How Do Retatrutide, Tirzepatide, and Semaglutide Compare? A Chemical & Receptor-Selectivity Reference

WARNING: LABORATORY RESEARCH USE ONLY. This material is synthesized and distributed strictly for B2B development, laboratory research, and analytical studies. It is not a drug, dietary supplement, or cosmetic. Under no circumstances are these chemical compounds intended for human or veterinary diagnostic or therapeutic application. All research must be conducted by qualified laboratory personnel using appropriate safety controls.

Last reviewed: 2026-09-22 · Research use only.

Retatrutide, Tirzepatide, and Semaglutide are frequently grouped together, but from a chemical and pharmacological standpoint their defining difference is receptor selectivity — how many, and which, incretin-family receptors each peptide engages. This reference compares them strictly on chemistry: receptor targets, CAS identity, molecular parameters, and in vitro potency as reported in the primary literature. It makes no comparison of clinical efficacy, human outcomes, or suitability for any use — those are outside the scope of research-reagent characterization.

For the full chemical profile of our anchor compound, see the Retatrutide chemical reference.

1. At-a-Glance Chemical Comparison

On comparing molecular weights across this table: each figure is the average mass of the whole molecule as the free base. Quote all three on the same basis or the comparison is meaningless — masses for these peptides also circulate as monoisotopic values, as salt forms (each TFA counterion adds ≈114.02, each acetate ≈60.05, each sodium ≈21.98 replacing a proton), and, for Retatrutide, as a backbone-only figure of ≈4090–4092 Da that omits its C20-diacid side chain entirely. Sodium salts of these peptides are separately registered substances, not notational variants — FDA GSRS carries Retatrutide sodium (UNII LQ42M82ZU6) as a record distinct from Retatrutide (UNII NOP2Y096GV). The Retatrutide reference maps every circulating Retatrutide value to what it is the mass of.

The ≈0.07 Da spread you will see on every one of these three compounds

Each compound in the table above circulates in the literature and on supplier datasheets under two average masses that differ by less than a tenth of a Dalton:

This is not a disagreement about the molecules. Both figures in each row are computed from the same molecular formula — the formulas in the table above are undisputed and confirmed against PubChem and FDA GSRS. The difference is which standard atomic-weight table the calculation used: current IUPAC values give the higher figure, superseded tables give the lower one. We reproduced all six values from the formulas directly; the residual error is ≤0.02 Da in every case, and the relative offset is the same ≈15 ppm for all three compounds — which is the signature of a units convention, not of a structural difference.

⚠️ A widely-repeated explanation for the Tirzepatide pair is arithmetically impossible. Several sources attribute the 4813.45 / 4813.53 difference to whether sodium is counted. It cannot be: substituting one sodium for a proton adds ≈21.98 Da, roughly 290× the observed 0.08 Da gap. A salt-form explanation for a sub-Dalton discrepancy fails on inspection, and checking that arithmetic is the difference between reporting a specification and repeating one.

Practical consequence for a specifying chemist: at ≈15 ppm this spread is far below the mass accuracy of routine peptide LC-MS identity work, so it will not change an identity call. It matters for paperwork consistency — a specification quoting 4813.53 against a certificate quoting 4813.45 is not a deviation, and should not be raised as one.

On the "Purity / identity verification" row: vendor listings for research peptides often advertise a single, unqualified purity percentage with mass-spectrometry identity, as though both were fixed properties of the compound. Neither is. A purity result is a per-batch analytical result, and the identity method a certificate actually uses can be MS or FTIR depending on the lab. This table does not assert a purity or identity-method figure for any of the three compounds; confirm the assayed value and method against the batch-specific Certificate of Analysis for the material received. For a worked example, see the Certificate of Analysis for Boss BioTech USA's Retatrutide lot 06242026 — HPLC peptide purity 99.8%, identity confirmed by FTIR, no MS data reported.

2. Receptor Selectivity — the Core Chemical Distinction

The three peptides sit on a clear axis of increasing receptor breadth:

Figure: receptor-engagement map showing increasing receptor breadth from mono- to triple-agonist. Text-only version:

  Semaglutide      Tirzepatide          Retatrutide
   (mono)            (dual)               (triple)
      │                │  │                 │  │  │
   GLP-1R           GIP  GLP-1          GIP  GLP-1  GCGR

• Semaglutide is a highly selective GLP-1 receptor agonist (a GLP-1 analogue sharing high sequence homology with native GLP-1).
• Tirzepatide adds GIP receptor engagement — a dual GIP/GLP-1 agonist built on a 39-amino-acid lipidated scaffold.
• Retatrutide adds a third target, the glucagon receptor (GCGR) — a triple GIP/GLP-1/glucagon agonist, also a 39-amino-acid lipidated scaffold.

This selectivity difference — mono → dual → triple — is the single most useful chemical distinction when selecting reference material for a receptor-signaling study.

3. In Vitro Potency, As Reported (read the caveat first)

Important methodological caveat: the EC50 values below come from different studies using different assay systems and cell lines. They characterize each molecule within its own report and are not directly head-to-head comparable; a smaller number here does not mean one compound is "stronger" or "better." Values are presented only to document each peptide's published in vitro receptor engagement.

*We do not assert a single cross-assay GLP-1R EC50 for Semaglutide here, because a value measured under a different protocol would invite an invalid head-to-head reading. Semaglutide's defining characteristic in this comparison is its mono-selectivity for GLP-1R.

4. Why These Distinctions Matter for Laboratory Research

Selecting reference material is an assay-design decision. A study modeling single-pathway GLP-1R signaling calls for a mono-agonist; work examining multi-receptor or GIP/glucagon-pathway cross-talk calls for a dual- or tri-agonist. Matching the peptide's receptor selectivity to the experimental question — and verifying its identity (by mass spectrometry or FTIR, per the batch certificate) and purity (by HPLC, against the certificate's stated specification) — is what keeps cell-based results interpretable and reproducible.

5. Frequently Asked Questions

Q: What is the difference between Retatrutide, Tirzepatide, and Semaglutide? A: Chemically, they differ in receptor selectivity: Semaglutide is a GLP-1 mono-agonist, Tirzepatide is a GIP/GLP-1 dual agonist, and Retatrutide is a GIP/GLP-1/glucagon triple agonist.

Q: How many receptors does each peptide target? A: Semaglutide targets one (GLP-1R), Tirzepatide two (GIPR, GLP-1R), and Retatrutide three (GIPR, GLP-1R, GCGR).

Q: What are the CAS numbers for Retatrutide, Tirzepatide, and Semaglutide? A: Retatrutide is CAS 2381089-83-2, Tirzepatide is CAS 2023788-19-2, and Semaglutide is CAS 910463-68-2.

Q: Are Retatrutide and Tirzepatide the same size? A: Both are 39–amino-acid lipidated peptides, but they differ in sequence, molecular formula, and receptor selectivity (triple vs dual). Semaglutide is a smaller 31–amino-acid GLP-1 analogue.

Q: Can in vitro EC50 values be compared directly across these three peptides? A: No. Reported EC50 values come from different studies and assay systems; they document each molecule within its own report and should not be read as a direct head-to-head potency ranking.

Q: Why do sources report two slightly different molecular weights for Tirzepatide (4813.45 and 4813.53)? A: Both are computed from the same undisputed formula, C₂₂₅H₃₄₈N₄₈O₆₈; they differ only in which standard atomic-weight table the calculation used — current IUPAC values give 4813.53, superseded tables give 4813.45. The same ≈15 ppm offset appears on Retatrutide (4731.33 vs 4731.42) and Semaglutide (4113.58 vs 4113.64). It is a units convention, not a structural difference. The explanation sometimes given — that sodium accounts for the gap — is arithmetically impossible: one sodium replacing a proton adds ≈21.98 Da, about 290× the observed difference.

Q: Which molecular weight basis should be quoted when comparing these three peptides? A: The average mass of the whole molecule as the free base, which is the basis used throughout this page. Masses for these peptides also circulate as monoisotopic values, as salt forms (TFA ≈+114.02, acetate ≈+60.05, sodium ≈+21.98 per counterion), and — for Retatrutide — as a backbone-only figure near 4090–4092 Da that omits the C20-diacid side chain. Comparing figures quoted on different bases is the most common error in this comparison.

6. Scholarly References

1. Coskun, T., et al. (2022). LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist… Cell Metabolism, 34(9), 1234–1247.e9. https://doi.org/10.1016/j.cmet.2022.07.013
2. Coskun, T., et al. (2018). LY3298176, a novel dual GIP and GLP-1 receptor agonist for the treatment of type 2 diabetes mellitus: from discovery to clinical proof of concept. Molecular Metabolism, 18, 3–14. https://doi.org/10.1016/j.molmet.2018.09.009
3. Lau, J., et al. (2015). Discovery of the once-weekly glucagon-like peptide-1 (GLP-1) analogue semaglutide. Journal of Medicinal Chemistry, 58(18), 7370–7380. https://doi.org/10.1021/acs.jmedchem.5b00726
4. National Center for Biotechnology Information. PubChem Compound Summaries for Retatrutide, Tirzepatide (CID 156588324), and Semaglutide (CID 56843331). https://pubchem.ncbi.nlm.nih.gov
5. U.S. Food & Drug Administration, Global Substance Registration System (GSRS). Substance records for Retatrutide (UNII NOP2Y096GV), Retatrutide sodium (UNII LQ42M82ZU6), Tirzepatide (UNII OYN3CCI6QE), and Semaglutide (UNII 53AXN4NNHX) — source for the CAS registry numbers and the 39/39/31 residue counts stated above. https://gsrs.ncats.nih.gov
6. European Molecular Biology Laboratory–EBI. ChEMBL database, Semaglutide (CHEMBL4556788, average mass 4113.64) and Tirzepatide (CHEMBL4297839). https://www.ebi.ac.uk/chembl/

Clinical-development references above are cited strictly for the historical chemistry of each molecule, never as evidence of any benefit.

Related references: What is Retatrutide? (full chemical reference) · Certificate of Analysis data for Retatrutide lot 06242026 · a B2B sourcing & verification guide is in development and will be linked here at launch.

Disclaimer: Boss BioTech USA distributes chemical compounds solely for laboratory research and analytical development. All products discussed above are not approved for human clinical use, diagnosis, prevention, treatment, or cure of any medical condition or disease. The purchaser assumes all risks associated with the handling, testing, and use of these materials.