Semaglutide vs tirzepatide

Semaglutide is a single-receptor GLP-1 agonist built on the human GLP-1 sequence. Tirzepatide is a dual GIP and GLP-1 agonist built on the GIP sequence. Tirzepatide is eight residues longer, its fatty diacid is two carbons longer, and its evidence base is younger. Both are lipidated peptides with human trial programmes run by their developers.

Two companies, two design problems. Novo Nordisk set out to make GLP-1 last a week by raising its albumin affinity and blocking its breakdown. ^[10] Eli Lilly set out to test whether GIP activity adds to what a selective GLP-1 agonist does, and engineered GLP-1 activity into a GIP-based chain. ^[8][11] The table below puts the results side by side.

Semaglutide and tirzepatide differ on receptor count, length, mass and evidence base. They match on Australian scheduling and on sport status. We checked every row against PubChem, PubMed, the June 2026 Poisons Standard and the WADA 2026 List on 29 September 2026.

Identifiers come from PubChem. ^[5][6] The semaglutide design is from the Novo Nordisk discovery paper ^[10] and the tirzepatide design and half-life from Lilly's. ^[8] The semaglutide half-life of about 1 week is from SUSTAIN-6. ^[12]

The sequence rows hold the least-known fact in this comparison. Line the two chains up from residue 1 and 16 of the first 31 positions match. Positions 2 to 9 are identical (Aib-E-G-T-F-T-S-D), and both molecules hang their fatty acid on the lysine at position 20. PubChem's structures show the same spacer in both: a gamma-glutamic acid and two AEEA units. The fatty acid is where they part: 18 carbons in semaglutide, 20 in tirzepatide. ^[5][6]

One trap for anyone reading the papers. Semaglutide literature uses GLP-1 numbering, where the peptide starts at residue 7. Its Aib8, Lys26 and Arg34 are positions 2, 20 and 28 of the 31-residue chain above. ^[10] Tirzepatide papers count from 1. Both molecules have their Aib at position 2 and their lipidated lysine at position 20 in their own chains, which the two numbering systems hide.

Semaglutide is a GLP-1 analogue with two substitutions and a lipid chain. Novo Nordisk's 2015 paper describes Aib at position 8 and Arg at position 34 of human GLP-1, with a fatty acid attached at lysine 26. ^[10] Native GLP-1 has a half-life of 1.5 minutes after intravenous delivery in humans. Semaglutide's is about a week. ^[7][12]

Each change solves one problem. The enzyme DPP-IV clips native GLP-1 at its N-terminus, and Aib at position 8 was the only substitution tested that kept both DPP-IV stability and high receptor affinity. The C18 diacid on a gamma-Glu and two-OEG linker gave the highest albumin affinity while keeping GLP-1 receptor potency. ^[7] Albumin-bound peptide avoids both breakdown and filtration by the kidney.

The GLP-1 receptor sits in the pancreas, gut, heart, lungs, kidneys and brain. A 2019 review by Novo Nordisk scientists links the pancreatic receptors to glucose control and the brain receptors to energy intake. It cites a 12-week study in people with obesity that reported a 24% average reduction in energy intake over three meals and snacks. ^[7] The review's authors work for the developer. Our semaglutide explainer covers the molecule and its trials in more depth.

Tirzepatide binds and activates both the GIP receptor and the GLP-1 receptor, and it leans towards GIP. Lilly's 2018 paper reports a Ki of 0.135 nM at the GIP receptor, comparable to native GIP, and 4.23 nM at the GLP-1 receptor, about 5-fold weaker than native GLP-1. ^[8]

A 2020 Lilly study in JCI Insight found greater engagement of the GIP receptor than the GLP-1 receptor. It called the molecule imbalanced. At the GLP-1 receptor, tirzepatide favoured cAMP generation over beta-arrestin recruitment and drove less receptor internalisation than GLP-1, which the authors called biased. ^[11]

That profile puts the two compounds in different places at the shared receptor. In the 2018 assays, tirzepatide was less potent at the GLP-1 receptor than semaglutide: a cAMP EC50 of 0.934 nM against 0.0571 nM. ^[8] Compare Ki values only within one assay. Novo Nordisk's own paper reported a semaglutide GLP-1 receptor affinity of 0.38 nM in a different system. ^[10] The tirzepatide explainer covers the GIP pharmacology and the full trial list.

Three randomised human trials and one set of mouse experiments compared semaglutide and tirzepatide directly. Eli Lilly funded all of them. We searched PubMed on 29 September 2026 for randomised controlled trials naming both compounds in the title and found 13 records: the three primary trials below, plus post hoc analyses of them and indirect comparisons. ^[13]

The mouse work adds a mechanistic check. When Lilly blocked the GLP-1 receptor in mice, semaglutide's glucose lowering disappeared and tirzepatide's barely changed, which fits a second receptor doing work. ^[8]

The Heise trial is the only blinded comparison. It measured insulin secretion and insulin sensitivity with clamp studies, and it reported larger changes in both with tirzepatide. ^[2] A 2024 model-based analysis of the same trial's meal tests reported greater glucagon suppression with tirzepatide. ^[14]

The head-to-head evidence is randomised and published in major journals, and it has four limits a careful reader should hold onto.

1. One sponsor. Eli Lilly funded SURPASS-2, SURMOUNT-5 and the Heise trial. ^[1][2][3] We found no independent head-to-head trial.
2. Open-label design. SURPASS-2 and SURMOUNT-5 both state it in their methods. Participants and investigators knew which drug was given. ^[1][3]
3. Unequal comparator strength. SURPASS-2 used three tirzepatide strengths and one semaglutide strength, lower than the semaglutide strengths used in STEP 1 and SURMOUNT-5. SURMOUNT-5 gave each group the highest strength participants tolerated. ^[1][15][3]
4. Different outcome records. Semaglutide has placebo-controlled cardiovascular outcome trials in type 2 diabetes (SUSTAIN-6, 3,297 participants) and in obesity without diabetes (SELECT, 17,604 participants). ^[12][16] None of the direct comparisons measured cardiovascular events.

Every trial used the manufacturers' pharmaceutical products.

Gastrointestinal events were the most common adverse events reported for both compounds, mostly mild to moderate. The clearest comparison comes from trials that ran both in the same population.

The Heise figures run the other way to SURPASS-2 for nausea and diarrhoea. The Heise groups were small (45 tirzepatide and 44 semaglutide participants), so small differences carry little weight. ^[2]

Each compound's own trials add these records:

• Semaglutide, STEP 1 (1,961 adults with overweight or obesity): nausea and diarrhoea were most common. Gastrointestinal events led to discontinuation in 4.5% against 0.8% on placebo. ^[15]
• Semaglutide, SUSTAIN-6 (type 2 diabetes): retinopathy complications were more frequent than with placebo (hazard ratio 1.76). ^[12]
• Semaglutide, SELECT: adverse events led to permanent discontinuation in 16.6% against 8.2% on placebo. ^[16]
• Semaglutide, STEP 5 (104 weeks): gastrointestinal adverse events in 82.2% against 53.9% on placebo. ^[17]
• Tirzepatide, SURPASS-1: nausea 12% to 18% against 6% on placebo, with no clinically significant or severe hypoglycaemia. ^[18]
• Tirzepatide, SURMOUNT-1: adverse events led to discontinuation in 4.3% to 7.1% of tirzepatide groups against 2.6% on placebo. ^[19]
• Both, in rats: each compound caused thyroid tumours in rats. It is not known whether this happens in humans. ^[20][21]

These figures describe trial populations under trial conditions. Novo Nordisk funded the semaglutide trials listed and Eli Lilly the tirzepatide trials. ^[12][19]

Semaglutide and tirzepatide are both listed in Schedule 4 of the Poisons Standard (prescription only). Both entries sit in the June 2026 instrument (F2026L00633), which commenced on 1 June 2026. The index also links tirzepatide to Appendix L, the dispensing-label requirements. Semaglutide's index entry shows Schedule 4 only. ^[4]

Neither compound is named on the WADA 2026 Prohibited List, effective 1 January 2026. ^[9] Class S0 covers substances with no current governmental approval for human therapeutic use. WADA revises the List every year. Our guide to peptide scheduling under Australian law explains what each schedule means.

Researchers compare semaglutide and tirzepatide. We found no trial that gave both together. Our PubMed search for clinical trials naming both alongside terms for combined use returned one record, a post hoc analysis of tirzepatide trials. ^[13]

Peptides Collective sells no vial that holds both compounds. They sit together in our metabolic research peptides range with retatrutide, the triple agonist. The tirzepatide vs retatrutide comparison sets out the next step up in receptor count.

Semaglutide is cheaper per vial in our catalogue: A$88 for 10mg against A$98 for tirzepatide 10mg. Per milligram, the order changes with vial size. Semaglutide 10mg works out at A$8.80 per mg, tirzepatide 10mg at A$9.80 and tirzepatide 20mg at A$8.40.

Prices are in Australian dollars as listed on 29 September 2026. Prescription medicine pricing follows a separate system with its own rules.

Peptides Collective sells semaglutide 10mg for A$88 and tirzepatide in 10mg and 20mg vials for laboratory research use only. Both ship from Western Australia with tracking, as lyophilised powder.

An independent lab tests every batch before sale for purity by HPLC and identity by mass spectrometry. The certificate is published under the batch number printed on the vial, and you can search the certificate of analysis library by that number.

The identity result is the quickest way to tell the two apart. PubChem lists semaglutide at 4114 g/mol and tirzepatide at 4813 g/mol, a gap of 699. ^[5][6] A mass spectrum for a tirzepatide batch should sit near the higher figure. Our guide on how to read a peptide certificate of analysis explains the HPLC trace and the mass spectrum.

Semaglutide and tirzepatide share an N-terminal motif, a lysine-20 lipid anchor and a linker, and they differ in receptor count, length and diacid length. Every direct human comparison was funded by tirzepatide's developer, and two of the three were open label. Both are Schedule 4 in Australia and absent from the WADA 2026 List. For a research vial, the batch certificate settles which molecule is inside.