Mounjaro (tirzepatide): What It Is, Trial History and Regulatory Status

Evidence tier: Established (FDA-approved; multiple randomized controlled trials).

This page is for educational purposes only and is not medical advice; consult a licensed physician or your prescriber about whether any prescription medication is appropriate. Nothing here describes how a medication should be used, and every dose mentioned below is described only as it appeared in a published trial or on an approved label.

What Mounjaro Is

Mounjaro is a brand name used by Eli Lilly and Company for tirzepatide, a synthetic 39-amino-acid peptide administered as a once-weekly subcutaneous injection. Because tirzepatide is a peptide, it is sometimes described informally as a "Mounjaro peptide" — but it is a fully approved prescription medicine manufactured under pharmaceutical regulation, not an experimental or unapproved compound.

Tirzepatide belongs to a drug class usually called dual incretin receptor agonists. Incretins are gut hormones released after eating that influence insulin secretion, glucagon, gastric emptying and appetite. Older medicines in this space act on the glucagon-like peptide-1 (GLP-1) receptor alone. Tirzepatide engages two receptors: the glucose-dependent insulinotropic polypeptide (GIP) receptor and the GLP-1 receptor.

How the molecule works, as described in the literature

A 2020 pharmacology study in JCI Insight characterised tirzepatide as an imbalanced and biased dual agonist, reporting that it bound the GIP receptor with affinity similar to native GIP while binding the GLP-1 receptor more weakly than native GLP-1, and that it produced biased signalling at the GLP-1 receptor with reduced beta-arrestin recruitment (PMID 32730231). Researchers framed this receptor imbalance as a possible explanation for the metabolic effects seen in clinical studies.

Downstream physiology has also been studied directly. A phase 2 analysis reported that tirzepatide improved markers of beta-cell function and insulin sensitivity in people with type 2 diabetes over 26 weeks of treatment (PMID 33236115). A separate mechanistic study in Diabetes Care reported that once-weekly tirzepatide reduced appetite scores, energy intake and fat mass in adults with type 2 diabetes compared with placebo (PMID 36857477). Taken together, the study authors described a combination of improved insulin secretion, improved insulin sensitivity and reduced food intake.

Approved Indications and Dates

In the United States, Mounjaro is the tirzepatide brand indicated for type 2 diabetes, while Zepbound is the separately branded tirzepatide product for weight-related indications. In several other jurisdictions, including the European Union and the United Kingdom, a single Mounjaro brand carries both the diabetes and the weight-management indications. Which product and indication apply in a given country is a regulatory matter, and any question about eligibility belongs with a licensed prescriber.

Trial History: The Pivotal Studies as Published

Tirzepatide was developed through two large phase 3 programmes — SURPASS in type 2 diabetes and SURMOUNT in obesity — plus mechanistic and disease-specific trials. The summaries below reflect only what the cited publications reported.

SURPASS-1: monotherapy in type 2 diabetes

SURPASS-1 was a double-blind, randomised phase 3 trial in which adults with type 2 diabetes not taking other glucose-lowering drugs received once-weekly subcutaneous tirzepatide 5 mg, 10 mg or 15 mg, or placebo, for 40 weeks (PMID 34186022). The study reported HbA1c reductions from baseline of roughly 1.9 to 2.1 percentage points across the three tirzepatide doses compared with essentially no change on placebo, together with body-weight reductions of approximately 7 kg to 9.5 kg across doses (PMID 34186022). Researchers reported that the most frequent adverse events were mild-to-moderate gastrointestinal events, and that no clinically significant hypoglycaemia or severe hypoglycaemia occurred in the tirzepatide groups (PMID 34186022).

SURPASS-5: add-on to titrated insulin glargine

SURPASS-5 randomised adults with type 2 diabetes inadequately controlled on insulin glargine to once-weekly tirzepatide 5 mg, 10 mg or 15 mg or placebo for 40 weeks, with insulin titrated in all groups (PMID 35133415). The study reported HbA1c reductions of roughly 2.1 to 2.4 percentage points with tirzepatide versus approximately 0.9 percentage points with placebo, and body-weight reductions with tirzepatide alongside weight gain in the placebo group (PMID 35133415). Gastrointestinal adverse events were again the most commonly reported, and hypoglycaemia was documented in a setting where background insulin was used (PMID 35133415).

A published review of the SURPASS programme

A 2024 review in Drugs summarised the tirzepatide type 2 diabetes programme, describing once-weekly subcutaneous maintenance doses of 5 mg, 10 mg and 15 mg reached through gradual dose escalation, and reporting glycaemic and weight outcomes across the SURPASS trials along with predominantly gastrointestinal tolerability findings (PMID 38388874). Reviews of this kind are useful for orientation but are secondary literature; the individual trial reports remain the primary source.

SURMOUNT-4: continued treatment versus withdrawal

SURMOUNT-4 used a randomised-withdrawal design in adults with obesity or overweight. Participants first received open-label tirzepatide at a maximum tolerated dose of 10 mg or 15 mg once weekly for 36 weeks, after which the study reported a mean weight reduction of about 21% during that lead-in period (PMID 38078870). They were then randomised to continue tirzepatide or switch to placebo for a further 52 weeks; researchers reported an additional mean weight reduction of roughly 5.5% in those continuing tirzepatide versus a mean weight regain of about 14% in those switched to placebo (PMID 38078870). The most common adverse events during the trial were gastrointestinal and were mostly mild to moderate (PMID 38078870).

SURMOUNT-5: head-to-head with semaglutide

A 2025 New England Journal of Medicine trial compared tirzepatide with semaglutide in adults with obesity and without diabetes over 72 weeks, using maximum tolerated doses of each drug (PMID 40353578). The study reported a mean weight reduction of approximately 20% with tirzepatide compared with approximately 14% with semaglutide, and researchers reported that gastrointestinal adverse events were common in both groups and mostly mild to moderate (PMID 40353578). Head-to-head data of this kind describe average group differences, not what any individual would experience.

Metabolic dysfunction-associated steatohepatitis (MASH)

A 2024 phase 2 trial published in the New England Journal of Medicine randomised adults with biopsy-confirmed MASH and moderate or severe fibrosis to tirzepatide 5 mg, 10 mg or 15 mg once weekly or placebo for 52 weeks (PMID 38856224). The study reported resolution of steatohepatitis without worsening of fibrosis in roughly 44%, 56% and 62% of participants across the ascending tirzepatide doses compared with about 10% on placebo, with gastrointestinal events again the most frequently reported adverse events (PMID 38856224). MASH is not an approved indication for Mounjaro; this was an investigational use studied in a trial setting.

Paediatric type 2 diabetes

SURPASS-PEDS was a randomised, double-blind, placebo-controlled phase 3 trial of tirzepatide in children and adolescents with type 2 diabetes, published in The Lancet in 2025, in which researchers reported greater improvement in glycaemic control with tirzepatide than with placebo and a safety profile in which gastrointestinal events predominated (PMID 40975112). Whether paediatric labelling follows from such a trial is a regulatory decision made by agencies, not by the trial authors.

Cardiometabolic research beyond the label

Tirzepatide continues to be studied in populations outside its approved indications. A 2025 JAMA report examined semaglutide and tirzepatide among patients with heart failure with preserved ejection fraction, a condition in which incretin-based therapy has become a subject of active investigation (PMID 40886075). Findings in such analyses do not extend the approved indications of Mounjaro.

Adverse Events: What Studies Report

Across the published trials, the dominant pattern reported was gastrointestinal. SURPASS-1 researchers reported that mild-to-moderate gastrointestinal adverse events — including nausea, diarrhoea and vomiting — were the most common events, and that discontinuation due to adverse events occurred in a minority of participants (PMID 34186022). SURPASS-5 reported the same broad pattern when tirzepatide was added to background insulin glargine, in a setting where hypoglycaemia is also relevant because of the insulin itself (PMID 35133415).

In the obesity trials, SURMOUNT-4 reported gastrointestinal adverse events as the most frequent and mostly mild to moderate in severity across the lead-in and randomised phases (PMID 38078870), and the 2025 head-to-head trial reported common, mostly mild-to-moderate gastrointestinal events with both tirzepatide and semaglutide (PMID 40353578). In the MASH trial, researchers again reported gastrointestinal events as the most common adverse events over 52 weeks (PMID 38856224).

Rarer events appear in the case-report literature rather than in trials. A 2024 case report in ACG Case Reports Journal described colonic ischemia occurring in a patient receiving tirzepatide (PMID 39507503). A single case report cannot establish causation or frequency; it documents an observation that clinicians considered worth publishing.

Label warnings, stated factually

The US prescribing information for Mounjaro carries a boxed warning regarding thyroid C-cell tumours, based on findings in rodents, and states that the human relevance of those findings has not been determined. The label also contains warnings and precautions addressing pancreatitis, hypoglycaemia when used with insulin or insulin secretagogues, hypersensitivity reactions, acute kidney injury in the setting of dehydration from gastrointestinal adverse reactions, severe gastrointestinal disease, diabetic retinopathy complications in patients with a history of retinopathy, gallbladder disease, and pulmonary aspiration during general anaesthesia or deep sedation. These are label facts; interpreting them for an individual is the role of a prescriber.

Regulatory Status

Mounjaro received FDA approval in May 2022 as an adjunct to diet and exercise for glycaemic control in adults with type 2 diabetes. European and UK regulators authorised tirzepatide under the Mounjaro name later in 2022 for type 2 diabetes, with the label subsequently extended to weight management. In the United States, Lilly markets tirzepatide for weight-related indications under the separate Zepbound brand, approved in November 2023 for chronic weight management and extended in December 2024 to include moderate-to-severe obstructive sleep apnoea in adults with obesity.

Shortage and compounding

Tirzepatide appeared on the FDA drug shortage list during a period of rapid demand growth following approval. While a drug is listed as in shortage, US law permits certain compounding of copies that would otherwise be restricted. The FDA subsequently declared the tirzepatide shortage resolved and, after a transition period, that pathway for compounded tirzepatide copies closed. Compounded versions are not FDA-approved products, have not been evaluated for safety, effectiveness or quality by the agency, and are not the same as the branded product described in the trials above. Questions about the legal status of a specific product in a specific state are matters for a licensed pharmacist, prescriber or attorney; nothing on this page is legal advice.

How Mounjaro Differs From Sibling Products

Mounjaro and Zepbound contain the same active ingredient — tirzepatide — from the same manufacturer. The practical differences are regulatory and presentational rather than chemical:

• Indication. Mounjaro is the US brand approved for type 2 diabetes; Zepbound is the US brand approved for chronic weight management and for obstructive sleep apnoea in adults with obesity.
• Geography. Outside the US, a single Mounjaro brand may carry both diabetes and weight-management indications, so the brand name alone does not indicate the indication.
• Trial programmes. The evidence supporting the diabetes indication comes largely from the SURPASS trials, including SURPASS-1 (PMID 34186022) and SURPASS-5 (PMID 35133415), while the weight-management evidence comes from the SURMOUNT trials, including SURMOUNT-4 (PMID 38078870).
• Class comparison. Tirzepatide is a dual GIP/GLP-1 receptor agonist, whereas semaglutide products act at the GLP-1 receptor alone; the two were compared directly in a 72-week obesity trial (PMID 40353578).

What the Literature Does Not Settle

Several limits are worth noting. The randomised-withdrawal design of SURMOUNT-4 reported substantial weight regain after treatment was stopped, which researchers framed as evidence about the durability of effect rather than about a permanent change (PMID 38078870). The MASH results came from a phase 2 trial with histological endpoints over 52 weeks rather than from long-term outcome data (PMID 38856224). Trial populations are selected by inclusion criteria and do not represent every patient. Rare events such as the colonic ischemia case described in 2024 remain uncharacterised in terms of frequency (PMID 39507503).

Decisions Belong With a Prescriber

Mounjaro is a prescription-only medicine. Whether it is appropriate, which brand and indication apply, how it interacts with other medicines, and how it is monitored are all clinical decisions that belong to a patient and their prescribing clinician. This page summarises published literature and label facts so that those conversations can be better informed; it does not recommend, instruct or endorse any course of action.

PeptideU is not affiliated with or endorsed by Eli Lilly and Company. Mounjaro is a trademark of its owner. This page is for educational purposes only and is not medical advice; consult a licensed physician or your prescriber about any medication.

References

• Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist (JCI Insight, 2020)
• Dual GIP and GLP-1 Receptor Agonist Tirzepatide Improves Beta-cell Function and Insulin Sensitivity in Type 2 Diabetes (The Journal of Clinical Endocrinology and Metabolism, 2021)
• 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 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 (The New England Journal of Medicine, 2024)
• Tirzepatide-Associated Colonic Ischemia (ACG Case Reports Journal, 2024)
• Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (The New England Journal of Medicine, 2025)
• Semaglutide and Tirzepatide in Patients With Heart Failure With Preserved Ejection Fraction (JAMA, 2025)
• Efficacy and safety of tirzepatide in children and adolescents with type 2 diabetes (SURPASS-PEDS): a randomised, double-blind, placebo-controlled, phase 3 trial (Lancet, 2025)