trial comparison is so slippery. Two compounds can look separated by four points purely because one press release quoted its efficacy estimand and the other did not. Where a real head-to-head exists, it settles the question properly: in REDEFINE 4, an open-label trial of 809 adults over 84 weeks, CagriSema reached 23.0% against tirzepatide’s 25.5%, and missed its non-inferiority endpoint. That is a different kind of evidence from two separate trials placed side by side.
How we tier the evidence behind these claimsWhether a licensed compounding pharmacy may legally prepare these substances — a separate question from FDA drug approval
The incretin agonists reached the compounding question by a different road than the research peptides. These are approved drugs, so the bulks list never applied to them. What applied instead is the rule against compounding an essentially-identical copy of a commercially available product — a rule suspended while a drug is in shortage, and reinstated when the shortage ends. That is what closed the compounded-GLP-1 era in 2025, and it is why a compounded version is not simply a cheaper equivalent.
FDA declared the shortage resolved on February 21, 2025. Enforcement discretion for compounded copies ended April 22, 2025 for 503A pharmacies and May 22, 2025 for 503B outsourcing facilities. In April 2026 FDA proposed excluding semaglutide from the 503B bulks list, finding no clinical need for bulk compounding.
FDA declared the shortage resolved on December 19, 2024 — the first of the class. Enforcement discretion ended February 18, 2025 for 503A pharmacies and March 19, 2025 for 503B outsourcing facilities.
An approved drug covered by the same copy rule. FDA included it in the April 2026 proposal to exclude the GLP-1 drugs from the 503B bulks list.
Investigational and not approved anywhere. There is no lawful compounding route for an investigational compound, and it is not on the 503A bulks docket.
None of these statuses is FDA approval, and none establishes efficacy, dosing, or a benefit-risk profile. Legal 503A compounding requires FDA to place a substance on the bulks list through notice-and-comment rulemaking. That step had not been completed for any peptide on this page as of the review date.
Regulatory status reviewed
Every side-by-side page covering a compound on this hub
Semaglutide vs Tirzepatide
mechanism, trials, and key differences
Retatrutide vs Tirzepatide
triple vs dual incretin agonism
Semaglutide vs Liraglutide
the same GLP-1 template, weekly versus daily
Retatrutide vs Semaglutide
three receptors versus one
CagriSema vs Tirzepatide
two peptides combined versus one dual-receptor molecule
CagriSema vs Retatrutide
the combination versus the triple agonist
Survodutide vs Retatrutide
the glucagon dual versus the triple agonist
Mazdutide vs Tirzepatide
two dual agonists, two different second receptors
Survodutide vs Mazdutide
two roads to the same dual mechanism
Why research-grade purity is difficult and expensive to achieve
GLP-1 agonists are among the most chemically complex peptides in commercial-scale production. Semaglutide’s 31-residue chain carries a C-18 fatty-diacid on a mini-PEG/γGlu linker — manufactured via solid-phase peptide synthesis (SPPS) followed by solution-phase acylation. Tirzepatide is a 39-residue chimeric sequence derived from native GIP with GLP-1-like modifications; retatrutide adds glucagon-receptor pharmacophore elements to a 39-mer backbone.
Each added axis multiplies potential impurity species. Deletion sequences, racemization at sensitive residues, and incomplete acylation are the primary process-related impurities. Orthogonal analytical methods — reversed-phase HPLC for sequence identity, size-exclusion chromatography for aggregates, and high-resolution mass spectrometry for side-chain integrity — are all required for a credible certificate of analysis.
How research peptides are manufacturedWhat is a GLP-1 receptor agonist?
A GLP-1 receptor agonist is a synthetic peptide engineered to mimic and prolong the action of glucagon-like peptide-1 — a gut hormone released after meals. By binding the GLP-1 receptor in the pancreas, brain, and gut, these compounds promote glucose-dependent insulin secretion, slow gastric emptying, and suppress appetite. Fatty-acid acylation and DPP-4-resistant amino-acid substitutions extend half-life from minutes to days or weeks, enabling once-weekly subcutaneous dosing.
What is the difference between semaglutide and tirzepatide?
Semaglutide activates only the GLP-1 receptor (mono-agonism). Tirzepatide simultaneously activates both the GLP-1 and GIP receptors (dual agonism), a mechanism associated with larger body-weight reductions in head-to-head trials. In SURMOUNT-1, tirzepatide achieved up to 22.5% mean body-weight reduction vs approximately 15% for semaglutide 2.4 mg in STEP-1. Clinical tolerability and dosing profiles differ; neither compound is a direct substitute for the other.
What is retatrutide and how does it compare?
Retatrutide (LY3437943) is an investigational triple agonist targeting GIP, GLP-1, and glucagon receptors (GcgR) in a single molecule. Glucagon-receptor agonism adds a thermogenic signal — increased energy expenditure — on top of the incretin axes. A 2023 Phase 2 trial published in NEJM reported approximately 24.2% mean body-weight reduction at 48 weeks on the highest dose — the largest signal reported in any peptide trial class at that time. Retatrutide has not received FDA approval.
Are these peptides legal to research?
FDA-approved compounds (semaglutide, tirzepatide) are approved prescription medications with defined regulatory pathways. Investigational compounds (retatrutide, cagrilintide) remain in clinical trials. This platform is a research reference for scientists, researchers, formulators, and educators studying the mechanisms and evidence base of these compounds — it is not a medical device and does not constitute treatment, dosing, or purchasing advice.
Why are GLP-1 peptides complex to synthesize?
GLP-1 receptor agonists are long-chain modified peptides (31–39 amino acids) with lipophilic side chains for half-life extension. Each modification adds synthesis steps: solid-phase chain assembly, protection-group chemistry, post-chain fatty-acid acylation, multi-step HPLC purification to pharmaceutical-grade purity, lyophilization, and cold-chain distribution. Impurity profiles in long modified peptides are complex and require orthogonal analytical methods (RP-HPLC, SEC, MS) to characterize fully — which is why certificate-of-analysis documentation is the minimum bar for any research-grade material.
Where can I track GLP-1 clinical trials in real time?
The ClinicalPulse tool on this platform searches ClinicalTrials.gov in real time for trials involving GLP-1 agonists, amylin analogs, and related metabolic peptides. Filter by phase, recruiting status, and compound name to track the latest completed and active studies.
Why do two sources quote different weight-loss numbers for the same trial?
Because modern obesity trials report more than one estimand — the pre-declared rule for how to count participants who stop the drug or start something else. In retatrutide’s Phase 3 TRIUMPH-1 trial, the 12 mg arm reduced mean body weight by 28.3% under the efficacy estimand and 25.0% under the treatment-regimen estimand, over 80 weeks in 2,339 adults. The efficacy estimand describes people who stayed on treatment as assigned; the treatment-regimen estimand keeps everyone randomised regardless of what they did next. Both are legitimate. A figure quoted without saying which one it is cannot be compared to anything.
Has retatrutide reported Phase 3 results?
Yes. TRIUMPH-1 randomised 2,339 adults with obesity, or overweight with a weight-related comorbidity and without diabetes, to once-weekly retatrutide at 4 mg, 9 mg or 12 mg or to placebo over 80 weeks. Mean body-weight reduction at 12 mg was 28.3% under the efficacy estimand against 2.2% for placebo. A subset with a baseline BMI of 35 or above continued to 104 weeks and reached about 30%. Retatrutide remains investigational and is not FDA-approved.
How does CagriSema compare with tirzepatide head-to-head?
REDEFINE 4 is the direct comparison — an open-label randomised trial of 809 adults with obesity and comorbidities over 84 weeks. CagriSema reduced mean body weight by 23.0% against 25.5% for tirzepatide, and did not meet its primary endpoint of non-inferiority. Under the treatment-regimen estimand the figures were 20.2% and 23.6%. CagriSema is not FDA-approved; Novo Nordisk submitted a New Drug Application in December 2025.
What is the incretin axis?
The incretin axis refers to the coordinated signaling of gut-derived hormones — primarily GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide) — that amplify insulin secretion in response to nutrient ingestion. Together they account for roughly 50–70% of postprandial insulin release. Both are rapidly degraded by DPP-4; engineered resistance to this degradation is the defining pharmaceutical chemistry challenge that modern incretin agonists solve.