Tirzepatide Research: SURPASS, SURMOUNT, and the Phase 3 Clinical Programme
The short version
Tirzepatide research centres on two large clinical trial programmes: SURPASS (type 2 diabetes, five trials) and SURMOUNT (obesity and overweight, four trials). In type 2 diabetes, the 40-week SURPASS-2 trial found tirzepatide superior to semaglutide 1 mg at all three doses — reducing HbA1c (blood sugar marker) by up to 2.30 percentage points versus 1.86 with the comparator. In obesity, the 72-week SURMOUNT-1 trial found weight reductions of up to 20.9% at the highest dose versus 3.1% with placebo — the largest weight reduction measured in any large controlled trial for obesity treatment. A 2025 head-to-head comparison (SURMOUNT-5) confirmed superiority over semaglutide in the obesity setting as well. More recent trials have extended into heart failure, liver disease, and sleep apnea. This page summarises the key findings from each trial, with full citations.
Discovery and mechanism — the preclinical and Phase 1 foundation
The discovery paper for LY3298176 (tirzepatide) reported the compound design, in vitro characterisation at the GIPR and GLP-1R, and a preclinical efficacy programme in mice in which tirzepatide reduced body weight and food intake significantly more than a selective GLP-1 receptor agonist. A Phase 1 programme in 142 subjects (healthy volunteers plus a T2D proof-of-concept cohort) confirmed the pharmacokinetics supporting once-weekly subcutaneous dosing and showed reductions in fasting glucose and body weight versus placebo [1].
Subsequent in vitro characterisation established that tirzepatide is an imbalanced and biased dual agonist — engaging the GIP receptor more fully than the GLP-1 receptor, and exhibiting biased GLP-1R signalling that favours cAMP generation over beta-arrestin recruitment. Beta-arrestin1 was shown to limit the insulin response to GLP-1 but not to GIP or tirzepatide in primary islet experiments, suggesting biased agonism contributes to enhanced insulin secretion [5].
A clinical pharmacology study found tirzepatide transiently delays gastric emptying to a degree similar to selective long-acting GLP-1 receptor agonists, an effect that attenuates with continued dosing [25]. Population pharmacokinetic analyses across the phase 3 programme confirmed the approximately five-day elimination half-life and once-weekly dosing pharmacokinetics [10].
SURPASS programme — type 2 diabetes trials
The SURPASS programme comprises five phase 3 randomised controlled trials evaluating tirzepatide in type 2 diabetes.
SURPASS-2 (n=1,879; 40 weeks; open-label; active comparator): Once-weekly tirzepatide 5, 10, and 15 mg versus semaglutide 1 mg once weekly in adults with type 2 diabetes inadequately controlled on metformin. Estimated reductions in HbA1c were 2.01, 2.24, and 2.30 percentage points with tirzepatide versus 1.86 percentage points with semaglutide — tirzepatide was non-inferior and superior at all three doses. Body weight reductions were -7.6, -9.3, and -11.2 kg with tirzepatide versus -5.7 kg with semaglutide, treatment differences of -1.9, -3.6, and -5.5 kg. Gastrointestinal adverse events were the most common side effects, mostly mild to moderate [2].
A StatPearls reference chapter confirms the type 2 diabetes approval and the dual GLP-1/GIP mechanism, noting the agent is not approved for type 1 diabetes [8]. A peer-reviewed overview characterised the compound as a new dual GIP/GLP-1 receptor agonist with a broad spectrum of metabolic activity [9]. A clinical pharmacokinetics study found that hepatic impairment did not produce clinically meaningful changes in tirzepatide exposure warranting dose adjustment [28].
For older adults (≥65 years) with type 2 diabetes and BMI below 30, a post hoc analysis of pooled SURPASS-1 to -5 data found tirzepatide reduced HbA1c by 1.97–2.10 percentage points across doses in this subgroup, without increasing hypoglycaemia risk [29].
SURMOUNT programme — obesity and overweight trials
The SURMOUNT programme comprises four phase 3 trials evaluating tirzepatide for obesity and weight management.
SURMOUNT-1 (n=2,539; 72 weeks; double-blind, placebo-controlled): Adults with obesity (BMI ≥30) or overweight (BMI ≥27) with a weight-related complication, without diabetes. Mean weight change at week 72: -15.0% (5 mg), -19.5% (10 mg), and -20.9% (15 mg) versus -3.1% with placebo. Gastrointestinal adverse events were most common, occurring primarily during dose escalation, and were mostly mild to moderate [3]. A DXA substudy (n=160) found approximately 75% of weight lost was fat mass and approximately 25% lean mass [19].
SURMOUNT-4 (phase 3 randomised withdrawal; n=670 randomised after a 36-week open-label lead-in achieving mean -20.9% weight reduction): Participants continuing tirzepatide achieved a further -5.5% weight change from week 36 to 88, versus +14.0% regain in those switched to placebo (difference -19.4%; P<0.001). 89.5% on tirzepatide maintained ≥80% of initial weight loss versus 16.6% on placebo. Overall mean reduction from baseline to week 88 was -25.3% with tirzepatide versus -9.9% with placebo [23]. A post hoc analysis showed weight regain after discontinuation tracked with worsening cardiometabolic parameters [24].
SURMOUNT-5 (n=751; 72 weeks; open-label, active comparator; phase 3b): Adults with obesity without type 2 diabetes, randomised to maximum tolerated tirzepatide (10 or 15 mg) or maximum tolerated semaglutide (1.7 or 2.4 mg). Least-squares mean weight change at week 72: -20.2% with tirzepatide versus -13.7% with semaglutide (P<0.001). Tirzepatide also produced greater reductions in waist circumference and higher proportions achieving ≥10%, ≥15%, ≥20%, and ≥25% weight loss [4].
A systematic review and meta-analysis for the ACP (Berg et al.) reported mean weight regain of approximately 9.69 kg in the pooled semaglutide/tirzepatide group after stopping, versus 2.20 kg for liraglutide, confirming the chronic-therapy framing [22].
Tirzepatide mechanism of action — the signalling basis for dual efficacy
Tirzepatide mechanism of action rests on the coordinated engagement of two incretin receptor pathways. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are gut-derived hormones that amplify meal-stimulated insulin secretion. Tirzepatide mimics both with a single 39-amino-acid peptide, producing: glucose-dependent insulin secretion enhancement, glucagon suppression, delayed gastric emptying, and central appetite reduction.
The in vitro characterisation by Willard et al. established the imbalanced and biased profile: preferential GIPR engagement, with GLP-1R signalling biased toward cAMP over beta-arrestin, and weaker GLP-1R internalisation than native GLP-1. This profile is proposed to enhance insulin secretion and contribute to the larger metabolic effects observed versus selective GLP-1 receptor agonists in trials [5]. A 2026 Lancet review confirmed the evidence base for tirzepatide's established benefits in type 2 diabetes prevention, heart failure with preserved ejection fraction, steatotic liver disease, and obstructive sleep apnea [12].
2024–2026 updates from the expanding trial programme
The 2026 American College of Physicians living clinical guideline for pharmacological treatment of overweight and obesity in non-pregnant adults placed semaglutide and tirzepatide as conditional first-line recommendations for obesity (BMI ≥30) and for overweight plus a comorbidity (BMI ≥27–30), based on moderate-certainty evidence, with guidance to use shared decision-making regarding benefits, harms, access, and contraindications [11].
A 2026 living systematic review and network meta-analysis underlying the ACP guideline (69 studies, 112,511 participants; search through October 2025) found nearly all studied interventions more effective than placebo for weight reduction, with semaglutide and tirzepatide producing the greatest weight loss. Semaglutide probably reduced mortality and major adverse cardiovascular events; evidence for mortality and MACE for tirzepatide was characterised as limited [11].
A 2026 Lancet review of GLP-1 receptor agonists and next-generation incretin therapies confirmed tirzepatide among agents with trial-established benefits in metabolic cardiovascular organ systems, and identified emerging investigation directions including neurodegenerative diseases, substance use disorders, and oral small-molecule GLP-1 receptor agonists [12].
The 10th CVOT Summit (2025) discussed the tirzepatide SURMOUNT-OSA and SUMMIT trial results as significant advances in reducing metabolic risk in obstructive sleep apnea and HFpEF, respectively, as part of the expanding incretin evidence base [13].