GLP-1 Evidence

Drug

Tirzepatide: what the trials show

Tirzepatide produced the largest mean weight losses ever recorded for a drug in a randomised obesity trial. It is also the one with the most interesting non-weight results.

Updated 3 min read 12 citations Evidence strength 5/5

SURMOUNT-1: the largest effect size in the field

SURMOUNT-1 randomised adults with obesity and without diabetes to tirzepatide 5, 10 or 15 mg weekly, or placebo, for 72 weeks [8]. Mean weight change at the top dose was about −20.9%, with the 5 mg dose already around −15% — meaning the lowest tirzepatide dose roughly matched the best semaglutide result. More than half of participants at the higher doses lost at least 20% of body weight.

A continuation of that programme extended follow-up and reported on diabetes prevention alongside sustained weight reduction [5], which matters because it moves the endpoint from a number on a scale to a clinical event.

Weight loss efficacy

The diabetes penalty applies here too

SURMOUNT-2 ran the same protocol in adults with obesity and type 2 diabetes and found roughly 14.7% mean weight loss at the top dose [7]. Again: a large effect, and again noticeably smaller than in the non-diabetic population. Meta-analysis comparing the two drugs in type 2 diabetes puts the relative ordering on firmer ground [1][3].

Tirzepatide — mean weight change by trial and population
TrialPopulationDoseDurationMean weight change
SURMOUNT-1 [8]Obesity, no diabetes15 mg72 weeks≈ −20.9%
SURMOUNT-1 [8]Obesity, no diabetes5 mg72 weeks≈ −15%
SURMOUNT-2 [7]Obesity + type 2 diabetes15 mg72 weeks≈ −14.7%
SURMOUNT-5 [10]Obesity, head-to-head vs semaglutide≤15 mg72 weeksSuperior to semaglutide

Beating semaglutide directly

Indirect comparisons and real-world cohorts had pointed this way for a while [12], but the trial that settled it compared the two drugs head-to-head at obesity doses in the same population, and tirzepatide won on mean weight loss by a substantial margin [10]. In type 2 diabetes, SURPASS-2 had already shown superiority over semaglutide 1 mg on both glycaemic and weight endpoints [9].

The results that are not about weight

Two dedicated trials moved tirzepatide beyond obesity as a category.

  • Obstructive sleep apnoea. In adults with moderate-to-severe OSA and obesity, tirzepatide reduced the apnoea-hypopnoea index substantially against placebo [6]. This is the first pharmacological treatment with randomised evidence in a condition previously managed almost entirely with CPAP.
  • Heart failure with preserved ejection fraction. In HFpEF with obesity — a phenotype with very few effective treatments — tirzepatide improved outcomes against placebo [4].
  • Cardiovascular events. A dedicated comparison against dulaglutide reported cardiovascular outcomes in type 2 diabetes [11], adding to the earlier pre-specified meta-analysis that found no signal of cardiovascular harm.

It also appears in the European guidance on steatotic liver disease alongside semaglutide [2].

Common questions

Is Mounjaro the same as Zepbound?
Same molecule, different brand names for the diabetes and obesity indications respectively.
Is tirzepatide simply better than semaglutide?
On mean weight loss in a head-to-head obesity trial, yes [10]. On everything else — tolerability for a given person, cost, availability, and the specific comorbidity you are treating — the answer is individual. Semaglutide has the larger cardiovascular outcome dataset.
Does the GIP component cause the extra weight loss?
It is the obvious candidate, but the mechanism is genuinely unresolved. Blocking GIP also produces weight loss in some models, which is one of the odder open problems in the field.
Do I have to reach 15 mg?
No. The 5 mg dose produced around 15% mean loss in SURMOUNT-1 [8], comparable to the best semaglutide result. Dose is titrated against tolerability.

References

Every citation below links to the original peer-reviewed record on PubMed or via DOI. Nothing here is a substitute for medical advice.

  1. Subcutaneously administered tirzepatide vs semaglutide for adults with type 2 diabetes: a systematic review and network meta-analysis of randomised controlled trials Karagiannis T, Malandris K, Avgerinos I, et al. · Diabetologia · 2024 · Meta-analysis DOIPubMed 38613667Full text
  2. EASL-EASD-EASO Clinical Practice Guidelines on the management of metabolic dysfunction-associated steatotic liver disease (MASLD) Journal of hepatology · 2024 · Clinical guideline DOIPubMed 38851997
  3. Comparative effectiveness of GLP-1 receptor agonists on glycaemic control, body weight, and lipid profile for type 2 diabetes: systematic review and network meta-analysis Yao H, Zhang A, Li D, et al. · BMJ (Clinical research ed.) · 2024 · Meta-analysis DOIPubMed 38286487Full text
  4. Tirzepatide for Heart Failure with Preserved Ejection Fraction and Obesity Packer M, Zile MR, Kramer CM, et al. · The New England journal of medicine · 2025 · Randomised controlled trial DOIPubMed 39555826
  5. Tirzepatide for Obesity Treatment and Diabetes Prevention Jastreboff AM, le Roux CW, Stefanski A, et al. · The New England journal of medicine · 2025 · Randomised controlled trial DOIPubMed 39536238
  6. Tirzepatide for the Treatment of Obstructive Sleep Apnea and Obesity Malhotra A, Grunstein RR, Fietze I, et al. · The New England journal of medicine · 2024 · Randomised controlled trial DOIPubMed 38912654Full text
  7. Tirzepatide once weekly for the treatment of obesity in people with type 2 diabetes (SURMOUNT-2): a double-blind, randomised, multicentre, placebo-controlled, phase 3 trial Garvey WT, Frias JP, Jastreboff AM, et al. · Lancet (London, England) · 2023 · Randomised controlled trial DOIPubMed 37385275
  8. Tirzepatide Once Weekly for the Treatment of Obesity Jastreboff AM, Aronne LJ, Ahmad NN, et al. · The New England journal of medicine · 2022 · Randomised controlled trial DOIPubMed 35658024
  9. Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes Frías JP, Davies MJ, Rosenstock J, et al. · The New England journal of medicine · 2021 · Randomised controlled trial DOIPubMed 34170647
  10. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity Aronne LJ, Horn DB, le Roux CW, et al. · The New England journal of medicine · 2025 · Clinical trial DOIPubMed 40353578
  11. Cardiovascular Outcomes with Tirzepatide versus Dulaglutide in Type 2 Diabetes Nicholls SJ, Pavo I, Bhatt DL, et al. · The New England journal of medicine · 2025 · Clinical trial DOIPubMed 41406444
  12. Semaglutide vs Tirzepatide for Weight Loss in Adults With Overweight or Obesity Rodriguez PJ, Goodwin Cartwright BM, Gratzl S, et al. · JAMA internal medicine · 2024 · Journal article DOIPubMed 38976257Full text