What Is Tirzepatide? Definition, Mechanism and Regulatory Status

Definition

Tirzepatide is a synthetic 39–amino-acid peptide engineered from the backbone of native glucose-dependent insulinotropic polypeptide (GIP) and modified so that a single molecule activates both the GIP receptor and the glucagon-like peptide-1 (GLP-1) receptor. A fatty diacid side chain promotes albumin binding and extends the circulating half-life, which is the basis for once-weekly subcutaneous administration; a systematic review of the molecule described this structural design and its incretin pharmacology (PMID 36498958). Because it engages two receptors rather than one, tirzepatide is usually described in the literature as a dual agonist or twincretin, distinguishing it from single-target GLP-1 receptor agonists such as semaglutide and liraglutide.

This page is for educational purposes only and is not medical advice; consult a licensed physician about any medical question, medication or health condition. Nothing here is a protocol, and no product is offered or recommended.

Mechanism: What "Dual Agonist" Means

Incretins are gut hormones released after eating that amplify glucose-dependent insulin secretion. GLP-1 receptor activation has been linked in the literature to insulin secretion, glucagon suppression, slowed gastric emptying and central appetite signalling, while GIP receptor activation has been studied for its effects on insulin secretion and adipose tissue biology. Tirzepatide was designed to engage both pathways with one peptide.

Imbalanced and biased signalling

A preclinical pharmacology study characterised tirzepatide as an imbalanced and biased dual agonist: researchers reported that the peptide bound the GIP receptor with affinity comparable to native GIP but engaged the GLP-1 receptor with weaker relative affinity than native GLP-1, and that at the GLP-1 receptor it favoured cyclic AMP generation over β-arrestin recruitment, with correspondingly less receptor internalisation (PMID 32730231). That signalling profile is frequently cited as one hypothesis for why the dual agonist behaved differently from GLP-1-only agents in clinical comparisons, although the study itself was mechanistic and did not measure clinical outcomes.

Appetite and energy intake

A clinical sub-study in people with type 2 diabetes examined how the mechanism translated to eating behaviour. The study reported that tirzepatide reduced appetite scores, ad libitum energy intake and fat mass relative to placebo over the observation period (PMID 36857477). Investigators interpreted the reductions in body weight seen in larger trials as consistent with lower energy intake rather than with any single peripheral mechanism.

Approved Products and Regulatory Status

Tirzepatide is marketed by Eli Lilly under two brand names with separate indications:

• Mounjaro — approved by the US Food and Drug Administration in May 2022 as an adjunct to diet and exercise to improve glycaemic control in adults with type 2 diabetes, and subsequently authorised in the European Union and other jurisdictions. A drug-monograph overview summarised the approval, the once-weekly subcutaneous route and the stepwise titration schedule used in the programme (PMID 36751934).
• Zepbound — approved by the FDA in November 2023 for chronic weight management in adults with obesity, or with overweight plus at least one weight-related comorbid condition, alongside reduced-calorie diet and increased physical activity. A later FDA action extended the labelling to moderate-to-severe obstructive sleep apnoea in adults with obesity.

Both products contain the same active peptide; the brand distinction reflects indication and labelling rather than a different molecule. A comprehensive drug review covering the type 2 diabetes indication summarised the approved dose range, the once-weekly schedule and the safety information carried in labelling (PMID 38388874).

Separately, peptide material labelled "research use only" (RUO) is not an approved medicine, is not manufactured to pharmaceutical standards, and is not authorised for human administration. Compounded copies of tirzepatide were widely discussed during the period when the FDA listed the drug in shortage; after the agency declared the shortage resolved, the regulatory latitude for large-scale compounding of copies narrowed. This paragraph describes regulatory context only and is not legal advice.

Trial Programme: What the Studies Reported

SURPASS (type 2 diabetes)

The phase 3 diabetes programme was named SURPASS. In SURPASS-1, a double-blind randomised trial of tirzepatide monotherapy over 40 weeks, participants received 5 mg, 10 mg or 15 mg once weekly or placebo; researchers reported HbA1c reductions of roughly 1.9 to 2.1 percentage points across the tirzepatide groups compared with essentially no change on placebo, together with reductions in body weight of approximately 7 to 9.5 kg (PMID 34186022). The most frequently reported adverse events in that trial were gastrointestinal and were described as mostly mild to moderate.

SURPASS-5 tested the same three maintenance doses as an add-on to titrated insulin glargine over 40 weeks. The study reported HbA1c reductions of about 2.1 to 2.4 percentage points with tirzepatide versus roughly 0.9 percentage points with placebo added to insulin, alongside reductions in body weight in the tirzepatide arms (PMID 35133415). Other trials in the series compared tirzepatide with insulin degludec, insulin glargine, semaglutide 1 mg and placebo in different background-therapy settings; the Drugs review synthesised these outcomes (PMID 38388874).

SURMOUNT (obesity and weight maintenance)

The obesity programme was named SURMOUNT. SURMOUNT-4 examined what happened when treatment was continued or withdrawn: after a 36-week open-label lead-in on the maximum tolerated dose of 10 mg or 15 mg weekly, during which researchers reported a mean weight reduction of about 20.9%, participants were randomised to continue tirzepatide or switch to placebo for a further 52 weeks. The study reported an additional mean reduction of roughly 5.5% in those continuing treatment, compared with a mean regain of about 14% in the placebo group (PMID 38078870). That design is often cited as evidence that the observed effect depended on continued administration.

Head-to-head with semaglutide

SURMOUNT-5 was an open-label randomised trial in adults with obesity and without diabetes, comparing maximum tolerated tirzepatide (10 mg or 15 mg weekly) with maximum tolerated semaglutide (1.7 mg or 2.4 mg weekly) over 72 weeks. The study reported a mean weight reduction of approximately 20.2% with tirzepatide versus approximately 13.7% with semaglutide, with gastrointestinal adverse events common in both groups and mostly mild to moderate (PMID 40353578).

In people who had both obesity or overweight and type 2 diabetes, no direct head-to-head trial was available, so investigators conducted an indirect treatment comparison anchored on placebo across separate trials. Researchers reported greater weight reduction with tirzepatide 10 mg and 15 mg than with semaglutide 2.4 mg in that indirect analysis, with broadly comparable gastrointestinal tolerability signals (PMID 40537987). Indirect comparisons carry more uncertainty than randomised head-to-head designs, a limitation the authors acknowledged.

Metabolic dysfunction-associated steatohepatitis (MASH)

A phase 2, 52-week randomised trial evaluated tirzepatide 5 mg, 10 mg and 15 mg once weekly against placebo in adults with biopsy-confirmed metabolic dysfunction-associated steatohepatitis and moderate or severe fibrosis. Researchers reported that resolution of steatohepatitis without worsening of fibrosis occurred in a higher percentage of participants in each tirzepatide group than in the placebo group, and that the most common adverse events were gastrointestinal (PMID 38856224). The trial was explicitly described as phase 2 and hypothesis-generating; tirzepatide was not approved for MASH on the basis of that study.

How Tirzepatide Differs From Semaglutide

The clearest distinction is receptor coverage. The table below summarises differences as described in the cited literature.

Both molecules are prescription medicines in the jurisdictions where they are authorised, and both were studied in large randomised programmes with placebo or active comparators. Greater mean effect sizes in one trial do not imply suitability for any individual, and the trials differed in eligibility criteria, background therapy and duration.

Adverse Events: What Studies Report

Across the published programme, gastrointestinal events dominated the adverse-event tables. In SURPASS-1, researchers reported nausea, diarrhoea and vomiting as the most common events, generally mild to moderate in severity and most frequent during dose escalation (PMID 34186022). SURPASS-5, in which tirzepatide was added to insulin glargine, similarly reported gastrointestinal events as the most common adverse events and documented hypoglycaemia rates in the context of concomitant insulin therapy (PMID 35133415).

In the obesity setting, the head-to-head trial against semaglutide reported that gastrointestinal adverse events were common with both agents and mostly mild to moderate (PMID 40353578). The phase 2 MASH trial also identified gastrointestinal events as the most frequently reported adverse events (PMID 38856224). Regulatory labelling for approved tirzepatide products carries additional warnings, including a boxed warning regarding thyroid C-cell tumours observed in rodents, as summarised in the drug monograph literature (PMID 36751934). Safety information reported in trials reflects supervised clinical settings with defined titration, monitoring and eligibility criteria.

Limitations of the Evidence

• Most large trials ran 40 to 72 weeks; long-term outcome data accumulate more slowly than short-term metabolic endpoints.
• Trial populations were selected by eligibility criteria and do not represent all people with diabetes or obesity.
• Some comparisons between tirzepatide and semaglutide were indirect, anchored across separate trials rather than randomised head-to-head (PMID 40537987).
• The MASH evidence was phase 2 and biopsy-based over 52 weeks, not a regulatory approval dataset (PMID 38856224).
• Mechanistic receptor studies were conducted in cell systems and cannot be read directly as clinical predictions (PMID 32730231).

References

• Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist (JCI Insight, 2020)
• Efficacy and safety of a novel dual GIP and GLP-1 receptor agonist tirzepatide in patients with type 2 diabetes (SURPASS-1): a double-blind, randomised, phase 3 trial (Lancet, 2021)
• Effect of Subcutaneous Tirzepatide vs Placebo Added to Titrated Insulin Glargine on Glycemic Control in Patients With Type 2 Diabetes: The SURPASS-5 Randomized Clinical Trial (JAMA, 2022)
• Tirzepatide: A Systematic Update (International Journal of Molecular Sciences, 2022)
• New Drug: Tirzepatide (Mounjaro) (The Senior Care Pharmacist, 2023)
• Tirzepatide Reduces Appetite, Energy Intake, and Fat Mass in People With Type 2 Diabetes (Diabetes Care, 2023)
• Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial (JAMA, 2024)
• Tirzepatide: A Review in Type 2 Diabetes (Drugs, 2024)
• Tirzepatide for Metabolic Dysfunction-Associated Steatohepatitis with Liver Fibrosis (New England Journal of Medicine, 2024)
• Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (New England Journal of Medicine, 2025)
• Indirect comparative efficacy and safety of tirzepatide 10 and 15 mg versus semaglutide 2.4 mg for the management of obesity and overweight in patients with type 2 diabetes (Diabetes, Obesity & Metabolism, 2025)