What Is Tirzepatide? The Dual GIP/GLP-1 Peptide Explained

The short version

What is tirzepatide? It is a synthetic peptide drug that copies two natural gut hormones at once — GIP and GLP-1 — both of which your body releases after eating to signal the pancreas to produce insulin. Most incretin drugs only copy GLP-1. Tirzepatide does both simultaneously, which is why the clinical trials show larger effects on blood sugar and body weight than earlier GLP-1-only drugs. The FDA approved it in 2022 for type 2 diabetes and in 2023 for chronic weight management. In the largest obesity trial, participants lost an average of nearly 21% of body weight over 72 weeks. This page explains the mechanism, the molecular structure that makes once-weekly dosing possible, and what Tirzepatide research has established about its cardiometabolic effects.

What is tirzepatide: definition and structure

Tirzepatide (development code LY3298176) is a synthetic 39-amino-acid peptide based on the native GIP (glucose-dependent insulinotropic polypeptide) sequence, engineered by Eli Lilly and reported in 2018 ^[5]. Its defining structural feature is a C20 fatty diacid (eicosanedioic acid) modification attached via a glutamic-acid linker and two AEEA (2-(2-aminoethoxy)ethoxy)acetic acid) spacer units to a lysine side chain on the peptide backbone. This fatty-diacid arm binds tightly to albumin — the most abundant protein in blood plasma — effectively extending the drug's elimination half-life to approximately 5 days ^[5]. Once-weekly subcutaneous dosing follows directly from that half-life.

Molecular formula: C225H348N48O68. Molecular weight: 4813.53 Da. CAS number: 2023788-19-2. ATC code: A10BX16. International nonproprietary name: tirzepatide.

Both GIP and GLP-1 are incretin hormones — gut-derived peptides released after eating that amplify insulin secretion in a glucose-dependent fashion (meaning the insulin signal is stronger when blood sugar is high, weaker when it is low, which reduces the risk of dangerous hypoglycaemia from the drug itself). GLP-1 also suppresses glucagon (a hormone that raises blood sugar), slows gastric emptying, and reduces appetite. GIP influences both fat metabolism and insulin sensitivity.

By engaging both receptors simultaneously, tirzepatide combines all of these effects with a single molecule ^[5].

Discovery and phase 1 proof of concept

The 2018 discovery paper (Coskun et al., Molecular Metabolism) established three things ^[5]. First, tirzepatide activated both the GIP receptor and the GLP-1 receptor in vitro, improving glucose-dependent insulin secretion and glucose tolerance. Second, in mice with chronic administration it decreased body weight and food intake significantly more than a selective GLP-1 receptor agonist. Third, a phase 1 programme in 142 human subjects (healthy volunteers and adults with type 2 diabetes) confirmed PK supporting once-weekly subcutaneous dosing and demonstrated reductions in fasting glucose and body weight versus placebo.

Subsequent in vitro characterisation (Willard et al., JCI Insight, 2020) classified tirzepatide as an imbalanced dual agonist: it engages the GIP receptor more fully than the GLP-1 receptor and shows biased GLP-1 receptor signalling that favours cAMP (cyclic AMP — the intracellular messenger that triggers insulin secretion) generation over beta-arrestin recruitment (a pathway associated with receptor internalisation and signal termination). In primary islet experiments, beta-arrestin 1 limited the insulin response to GLP-1 but not to GIP or tirzepatide — a proposed mechanism for the enhanced efficacy ^[7].

Approved indications and what the trials established

FDA approvals:

• Type 2 diabetes mellitus — May 2022 ^[9]
• Chronic weight management in adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with at least one weight-related condition — November 2023
• Moderate-to-severe obstructive sleep apnea in adults with obesity

Key phase 3 results:

• SURPASS-2 (type 2 diabetes, n=1,879, 40 weeks): HbA1c -2.01/-2.24/-2.30 percentage points (5/10/15 mg) versus -1.86 percentage points with a selective GLP-1 receptor agonist; weight differences -1.9/-3.6/-5.5 kg in favour of tirzepatide ^[1]
• SURMOUNT-1 (obesity without diabetes, n=2,539, 72 weeks): body weight -15.0%/-19.5%/-20.9% (5/10/15 mg) versus -3.1% with placebo ^[6]
• SURMOUNT-5 (obesity without diabetes, head-to-head, n=751, 72 weeks): tirzepatide -20.2% versus comparator -13.7% (p<0.001) ^[8]
• SURPASS meta-analysis (7 trials, n=7,215): MACE-4 HR 0.80 (95% CI 0.57-1.11); cardiovascular death HR 0.90; all-cause death HR 0.80 ^[2]
• Lipid meta-analysis (13 RCTs): significant improvement in all lipid markers with dose-response trend ^[3]

Tirzepatide vs semaglutide: the evidence

The evidence for tirzepatide vs semaglutide is now head-to-head across two major trials.

SURPASS-2 (type 2 diabetes): tirzepatide was non-inferior and superior to semaglutide 1 mg at all three doses on HbA1c reduction, with greater weight reduction at all doses ^[1].

SURMOUNT-5 (obesity without diabetes): tirzepatide at the maximum tolerated dose produced -20.2% body weight versus -13.7% with semaglutide at the maximum tolerated dose over 72 weeks, a difference reaching p<0.001; tirzepatide also achieved higher proportions of participants reaching ≥10%, ≥15%, ≥20%, and ≥25% weight-loss thresholds ^[8].

This is an FDA-approved drug-to-drug comparison in a published, peer-reviewed phase 3 trial — not an indirect comparison across separately designed studies.