Is tirzepatide a GLP-1 drug?

It is more than one. Tirzepatide activates both the GIP and GLP-1 receptors, which is why it is described as a dual agonist rather than a GLP-1 agonist.

Why is SURMOUNT-1 quoted as both 20.9% and 22.5%?

Different estimands. 20.9% covers everyone randomised on 15 mg; 22.5% covers those who adhered to treatment.

Is tirzepatide better than semaglutide?

On weight reduction, SURMOUNT-5 showed a clear head-to-head advantage: 20.2% against 13.7% at 72 weeks. Semaglutide has the larger cardiovascular outcome evidence base.

What Is Tirzepatide? The First Dual GIP/GLP-1 Agonist

The short answer

Tirzepatide is a 39-residue synthetic peptide that activates both the GIP and GLP-1 receptors from a single molecule, the first dual incretin agonist to reach the market. Built on a modified GIP backbone with a C20 fatty diacid for weekly dosing, it reported up to 22.5% weight reduction over 72 weeks in SURMOUNT-1.

Key facts

Class: Dual GIP / GLP-1 receptor agonist
Developer: Eli Lilly
Residues: 39 amino acids
Molecular weight: ~4,813 Da
Molecular formula: C225H348N48O68
PubChem CID: 166567236
Backbone: Modified GIP scaffold
Brands: Mounjaro, Zepbound

Why a GIP backbone rather than a GLP-1 one

This is the design decision that defines tirzepatide. Rather than starting from GLP-1 and adding GIP activity, Lilly started from GIP and engineered in GLP-1 receptor binding. The resulting molecule is a full GIP receptor agonist with somewhat weaker GLP-1 activity relative to native GLP-1: an imbalanced dual agonist by design, not an evenly split one. Retatrutide later used the same GIP-scaffold strategy to add a third receptor.

What GIP adds

Glucose-dependent insulinotropic polypeptide is the other major incretin hormone, and it accounts for a substantial share of the incretin effect in healthy physiology. Its contribution to weight reduction is still debated. GIP receptor agonism and antagonism have both been argued to help, which is an unusual state for a field to be in. What is not in dispute is the empirical result: adding GIP activity to GLP-1 activity produces more reduction than GLP-1 alone.

Research material referenced

Retatrutide 10mg, third-party HPLC tested

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SURMOUNT-1 and the estimand distinction

SURMOUNT-1 ran 72 weeks testing 5 mg, 10 mg and 15 mg once weekly. It is commonly quoted two different ways. On the treatment-regimen estimand, covering everyone randomised, 10 mg produced 19.5% and 15 mg 20.9%. On the efficacy estimand, covering those who adhered, the same arms produced 21.4% and 22.5%. Both sets appear in the literature; the higher pair is quoted far more often. Published in the New England Journal of Medicine in June 2022.

Head to head against semaglutide

SURMOUNT-5 randomised 751 adults with obesity and without type 2 diabetes to maximum tolerated tirzepatide (10 or 15 mg) or maximum tolerated semaglutide (1.7 or 2.4 mg) for 72 weeks. Tirzepatide produced 20.2% mean reduction against semaglutide's 13.7%, and 18.4 cm waist reduction against 13.0 cm, both significant at p<0.001. Gastrointestinal adverse events causing discontinuation were lower on tirzepatide at 2.7% than semaglutide at 5.6%.

Where it sits in the landscape

Tirzepatide is currently the most effective incretin with a marketing authorisation. Compounds reporting larger figures (retatrutide at 28.3%, CagriSema at 22.7%) are investigational or under review. It is marketed as Mounjaro for type 2 diabetes and Zepbound for weight management.

Frequently asked questions

Is tirzepatide a GLP-1 drug?: It is more than one. Tirzepatide activates both the GIP and GLP-1 receptors, which is why it is described as a dual agonist rather than a GLP-1 agonist.
Why is SURMOUNT-1 quoted as both 20.9% and 22.5%?: Different estimands. 20.9% covers everyone randomised on 15 mg; 22.5% covers those who adhered to treatment.
Is tirzepatide better than semaglutide?: On weight reduction, SURMOUNT-5 showed a clear head-to-head advantage: 20.2% against 13.7% at 72 weeks. Semaglutide has the larger cardiovascular outcome evidence base.

Extended research context

The GLP-1 & Incretin Science deep dive

Deep dive: the two routes to a bigger effect

Every compound trying to beat GLP-1 alone has taken one of two routes. The first adds more receptors from the same hormone family: GIP in tirzepatide, GIP and glucagon in retatrutide. The second adds a non-incretin satiety hormone, which in practice means amylin: CagriSema combines cagrilintide with semaglutide, and amycretin engages both receptors from one molecule. Both routes work, because they recruit signalling pathways that do not fully overlap. Neither has escaped the constraint that binds all of them, which is that gastrointestinal tolerability worsens as effect size grows.

Deep dive: why a percentage is not a result

The most-quoted numbers in this field are the least comparable. REDEFINE 1 reported 22.7% and 20.4% for the same compound in the same trial, the first among participants who adhered to treatment, the second across everyone randomised. TRIUMPH-1 reported 28.3% in an uncomplicated obesity population while TRIUMPH-3 reported up to 22.6% in adults with established cardiovascular disease, using the same compound. Before any two figures can be compared they have to match on estimand, population, duration, comparator and whether the number is placebo-adjusted. Most published comparisons match on none of them.

Deep dive: what happens after the trial stops

Every headline figure describes weight while treatment continues. The STEP-1 extension found that a year after semaglutide was stopped, participants had given back roughly two-thirds of what they lost, moving from 17.3% mean reduction to a net 5.6%, though average weight remained below baseline and nearly half stayed at least 5% down. Meta-analysis puts regain at around 0.8 kg per month. This is why maintenance studies such as TRIUMPH-6 matter more to the field's future than another two points of peak reduction.

Research applications

▸Comparing incretin and amylin compounds on a like-for-like basis
▸Interpreting estimands, thresholds and placebo-adjusted figures in trial reports
▸Tracking the obesity pipeline across sponsors and jurisdictions
▸Understanding receptor pharmacology behind GLP-1, GIP, glucagon and amylin
▸Distinguishing licensed medicines from investigational compounds

Handling checklist

✓Identify which estimand a quoted percentage comes from before citing it
✓Check the trial population and baseline BMI against the comparison you are making
✓Confirm the duration and whether the reduction curve had plateaued
✓Read discontinuation rates alongside efficacy figures
✓Verify every NCT identifier against ClinicalTrials.gov rather than secondary reporting

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Comparing headline percentages across different trials

Fix: Population, duration, estimand and comparator all differ; the numbers are not interchangeable.

✗ Quoting the larger of two figures from the same trial

Fix: Name the estimand. Efficacy and treatment-policy answer different questions.

✗ Treating peak reduction as a durable outcome

Fix: Substantial regain follows cessation across the class; peak figures describe a maintained state.

✗ Assuming an oral route means a weaker mechanism

Fix: Route and receptor count are independent. Orforglipron is weaker because it hits one receptor, not because it is a tablet.

✗ Reading investigational compounds as available treatments

Fix: Most of this pipeline holds no authorisation anywhere; mazdutide is approved only in China.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

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What Is Tirzepatide? The First Dual Incretin Agonist