Peptide Briefs

Tirzepatide: Evidence Grade A | Human Trials, Studies, and Sources

Evidence grade: A — reviewed September 2026

Controlled human trials: more than 20 Phase 3 trials completed and published across three programs (SURPASS, SURMOUNT, SUMMIT), plus a 13,299-participant cardiovascular outcomes trial; roughly 30,000 participants in total. Phase 1 and 2 trials: multiple, published. Pilot or open-label human studies: several (post-marketing and extension studies). Animal studies: yes (mouse, rat, monkey pharmacology and toxicology, including the rodent thyroid C-cell finding that carries the boxed warning).

Also known as: LY3298176, Mounjaro (type 2 diabetes brand), Zepbound (weight management and sleep apnea brand), “tirz” (vendor and forum shorthand)

Sequence: 39-amino-acid synthetic peptide built on the GIP backbone, with amino-acid substitutions that add GLP-1 receptor activity and a C20 fatty diacid attached at lysine-20 to extend half-life to about five days. Two Aib substitutions protect it from DPP-4 breakdown. The full sequence is published in the FDA label and the original Coskun 2018 paper.

One-line rationale: The only dual GIP/GLP-1 agonist with a completed, published Phase 3 program, a published cardiovascular outcomes trial, and regulatory approval in four indications. It grades A because every major claim about it rests on peer-reviewed, regulator-reviewed data; the open questions are about long-term use, not whether it works.

Note on what an A means here: it is a grade on the evidence, not a safety endorsement. Tirzepatide has a boxed warning, a heavy gastrointestinal side-effect burden, and weight that returns when it is stopped. See Safety signals.

What it is

Tirzepatide is Eli Lilly’s once-weekly injectable that activates two receptors: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). Semaglutide hits one of those. Retatrutide hits both plus glucagon.

It was approved by the FDA for type 2 diabetes in May 2022 as Mounjaro, for chronic weight management in November 2023 as Zepbound, for moderate-to-severe obstructive sleep apnea in adults with obesity in December 2024, and for type 2 diabetes in children aged 10 and older in early 2026 on the basis of SURPASS-PEDS. A cardiovascular risk-reduction indication in type 2 diabetes followed the SURPASS-CVOT publication.

Trial program

TrialPopulationPublishedKey result
SURPASS-1T2D, drug-naive, n=478Lancet, 2021A1C −1.87 to −2.07 pts vs +0.04 placebo, 40 wk
SURPASS-2T2D on metformin, n=1,879NEJM, 2021Beat semaglutide 1 mg on A1C and weight at all three doses
SURPASS-3T2D vs insulin degludec, n=1,444Lancet, 2021Superior A1C and weight; liver-fat MRI substudy
SURPASS-4T2D with CV risk vs insulin glargine, n=2,002Lancet, 2021Superior A1C; no excess CV events
SURPASS-5T2D on basal insulin, n=475JAMA, 2022A1C −2.1 to −2.4 pts vs −0.9 placebo
SURPASS-6T2D vs insulin lispro, n=1,428JAMA, 2023Superior A1C with weight loss instead of gain
SURPASS-PEDST2D age 10–17, n=99Lancet, 2025Met primary A1C endpoint; basis for pediatric approval
SURPASS-CVOTT2D with established CVD, n=13,2992025Non-inferior to dulaglutide on MACE-3 (HR 0.92); all-cause mortality 16% lower
SURMOUNT-1Obesity without T2D, n=2,539NEJM, 2022−15.0% / −19.5% / −20.9% at 5 / 10 / 15 mg vs −3.1% placebo, 72 wk
SURMOUNT-2Obesity with T2D, n=938Lancet, 2023−12.8% / −14.7% vs −3.2%, 72 wk
SURMOUNT-3After 12-wk lifestyle lead-in, n=579Nat Med, 2023−26.6% total from study start at 84 wk
SURMOUNT-4Maintenance after 36-wk lead-in, n=670JAMA, 2024Continued drug −5.5% further; switch to placebo regained +14%
SURMOUNT-5Head-to-head vs semaglutide 2.4 mg, n=751NEJM, 2025−20.2% vs −13.7% at 72 wk
SURMOUNT-OSAObstructive sleep apnea with obesity, n=469NEJM, 2024AHI reduced up to 62.8%; basis for OSA approval
SUMMITHFpEF with obesity, n=731NEJM, 202438% reduction in HF events or CV death

Regional Phase 3 trials (SURPASS J-mono, J-combo, AP-Combo; SURMOUNT-J, SURMOUNT-CN) are also published and consistent with the above.

Still outstanding: SURMOUNT-MMO (morbidity and mortality in obesity without diabetes, ~15,000 participants, expected 2027); SURMOUNT-ADOLESCENTS-1 (obesity aged 12–17, primary completion June 2026, publication pending); SURMOUNT-MAINTAIN and the Foundayo switch studies (dose reduction and maintenance, topline May 2026); TREASURE-CKD (kidney, Phase 2); TRIUMPH-5 (retatrutide vs tirzepatide, ongoing).

Safety signals

The adverse-event profile is the incretin-class profile. In SURMOUNT-1 at 15 mg: nausea 31%, diarrhea 23%, vomiting 12%, constipation 12%, mostly during dose escalation. Discontinuation for adverse events was 4.3% to 7.1% across doses vs 2.6% on placebo.

Signals specific enough to name:

  • Thyroid C-cell tumors. Boxed warning, based on rodent studies. No causal link in humans has been shown; contraindicated in people with a personal or family history of medullary thyroid carcinoma or MEN 2.
  • Pancreatitis and gallbladder disease. Reported at low rates across the program, consistent with the class.
  • Hypoglycemia when combined with insulin or sulfonylureas.
  • Dehydration and acute kidney injury secondary to severe GI events.
  • Weight regain on stopping. SURMOUNT-4 is the cleanest demonstration: participants switched to placebo after 36 weeks regained about 14 percentage points over the following year.
  • Lean mass. DXA substudies show roughly a quarter of weight lost is lean mass, in line with other incretin drugs and with diet-induced weight loss.

Class-level questions under review for GLP-1 drugs generally, including non-arteritic anterior ischemic optic neuropathy and suicidal ideation, have not shown a signal for tirzepatide in the published program; post hoc psychiatric-safety analyses of SURMOUNT (2026) found none.

Research-use-only tirzepatide sold by vendors is not the approved product. It is not covered by any of the trial data above unless the vial contains what the label says; see Reading a COA.

Why A, not lower

Every criterion on the Grade Key is met: multiple large randomized controlled trials, peer-reviewed publication of the full pivotal program, a completed outcomes trial, and regulatory review of the safety file in more than one jurisdiction. The open questions are real but are about duration and population, not about whether the effect exists.

See also

Retatrutide (grade B; adds glucagon receptor activity, three of five Phase 3 results published). Semaglutide (grade A; comparator in SURPASS-2 and SURMOUNT-5). Grade Key.

Sources

  1. Coskun T, et al. LY3298176, a novel dual GIP and GLP-1 receptor agonist for the treatment of type 2 diabetes mellitus. Mol Metab. 2018.
  2. Frias JP, et al. Tirzepatide versus semaglutide once weekly in patients with type 2 diabetes (SURPASS-2). N Engl J Med. 2021.
  3. Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med. 2022. ClinicalTrials.gov NCT04184622.
  4. Garvey WT, et al. Tirzepatide once weekly for the treatment of obesity in people with type 2 diabetes (SURMOUNT-2). Lancet. 2023. NCT04657003.
  5. Aronne LJ, et al. Continued treatment with tirzepatide for maintenance of weight reduction in adults with obesity (SURMOUNT-4). JAMA. 2024.
  6. Aronne LJ, et al. Tirzepatide as compared with semaglutide for the treatment of obesity (SURMOUNT-5). N Engl J Med. 2025. NCT05822830.
  7. Malhotra A, et al. Tirzepatide for the treatment of obstructive sleep apnea and obesity (SURMOUNT-OSA). N Engl J Med. 2024. NCT05412004.
  8. Packer M, et al. Tirzepatide for heart failure with preserved ejection fraction and obesity (SUMMIT). N Engl J Med. 2024. NCT04847557.
  9. Nicholls SJ, Pavo I, Bhatt DL, et al. Cardiovascular outcomes with tirzepatide versus dulaglutide in type 2 diabetes (SURPASS-CVOT). N Engl J Med. 2025;393(24):2409-2420. doi:10.1056/NEJMoa2505928. NCT04255433. Post hoc cardiorenal analysis: Nissen SE, et al. JAMA Cardiol. 2026.
  10. SURPASS-PEDS: Efficacy and safety of tirzepatide in children and adolescents with type 2 diabetes. Lancet. 2025.
  11. Wadden TA, et al. Psychiatric safety of tirzepatide in people with obesity (SURMOUNT post hoc). Obesity. 2026.
  12. FDA. Zepbound and Mounjaro prescribing information; approval history 2022–2026, including the August 28, 2026 cardiovascular risk-reduction label expansion (Eli Lilly news release).
  13. ClinicalTrials.gov NCT06075667 (SURMOUNT-ADOLESCENTS-1), primary completion June 2026.