Tirzepatide vs retatrutide

Tirzepatide and retatrutide differ by one receptor. Both activate the GIP and GLP-1 receptors; retatrutide also activates the glucagon receptor. ^[1] Retatrutide is the newer molecule, with a smaller and earlier-stage trial record. In Australia tirzepatide is a Schedule 4 substance, and retatrutide has no entry in the Poisons Standard. ^[3]

Tirzepatide came first. Lilly researchers described it in Molecular Metabolism in 2018 under the code LY3298176, as a 39-amino-acid linear peptide conjugated to a C20 fatty diacid. ^[14] Retatrutide followed in Cell Metabolism in 2022 as LY3437943, a triple agonist peptide at the glucagon, GIP and GLP-1 receptors. ^[1]

Four years is a long gap in this field. By the time retatrutide's first phase 2 papers appeared in 2023, tirzepatide already had published phase 3 trials in type 2 diabetes and obesity. ^[15][16][9] The 2026 TRANSCEND-T2D-1 paper still describes retatrutide as under clinical development for type 2 diabetes, obesity and related complications. ^[11]

Tirzepatide and retatrutide share a length, a fatty acid and a developer, and differ in targets, sequence, mass, evidence depth and Australian scheduling.

Identifiers come from PubChem ^[5][6][7] and the tirzepatide architecture and half-life from Lilly's discovery paper. ^[14] The retatrutide half-life comes from its phase 1b trial, ^[17] and the legal rows from the June 2026 Poisons Standard ^[3] and the WADA 2026 List. ^[13]

We lined up the two sequences from their PubChem records on 29 September 2026:

• Tirzepatide: YXEGTFTSDYSIXLDKIAQKAFVQWLIAGGPSSGAPPPS
• Retatrutide: YXQGTFTSDYSIXLDKKAQXAFIEYLLEGGPSSGAPPPS

X marks a non-coded residue. The two chains match at 30 of 39 positions, including the last 11 (GGPSSGAPPPS). They differ at positions 3, 13, 17, 20, 23, 24, 25, 27 and 28. Position 13 looks identical in one-letter code, yet it holds Aib in tirzepatide and alpha-methyl-leucine in retatrutide. ^[7][14] The fatty acid also moves. Tirzepatide's C20 diacid hangs from lysine 20 on two AEEA spacers, while retatrutide's hangs from lysine 17 on one. ^[5][7]

Those small changes matter at the bench. The formulas differ by C4H6N2, and PubChem's molar masses differ by 82 g/mol. A mass spectrometry identity result on a certificate can tell the two vials apart. Our guide on how to read a peptide certificate of analysis shows where that figure sits on the report.

Tirzepatide is a dual agonist that activates the GIP receptor and the GLP-1 receptor. A 2020 receptor study described it as imbalanced, with greater engagement of the GIP receptor than the GLP-1 receptor. At the GLP-1 receptor it favoured cAMP generation over beta-arrestin recruitment, a property the authors called biased agonism. ^[4]

In Lilly's 2018 discovery work, tirzepatide activated both receptor pathways in cell lines and produced glucose-dependent insulin secretion in mice. The same paper reported phase 1 data from 142 human participants and a mean half-life of about 5 days. ^[14] The C20 fatty diacid binds albumin, which slows clearance.

Our explainer on what tirzepatide is covers the binding constants, the full trial table and Australian registration.

Retatrutide is a triple agonist that activates the glucagon, GIP and GLP-1 receptors. In Lilly's 2022 cell assays it showed balanced glucagon and GLP-1 receptor activity and more GIP receptor activity. ^[1] Tirzepatide has no glucagon receptor activity, so the glucagon arm is the mechanistic difference researchers study.

In obese mice, retatrutide lowered body weight and improved glycaemic control. The authors attributed part of the body weight effect to glucagon receptor-mediated increases in energy expenditure, added to the reduced calorie intake driven by the GIP and GLP-1 receptors. ^[1] That is a mouse finding. It describes the hypothesis behind the human programme.

The phase 1b trial in 72 adults with type 2 diabetes reported a half-life of approximately 6 days. ^[17] Retatrutide's fatty acid sits on lysine 17 with a shorter spacer than tirzepatide's. ^[7] The full profile is in our explainer on what retatrutide is.

No randomised trial comparing tirzepatide with retatrutide in people had been published by 29 September 2026. We searched PubMed that day. 106 records mention both compounds, and the eight retatrutide trial records compare it with placebo or dulaglutide. One head-to-head phase 3 trial is registered and still running. ^[2]

That trial, NCT06662383, is a double-blind phase 3 study of retatrutide against tirzepatide in 800 adults with obesity. Its primary outcome is percent change from baseline in body weight. Lilly sponsors it, and the registry lists primary completion in November 2026 and study completion in December 2026. ^[2] A second Lilly trial, SYNERGY-Outcomes (NCT07165028), has tirzepatide, retatrutide and placebo arms in 4,500 participants with metabolic dysfunction-associated steatotic liver disease, with completion listed for August 2032. ^[18]

Direct comparisons do exist in mice:

The mouse results point in different directions depending on the model. None of them predicts a human result.

Indirect comparisons pool separate trials. A 2026 network meta-analysis in BMJ Medicine combined 58 trials with 24,214 participants without diabetes. Against placebo, it estimated body weight change of -22.10% for retatrutide and -19.28% for tirzepatide, and its authors flagged limited head-to-head evidence. ^[12] A 2026 BMJ network meta-analysis rated the tirzepatide estimate (-14.9% against lifestyle modification at one year) as moderate to high certainty. It placed retatrutide among emerging agents at 13.1% to 14.6%, with very low to low certainty. ^[22] The two analyses used different comparators and certainty methods, so their numbers do not line up with each other.

Cross-trial reading has the same problem. SURMOUNT-1 followed 2,539 adults without diabetes for 72 weeks and reported mean body weight change of -15.0% to -20.9% against -3.1% on placebo. ^[16] Retatrutide's phase 2 obesity trial followed 338 adults for 48 weeks and reported -8.7% to -24.2% against -2.1% on placebo. ^[9] Different sizes, durations and phases make those two ranges hard to set side by side. Tirzepatide's own direct comparison was against semaglutide, covered in semaglutide vs tirzepatide.

Researchers study tirzepatide and retatrutide side by side as comparators. No published study has examined the two combined in one preparation. They share arms in the head-to-head trial and in SYNERGY-Outcomes, ^[2][18] and they appear together in three mouse studies of kidney disease and obesity. ^[19][20][21]

Our catalogue has no stack that contains either compound. Each one is sold as a single-compound vial with its own batch certificate.

Gastrointestinal events were the most common adverse events in the published trials of both compounds, mostly mild to moderate. The trials differ in population and length, so each rate describes its own trial.

Two signals are specific enough to note. The heart rate rise is reported for retatrutide's phase 2 obesity trial. ^[9] For tirzepatide, MedlinePlus states that it caused thyroid tumours in rats and that the human relevance is unknown. ^[23] The BMJ Medicine network meta-analysis also reported low certainty evidence of higher discontinuation rates with retatrutide. ^[12]

The comparison evidence cannot yet rank the two compounds on any human outcome. Four gaps explain why.

1. No published head-to-head trial. NCT06662383 is the only registered phase 3 comparison, and it has not reported. ^[2]
2. One sponsor funds almost everything. Lilly funded the tirzepatide phase 3 trials and every retatrutide trial listed here, and it sponsors both registered comparisons. ^{[15][17][11][2]}
3. Retatrutide's evidence is younger. Its first phase 3 paper appeared in 2026. The four TRIUMPH phase 3 trials, with over 5,800 participants across obesity, sleep apnoea and knee osteoarthritis, appear in PubMed as a design paper. ^[11][24] Results reported only in company announcements are outside the peer-reviewed record we checked.
4. No trial used research-grade material. Every human study used the sponsor's investigational or registered product. Purity and identity of a research vial are answered by its batch certificate.

Tirzepatide is listed in Schedule 4 of the Poisons Standard (prescription only), in the June 2026 instrument (F2026L00633) that commenced on 1 June 2026. Retatrutide is not named in the Poisons Standard; a text search of the same instrument found no entry. ^[3]

"Not named" is a narrow statement. It means the instrument has no individual entry for retatrutide, and it says nothing about approval. Our guide to Australian peptide law and the Poisons Standard explains what each schedule means and how the rules apply.

For sport, neither compound is named on the WADA 2026 Prohibited List, effective 1 January 2026. Class S0 prohibits at all times any pharmacological substance not addressed elsewhere on the List and without current governmental approval for human therapeutic use, which includes drugs under clinical development. ^[13] Tirzepatide is a registered prescription medicine in Australia, as our tirzepatide explainer sets out. For retatrutide we have not confirmed whether S0 applies, so athletes should check with Sport Integrity Australia.

Peptides Collective sells both for laboratory research use only. Tirzepatide in 10mg and 20mg vials costs A$98 and A$168. Retatrutide in 10mg and 20mg vials costs A$128 and A$228. Stock ships from Western Australia with tracking.

Per milligram, tirzepatide works out at A$9.80 (10mg) and A$8.40 (20mg), and retatrutide at A$12.80 and A$11.40. The retatrutide vial costs A$30 more at 10mg and A$60 more at 20mg.

An independent lab tests every batch of both compounds before sale. Each certificate reports purity by HPLC and identity by mass spectrometry, and it is published under the batch number printed on the vial. Search the certificate of analysis library by that number. For these two vials, check the identity result against the right molar mass: about 4813 g/mol for tirzepatide and about 4731 g/mol for retatrutide.

Tirzepatide and retatrutide are close structural relatives that differ by one receptor target, nine sequence positions and a relocated fatty acid. Tirzepatide has a large phase 3 record and a Schedule 4 entry; retatrutide has a younger record and no entry in the Poisons Standard. No published trial compares them directly, so the certificate for each batch is the evidence that settles what a research vial contains.