Head to head
Semaglutide vs Tirzepatide: 15% vs 22% Weight Loss in Trials
Published by Peptara Labs · Updated · How we research and check our pages
Semaglutide vs tirzepatide is a side-by-side research comparison by Peptara Labs (peptaralabs.io) of the two most-used GLP-1-class weight-loss compounds: mechanism, clinical trial results and side effects. The 15% (Semaglutide Phase 3 STEP-1) and 22% (Tirzepatide Phase 3 SURMOUNT-1) figures come from separate obesity trials with different durations and populations, not from a head-to-head study. Both are FDA-approved as brand pens.
Verdict in 4 lines
- Both are top-tier GLP-1 weight-loss compounds, FDA-approved as brand pens.
- Tirzepatide is the dual-agonist with stronger Phase 3 weight-loss data (22.5%, 72 weeks).
- Semaglutide is the single GLP-1 agonist with the longest real-world track record (14.9%, 68 weeks).
- Trial percentages cited below come from separate obesity studies, not a head-to-head trial.
TL;DR comparison
One table, every row that matters. The weight-loss figures come from two separate obesity trials, STEP-1 and SURMOUNT-1, not from one head-to-head study.
Mechanism: single vs dual agonist
Both peptides work by activating gut and pancreatic hormone receptors that regulate appetite, insulin secretion, and energy balance. The difference is how many of those receptors each one activates.
Semaglutide (single GLP-1 agonist)
Activates one receptor: GLP-1 (glucagon-like peptide 1, slows gastric emptying and suppresses appetite). It was the first GLP-1 agonist to drive large-scale weight loss in obesity trials and remains the most-prescribed compound in the class.
Tirzepatide (dual-agonist)
Activates two receptors: GLP-1 plus GIP (glucose-dependent insulinotropic peptide, boosts insulin sensitivity and may modulate fat storage). The second pathway is the leading explanation for why Tirzepatide trial fat-loss numbers exceeded single-agonist semaglutide.
Honest trade-off: more receptor activation can mean more potential side effects. The dual-agonist mechanism that drives Tirzepatide's stronger fat-loss percentage can amplify GI events at higher doses.
Clinical trial results
The headline figures, the trial citations, and the disclaimer that has to come with them.
Semaglutide Phase 3 (STEP-1)
- Reference: STEP-1, Wilding et al, NEJM 2021
- Phase: 3
- n: 1,961
- Duration: 68 weeks
- Cohort: semaglutide group
- Result: 14.9% mean body weight reduction
Tirzepatide Phase 3 (SURMOUNT-1)
- Reference: SURMOUNT-1, Jastreboff et al, NEJM 2022
- Phase: 3
- n: 2,539
- Duration: 72 weeks
- Cohort: highest dose group
- Result: 22.5% mean body weight reduction
Read this before quoting either number
These percentages come from two separate obesity trials with different durations (68 vs 72 weeks) and different patient populations. A direct percentage comparison should be read as descriptive, not causal. The head-to-head evidence comes from two other trials, set out below: SURPASS-2 (NEJM 2021) in type 2 diabetes and SURMOUNT-5 (NEJM 2025) in adults with obesity. The Peptara position: cite the numbers accurately, name the trials, and never present the 15% and 22% figures as a head-to-head result.
Head-to-head notes: SURPASS-2 and SURMOUNT-5
SURPASS-2 (Frias et al, NEJM 2021) compared Tirzepatide against semaglutide in type 2 diabetes. Tirzepatide produced greater A1c reduction and greater weight loss in all three of its dose groups. Two caveats: the semaglutide group received a lower dose than the one used in the obesity trials, and the population had type 2 diabetes, not obesity. SURMOUNT-5 (Aronne et al, NEJM 2025) later compared the two in adults with obesity without type 2 diabetes and reported greater weight loss with Tirzepatide.
Side effects
Both share the GLP-1 class profile. Each has compound-specific notes worth flagging.
Semaglutide-specific
- FDA boxed warning: thyroid C-cell tumors (rodent data)
- Rare reports of pancreatitis
- Gallbladder events (cholelithiasis)
- Largest cardiovascular outcomes dataset in class (SELECT)
Tirzepatide-specific
- FDA boxed warning: thyroid C-cell tumors (rodent data)
- Higher GI rate at top doses vs lower doses
- Rare reports of pancreatitis
- Gallbladder events (cholelithiasis, cholecystitis)
In both trials, gastrointestinal effects were mostly mild to moderate (Wilding et al., 2021; Jastreboff et al., 2022).
Frequently asked questions
Is Tirzepatide better than Semaglutide for weight loss?
In their respective Phase 3 obesity trials, Tirzepatide produced a higher mean body weight reduction (22.5% at 72 weeks in the highest dose group, SURMOUNT-1) than Semaglutide (14.9% at 68 weeks, STEP-1). Head-to-head trials also favored Tirzepatide: SURPASS-2 in type 2 diabetes, on weight and A1c, and SURMOUNT-5 in adults with obesity, on weight. Mechanistically, Tirzepatide adds GIP-receptor activation on top of GLP-1, which is the leading explanation for the larger effect. So on the trial data, Tirzepatide is the stronger fat-loss compound. Semaglutide has the longer real-world safety record and the larger cardiovascular outcomes dataset (SELECT).
Which has fewer side effects?
Both share the GLP-1 class side-effect profile reported in trials: nausea, vomiting, constipation and decreased appetite. In the SURPASS-2 diabetes trial, overall GI tolerability was broadly similar between the two, with Tirzepatide showing a slightly higher rate of some GI events at higher doses (the trade-off for the larger effect). Both carry an FDA boxed warning for thyroid C-cell tumors (rodent data, no confirmed human signal) and rare reports of pancreatitis and gallbladder events.
Why are these two compared so often?
They are the two most-used GLP-1-class weight-loss compounds in the world. Semaglutide (Ozempic, Wegovy) was the breakthrough that made GLP-1 weight loss mainstream. Tirzepatide (Mounjaro, Zepbound) followed with a dual-agonist mechanism and higher trial numbers, which is why "Semaglutide vs Tirzepatide" is one of the highest-volume comparison queries in the space.
Does Peptara Labs sell semaglutide or tirzepatide?
No. Peptara Labs does not sell semaglutide or tirzepatide. Both are FDA-approved as prescription brand medicines: semaglutide as Ozempic and Wegovy, tirzepatide as Mounjaro and Zepbound.
Are these trial numbers from the same study?
No. The 14.9% Semaglutide figure comes from STEP-1, Wilding et al, NEJM 2021, a Phase 3 obesity trial, n=1,961, 68 weeks. The 22.5% Tirzepatide figure comes from SURMOUNT-1, Jastreboff et al, NEJM 2022, a Phase 3 obesity trial, n=2,539, 72 weeks, highest dose group. These are separate trials with different durations and different patient populations. A direct percentage comparison should be read as descriptive, not causal. The head-to-head trials are SURPASS-2, in type 2 diabetes, and SURMOUNT-5, in adults with obesity without diabetes, and both favored tirzepatide.