# Tirzepatide Trials — SURPASS, SURMOUNT, SUMMIT and SYNERGY-NASH summaries > Trial-by-trial summary of the tirzepatide evidence base: mechanism, SURPASS diabetes trials, SURMOUNT obesity trials, SUMMIT in heart failure, SYNERGY-NASH in liver disease, and SURPASS-CVOT. A working summary of the published Phase 3 program — what each trial tested, what it found, and where the numbers came from. ## What the trials actually found The tirzepatide evidence base is unusually broad for a relatively new medicine. Two large Phase 3 programmes — SURPASS in type 2 diabetes and SURMOUNT in obesity — produced consistent results across tens of thousands of participants. SURPASS showed blood-sugar reductions of 1.9% to 2.6% on standard measures; SURMOUNT showed body-weight reductions from 13% to nearly 21% depending on dose. Two subsequent programmes extended the story: SUMMIT tested tirzepatide in heart failure with preserved ejection fraction and obesity, reducing serious heart events by 38%; SYNERGY-NASH tested it in liver disease, resolving the condition in more than half of treated participants. The cardiovascular outcomes trial, SURPASS-CVOT, enrolled over 13,000 people with type 2 diabetes and heart disease and reported results in late 2025. The sections below walk through each programme in order. ## Mechanism: imbalanced dual agonism Tirzepatide engages the GIP receptor with affinity comparable to native GIP, but it binds the GLP-1 receptor roughly 18-20-fold weaker than native GLP-1 [1]. That sounds like a weakness; it appears to be the point. At the GLP-1 receptor, tirzepatide biases signaling toward cyclic-AMP generation and away from β-arrestin recruitment, which slows the receptor's internalization and may sustain the signal longer per dose [1]. Cryo-EM structures published in 2022 confirmed that the modified N-terminus accommodates both receptor binding pockets and that the C20 fatty diacid tail extends into the extracellular space, where it tethers the drug to circulating albumin and shields it from renal filtration [13]. The result is a molecule with a 5-day half-life that can be dosed once weekly. This matters clinically because GIP and GLP-1 act on overlapping but non-identical hypothalamic circuits. GIP receptors are expressed in central regions that GLP-1 receptors do not reach, and the two together appear to suppress food intake more than either alone [6]. ## SURPASS: diabetes evidence The SURPASS program ran five pivotal Phase 3 trials in adults with type 2 diabetes. Every trial met its primary endpoint. Across SURPASS-1 through SURPASS-5, tirzepatide produced HbA1c reductions of 1.9% to 2.6% at the maintenance doses (5, 10, or 15 mg once weekly), and body-weight reductions ranging from 6.6% to 13.9% [4]. The most-cited entry in the series is **SURPASS-2**, a head-to-head trial against once-weekly semaglutide 1 mg in 1,879 patients. At 40 weeks, tirzepatide 15 mg reduced HbA1c by 2.30% versus 1.86% with semaglutide — a 0.45% absolute difference favoring tirzepatide. Body weight fell 12.4 kg with tirzepatide 15 mg versus 6.2 kg with semaglutide 1 mg [2]. A 2023 pooled analysis of the SURPASS program showed systolic blood pressure reductions of 4.0-5.6 mmHg across the dose range, broadly proportional to weight loss and independent of background antihypertensive medication [14]. Up to 97% of participants on the 15 mg dose achieved HbA1c below 7%, and up to 62% reached below 5.7% — the non-diabetic range [4]. ## SURMOUNT: obesity evidence SURMOUNT-1 (2022) randomized 2,539 adults with obesity, without diabetes, to tirzepatide 5, 10, or 15 mg or placebo for 72 weeks. Mean weight reductions were 15.0%, 19.5%, and 20.9% on the three doses, versus 3.1% with placebo. Approximately 9 in 10 tirzepatide participants lost weight. Fat-mass reduction outpaced lean-mass reduction by roughly three-to-one [5]. **SURMOUNT-2** (2023) tested the drug in adults who had both obesity and type 2 diabetes. Mean weight reductions were 13.4% (10 mg) and 15.7% (15 mg) over 72 weeks — smaller than SURMOUNT-1, consistent with the long-observed pattern that diabetes blunts obesity-pharmacotherapy response [15]. **SURMOUNT-3** (2023) added tirzepatide after a 12-week intensive lifestyle lead-in. Participants who had already lost ≥5% with lifestyle alone went on to lose an additional 21.1% with tirzepatide, for a total mean reduction of 26.6% from study entry [16]. **SURMOUNT-4** (published 2024) tested what happens when tirzepatide stops. After a 36-week open-label lead-in, participants randomized to placebo regained 14% over the next year; those who continued tirzepatide lost an additional 6.7%. The between-group difference was 20.4 percentage points [17]. **SURMOUNT-5** (NEJM, May 2025) was the first head-to-head obesity trial against semaglutide. Over 72 weeks, tirzepatide produced 20.2% mean weight loss versus 13.7% with semaglutide — a relative difference of 47% [6]. **SURMOUNT-OSA** (NEJM, 2024) extended the obesity evidence to obstructive sleep apnea: tirzepatide reduced the apnea-hypopnea index by roughly 25 events per hour and produced about 20% mean weight loss over 52 weeks, supporting the December 2024 FDA approval [18]. ## SUMMIT, SYNERGY-NASH and SURPASS-CVOT: adjacent specialties Three trials published in 2024-2025 extended the tirzepatide story beyond diabetes and obesity. **SUMMIT** (NEJM, 2025) enrolled 731 adults with heart failure with preserved ejection fraction (HFpEF) and obesity. Tirzepatide reduced the composite of cardiovascular death or worsening heart-failure events by 38% (hazard ratio 0.62, 95% CI 0.41-0.95) over a median of 104 weeks, and improved both Kansas City Cardiomyopathy Questionnaire scores and 6-minute walk distance [7]. A secondary analysis published in Nature Medicine suggested the benefit is not fully explained by weight loss — circulating hsCRP, NT-proBNP, and estimated plasma volume also fell [19]. **SYNERGY-NASH** (NEJM, 2024) treated 190 adults with biopsy-confirmed MASH (metabolic dysfunction-associated steatohepatitis) and stage 2 or 3 fibrosis for 52 weeks. Resolution of MASH without fibrosis worsening occurred in 51-62% of tirzepatide-treated participants across the three doses, versus 10% with placebo [8]. **SURPASS-CVOT** (NEJM, December 2025) — the cardiovascular outcomes trial — randomized 13,165 adults with type 2 diabetes and established atherosclerotic cardiovascular disease to tirzepatide or dulaglutide. Tirzepatide met non-inferiority for three-point MACE, with an 8% relative reduction in events and a 16% reduction in all-cause mortality favoring tirzepatide over a median follow-up of 4 years. Discontinuation for adverse events was higher in the tirzepatide arm (13.3% vs 10.2%) [9]. ## How to read the numbers Two cautions for the lay reader. First, the very large effect sizes in SURMOUNT and SURPASS are baseline-corrected and relative; absolute weight or HbA1c change varies widely by individual and depends heavily on dose tolerance during the 4-5 month titration. Second, the bulk of the evidence is industry-sponsored (Eli Lilly funded every pivotal trial cited above). Industry sponsorship does not invalidate the data — peer review, registration on ClinicalTrials.gov, and FDA review are all checks — but it is worth knowing as you read. For a complete citation list with DOIs and PMC links, see the [references page](/references). --- An independent editorial chronicle of the clinical literature — not a clinic, not a vendor, not a prescription.