What tirzepatide is and how it works
Tirzepatide is a 39-amino-acid peptide built on the sequence of glucose-dependent insulinotropic polypeptide, the other incretin hormone, and engineered until it also fits the GLP-1 receptor. An unnatural amino acid at the cleavage site protects it from dipeptidyl peptidase-4, and a C20 fatty diacid chain binds serum albumin, which slows renal clearance and gives a half-life of about five days. One molecule, two receptors, one injection a week.
The two receptors are not engaged equally. In receptor assays tirzepatide behaves much like native GIP at the GIP receptor, while at the GLP-1 receptor it is a weaker agonist than GLP-1 itself. Pharmacologists call this imbalanced agonism, and it is deliberate: the GLP-1 arm supplies glucose-dependent insulin release, glucagon suppression, slowed gastric emptying and the central appetite effect, while the GIP arm adds something the GLP-1 receptor cannot.
What that something is remains the open question of the field. GIP was the first incretin discovered and, for obesity, the least promising. Its insulin-releasing effect is blunted in type 2 diabetes. In fat tissue it promotes storage rather than release, which sounds like exactly the wrong direction. Awkwardly for the story, mouse work showed that blocking the GIP receptor also produced weight loss, so the literature contained a genuine paradox: agonism and antagonism appeared to point the same way. The best-supported explanation is that the useful GIP signal is in the brain rather than the fat cell, where receptors in appetite-regulating regions reduce food intake and appear to dampen the nausea driven by GLP-1 agonism. Prolonged agonism may also desensitise the receptor, which would make chronic stimulation resemble blockade. Neither account is settled: tirzepatide was shown to work before anyone could say precisely why the second receptor helps.
What the research shows for tirzepatide
The clinical record rests on two large randomised programmes: SURPASS in type 2 diabetes, where tirzepatide lowered HbA1c more than the comparators it was tested against, and SURMOUNT in obesity. SURMOUNT-1, reported in 2022, is the reference trial. Adults with obesity or overweight and without diabetes received weekly tirzepatide or placebo for 72 weeks. Mean weight change was −15.0% at 5 mg, −19.5% at 10 mg and −20.9% at 15 mg, against −3.1% on placebo. The dose response is continuous: more receptor activation kept producing more effect across the range tested.
The head-to-head: SURMOUNT-5
Cross-trial comparison is a weak form of evidence, because populations, durations and lifestyle programmes differ. SURMOUNT-5, reported in 2025, removed that objection for one pairing by randomising participants directly to tirzepatide or semaglutide. At 72 weeks mean weight loss was 20.2% on tirzepatide against 13.7% on semaglutide. It is the only direct comparison among the three incretin peptides profiled on this site, and it is the reason the dual agonist is generally described as the more effective of the two rather than merely the more recent.
What none of these trials measured is a cardiovascular outcome in people without diabetes, the result semaglutide already holds from SELECT. Until such a trial reports, the fair statement is that tirzepatide produces more weight loss in direct comparison, and that the hard-outcome evidence behind it is younger.
Doses used in published research
| Setting | Dose and route | Interval | Duration |
|---|---|---|---|
| SURMOUNT-1 (obesity, 2022) | 5, 10 or 15 mg subcutaneous | Weekly | 72 weeks |
| SURMOUNT-5 (vs semaglutide, 2025) | Maximum tolerated 10 or 15 mg | Weekly | 72 weeks |
| Label escalation | 2.5 mg start, +2.5 mg steps | Every 4 weeks | To 5, 10 or 15 mg |
The 2.5 mg starting dose is not a therapeutic dose at all: it exists to let the gut adapt, and the four-week step is the pattern every trial in the programme used. Participants who reached 15 mg lost the most weight; those who could not tolerate the escalation stayed lower.
These are the schedules used in supervised clinical trials and written into an approved product label. They are recorded here as study designs, not as instructions, and research material carries no authorisation for human use.
Safety and side effects of tirzepatide
The adverse event profile follows the incretin pattern, with the gut leading and the dose escalation shaping most of it.
- Gastrointestinal effects. Nausea, diarrhoea, vomiting and constipation are the most frequent findings in every SURMOUNT and SURPASS trial, mostly mild to moderate and concentrated in the escalation phase. A minority of participants stopped because of them.
- Gallbladder disease. Reported more often than on placebo, as with rapid weight loss generally.
- Pancreatitis. Uncommon, and a labelled warning rather than a frequent event.
- Thyroid C-cell tumours in rodents. A class finding in rats of unproven human relevance, which regulators act on through a family-history contraindication.
- Low blood sugar. Rare on tirzepatide alone, more likely with insulin or a sulfonylurea.
- Heart rate. A small mean increase was recorded, consistent with the class.
Regulatory status of tirzepatide in Europe
Tirzepatide is authorised in the EU as Mounjaro, following assessment by the European Medicines Agency, for type 2 diabetes and for weight management. It is a prescription-only medicine, made under pharmaceutical quality rules and dispensed through pharmacies. In the United States the same molecule is sold as Zepbound.
Research-grade tirzepatide is a different matter entirely. It has no marketing authorisation, has not been assessed by any regulator, and is supplied for laboratory use. Under EU law a medicinal product may only be placed on the market with an authorisation; national rules on possession and personal import vary between member states. Tirzepatide is not named on the current WADA Prohibited List, which is reissued annually, so athletes should consult the list in force.
Sourcing tirzepatide and checking quality
King Peptides sells this peptide as Trizzy (Tirzepatide) 10 mg, research material rather than Mounjaro. The distinction is not marketing language: an authorised medicine is a named product with a dossier behind it, and a research vial is a chemical supplied for laboratory work.
Two figures on the lot-specific certificate of analysis do most of the verification work. HPLC purity should be 99% or better, and the mass spectrometry peak should sit at 4813.5 Da, the figure in the molecular panel beside this article. That mass separates tirzepatide from semaglutide on paper, some 700 Da apart, so a certificate whose mass does not match the name on the label has told you something. Our guide to certificates of analysis covers lot numbers and the signs of a recycled document, and the storage and reconstitution guide covers lyophilised material at −20 °C. Orders are dispatched from the Netherlands, so EU deliveries stay inside the customs union: usually 1–2 business days within the Netherlands and 3–5 business days elsewhere in the EU, while parcels to the United Kingdom, Switzerland or Norway can attract import duties. Packaging is neutral and tracked, and buyers confirm at checkout that they order for research and are at least 18.
Frequently asked questions about tirzepatide
Is tirzepatide simply a stronger GLP-1 agonist? No. At the GLP-1 receptor it is a weaker agonist than semaglutide. The extra effect comes from the second receptor, not from a bigger push on the first.
Does the GIP arm cause the side effects or reduce them? The evidence points towards reducing them. Central GIP receptor signalling appears to blunt nausea in animal models, which may be why a compound with more total receptor activity is not proportionally harder to tolerate.
How does it compare with semaglutide and retatrutide? SURMOUNT-5 puts it ahead of semaglutide on weight at 72 weeks. Against retatrutide, which adds a third receptor, no direct comparison exists. Our side-by-side comparison sets out what each trial actually measured.
Is research-grade tirzepatide the same as Mounjaro? The peptide may be, the product is not. Mounjaro is an authorised medicine with a dossier, a patient leaflet and pharmacy supply. A research vial has none of these.
Research use only. Tirzepatide is described here for laboratory and scientific research. Research material carries no marketing authorisation, whatever the status of the molecule as a medicine elsewhere, and it is not intended for human or veterinary use. Nothing on this page is medical advice, and the dosing information above summarises published study designs. Always comply with the laws that apply in your country.