Zepbound (Tirzepatide): What It Is, Trial History and Regulatory Status

Zepbound is a brand name for tirzepatide, a once-weekly injectable peptide marketed by Eli Lilly and Company. Tirzepatide belongs to a class described in the literature as dual incretin receptor agonists: single molecules that act at both the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor. This page summarises what peer-reviewed studies and the prescribing information state about the molecule, its trial history and its regulatory position. It does not offer instructions, protocols or recommendations of any kind.

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

What Zepbound Is

Zepbound contains tirzepatide as its active ingredient, supplied as a subcutaneous injection administered once weekly. The molecule is a 39-amino-acid synthetic peptide based on the GIP sequence, modified with a fatty acid side chain that extends its half-life to support weekly administration, as described in a systematic update on tirzepatide (PMID 36498958). Because the active ingredient is the same as that in Eli Lilly's diabetes product, the two brands share a pharmacology but differ in the indications for which each was approved.

Mechanism as described in the literature

Pharmacology work published in 2020 characterised tirzepatide as an imbalanced and biased dual agonist: researchers reported that its affinity at the GIP receptor resembled that of native GIP while its activity at the GLP-1 receptor was weaker than that of native GLP-1, with signalling biased toward cyclic AMP generation over β-arrestin recruitment (PMID 32730231). A clinical pharmacology analysis in people with type 2 diabetes reported improvements in markers of beta-cell function and insulin sensitivity with tirzepatide treatment (PMID 33236115). Reviews have summarised the downstream effects described across the programme as enhanced glucose-dependent insulin secretion, reduced glucagon at elevated glucose, slowed gastric emptying and reduced energy intake (PMID 38388874).

Approved indications and approval years

Zepbound is not approved as a treatment for type 2 diabetes; that indication belongs to the separately branded tirzepatide product, which a drug-information summary introduced as a new dual GIP/GLP-1 receptor agonist for type 2 diabetes (PMID 36751934). Whether any approved indication applies to a particular individual is a clinical judgement for a licensed prescriber, not something a summary page can determine.

Trial History as Published

Tirzepatide was studied in two large phase 3 programmes — SURPASS in type 2 diabetes and SURMOUNT in obesity — plus phase 2 work in other cardiometabolic conditions. Dose levels described below are the regimens investigators used in those trials or those listed in the label; they are reported here as study facts, not as guidance.

Weight-reduction trials

The SURMOUNT-4 randomized clinical trial, published in JAMA in 2024, enrolled adults with obesity or overweight and used a 36-week open-label lead-in of once-weekly tirzepatide at the maximum tolerated dose of 10 mg or 15 mg before randomizing participants to continue tirzepatide or switch to placebo for 52 more weeks (PMID 38078870). Researchers reported a mean weight reduction of 20.9% during the open-label lead-in phase (PMID 38078870). From randomization to week 88, the study reported a mean additional change of −5.5% in participants who continued tirzepatide compared with +14.0% in those switched to placebo, a design intended to test maintenance of weight reduction rather than initial reduction (PMID 38078870).

A 2025 New England Journal of Medicine trial compared tirzepatide with semaglutide head-to-head in adults with obesity and without diabetes, using maximum tolerated doses of tirzepatide (10 mg or 15 mg weekly) and semaglutide (1.7 mg or 2.4 mg weekly) over 72 weeks (PMID 40353578). Researchers reported a greater mean percentage reduction in body weight with tirzepatide than with semaglutide at the end of the treatment period, with gastrointestinal adverse events common in both groups (PMID 40353578).

Type 2 diabetes trials (the tirzepatide molecule)

SURPASS-1, a double-blind phase 3 trial published in The Lancet in 2021, randomized adults with type 2 diabetes not taking glucose-lowering medication to once-weekly tirzepatide 5 mg, 10 mg or 15 mg or placebo for 40 weeks (PMID 34186022). The study reported HbA1c reductions of roughly 1.9 to 2.1 percentage points across the three tirzepatide groups versus little change with placebo, alongside body-weight reductions of approximately 7 to 9.5 kg (PMID 34186022).

SURPASS-5, published in JAMA in 2022, added once-weekly tirzepatide 5 mg, 10 mg or 15 mg or placebo to titrated insulin glargine for 40 weeks and reported greater reductions in HbA1c and in body weight with each tirzepatide dose than with placebo (PMID 35133415). An adjusted indirect treatment comparison published in 2024 compared tirzepatide 5, 10 and 15 mg with injectable semaglutide 0.5 mg in type 2 diabetes and reported larger estimated HbA1c and weight reductions for tirzepatide, while noting the limitations inherent to indirect comparisons (PMID 38777128). A 2024 review in Drugs synthesised the SURPASS programme and described tirzepatide as effective for glycemic and weight endpoints with a tolerability profile dominated by gastrointestinal events (PMID 38388874).

Investigational areas beyond the approved indications

A phase 2 trial published in the New England Journal of Medicine in 2024 randomized adults with metabolic dysfunction-associated steatohepatitis (MASH) and stage F2–F3 fibrosis to once-weekly tirzepatide 5 mg, 10 mg or 15 mg or placebo for 52 weeks, and researchers reported resolution of steatohepatitis without worsening of fibrosis more often in each tirzepatide group than in the placebo group (PMID 38856224). A 2025 JAMA analysis examined semaglutide and tirzepatide use among patients with heart failure with preserved ejection fraction, an area the authors framed as requiring further comparative study (PMID 40886075). Neither MASH nor heart failure is an approved Zepbound indication, and published phase 2 or observational findings do not establish approved uses.

Adverse Events: What Studies Report

Across the tirzepatide trials, the most frequently reported adverse events were gastrointestinal. In SURPASS-1, researchers reported that nausea, diarrhea and vomiting were the most common adverse events, described as mostly mild to moderate and occurring predominantly during dose escalation (PMID 34186022). SURPASS-5 similarly reported gastrointestinal events as the most common adverse events when tirzepatide was added to insulin glargine (PMID 35133415).

In the weight-management setting, SURMOUNT-4 reported that the most common adverse events with tirzepatide were gastrointestinal, mostly mild to moderate in severity (PMID 38078870), and the 2025 head-to-head trial against semaglutide reported gastrointestinal adverse events in both treatment groups (PMID 40353578). The phase 2 MASH trial also reported gastrointestinal events as the most common adverse events with tirzepatide (PMID 38856224). Reviews summarising the programme have described nausea, vomiting, diarrhea, constipation, decreased appetite and injection-site reactions among reported events, and have discussed gallbladder-related events, pancreatitis and hypoglycemia risk when tirzepatide is combined with insulin or insulin secretagogues (PMID 36498958, PMID 38388874).

Trial populations were selected by entry criteria and monitored under protocol, so reported event rates do not necessarily transfer to any individual. Only a prescriber with access to a person's history can weigh these reported risks.

Regulatory Status and Label Facts

Zepbound is a prescription drug approved by the US Food and Drug Administration and is a scheduled-free, non-controlled product dispensed by pharmacies. Tirzepatide has also been authorised by other regulators, including the European Medicines Agency, under separate brand names and indication wording.

Boxed warning and label sections

The US prescribing information for Zepbound carries a boxed warning regarding the risk of thyroid C-cell tumors, based on findings in rodents with GLP-1 receptor agonists; the label states that the human relevance of those findings has not been determined and contraindicates use in people with a personal or family history of medullary thyroid carcinoma or with multiple endocrine neoplasia syndrome type 2. The label's warnings and precautions sections address severe gastrointestinal disease, acute pancreatitis, acute gallbladder disease, hypoglycemia when used with insulin or insulin secretagogues, acute kidney injury in the setting of volume depletion, hypersensitivity reactions, diabetic retinopathy complications in people with type 2 diabetes, heart-rate increase, suicidal behaviour and ideation monitoring, and the risk of pulmonary aspiration during general anesthesia or deep sedation. The label also specifies that the product is not indicated for use in combination with other tirzepatide-containing products or other GLP-1 receptor agonists.

Label dosing is structured as a weekly subcutaneous injection with stepwise escalation intended to improve gastrointestinal tolerability, a strategy the trials themselves used when they titrated participants to maintenance doses of 5, 10 or 15 mg weekly (PMID 34186022, PMID 35133415). Any decision about initiation, escalation, continuation or discontinuation belongs to the reader and their prescriber.

Shortage and compounding context

Under US law, licensed pharmacies and outsourcing facilities may compound a drug that appears on the FDA drug shortage list or is not commercially available, subject to the conditions of sections 503A and 503B of the Federal Food, Drug, and Cosmetic Act. Tirzepatide appeared on FDA's shortage list during periods of constrained supply after launch, and FDA subsequently announced that the shortage was resolved, which removed that statutory basis for large-scale compounding of tirzepatide copies. Compounded preparations are not FDA-approved products and are not reviewed for safety, effectiveness or manufacturing quality in the way an approved product is; none of the trials summarised on this page studied compounded material. This section describes publicly stated regulatory positions and is not legal advice.

How Zepbound Differs from Other Tirzepatide Products

Zepbound and Mounjaro contain the same active ingredient from the same manufacturer but are separate approved products with separate labels. The practical distinctions described in regulatory and drug-information sources are:

• Indication: Zepbound is approved for chronic weight management and for moderate-to-severe obstructive sleep apnea in adults with obesity; the sibling product is approved for glycemic control in type 2 diabetes (PMID 36751934).
• Trial programme cited on each label: the diabetes label rests on the SURPASS trials, such as monotherapy and insulin-add-on studies (PMID 34186022, PMID 35133415), while the weight-management label rests on the SURMOUNT programme, including the maintenance trial SURMOUNT-4 (PMID 38078870).
• Molecule: the pharmacology is identical, as the dual GIP/GLP-1 receptor profile characterised in preclinical work applies to the single tirzepatide molecule regardless of brand (PMID 32730231).
• Presentation and coverage: the two brands are supplied in different devices and package configurations and are handled differently by insurers and formularies; these are commercial and administrative differences, not pharmacological ones.

Tirzepatide is also distinct from semaglutide-based products, which act at the GLP-1 receptor only; the 2025 head-to-head obesity trial and a 2024 indirect comparison in type 2 diabetes were designed specifically to quantify differences between the two molecules (PMID 40353578, PMID 38777128).

What the Published Evidence Does and Does Not Address

The tirzepatide literature is unusually large for a recently approved peptide, with randomized, placebo-controlled and active-comparator trials reporting weight and glycemic endpoints (PMID 38078870, PMID 34186022). Several limitations were nonetheless described by the authors themselves: SURMOUNT-4 measured maintenance after an open-label lead-in rather than blinded initiation (PMID 38078870); the 2025 comparative obesity trial was open-label (PMID 40353578); the MASH trial was phase 2 and used histological surrogate endpoints over 52 weeks (PMID 38856224); and the cross-molecule comparison in diabetes was indirect rather than randomized (PMID 38777128). Reviews have noted that long-term outcome data continue to accumulate (PMID 38388874).

Consulting a Prescriber

Zepbound is available only by prescription, and its label includes contraindications, a boxed warning and drug-interaction considerations that require individual assessment. Questions about eligibility, monitoring, interactions with other medicines, pregnancy, surgical or anesthesia planning, and what to do about any symptom should be directed to a licensed clinician who can review the full medical history. This page is for educational purposes only and is not medical advice; consult a licensed physician about any medication or medical condition.

PeptideU is not affiliated with or endorsed by Eli Lilly and Company. Zepbound 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

• Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial (JAMA, 2024)
• 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)
• Tirzepatide for Metabolic Dysfunction-Associated Steatohepatitis with Liver Fibrosis (New England Journal of Medicine, 2024)
• Dual GIP and GLP-1 Receptor Agonist Tirzepatide Improves Beta-cell Function and Insulin Sensitivity in Type 2 Diabetes (Journal of Clinical Endocrinology and Metabolism, 2021)
• Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (New England Journal of Medicine, 2025)
• Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist (JCI Insight, 2020)
• Tirzepatide: A Systematic Update (International Journal of Molecular Sciences, 2022)
• Tirzepatide 5, 10 and 15 mg versus injectable semaglutide 0.5 mg for the treatment of type 2 diabetes: An adjusted indirect treatment comparison (Diabetes Research and Clinical Practice, 2024)
• New Drug: Tirzepatide (Mounjaro) (The Senior Care Pharmacist, 2023)
• 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)
• Semaglutide and Tirzepatide in Patients With Heart Failure With Preserved Ejection Fraction (JAMA, 2025)
• Tirzepatide: A Review in Type 2 Diabetes (Drugs, 2024)