Tirzepatide Guide: The Dual GIP/GLP-1 Agonist Behind Mounjaro and Zepbound

Tirzepatide is a first-in-class dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist developed by Eli Lilly. It is a single synthetic peptide molecule that simultaneously activates receptors for two incretin hormones – a mechanism no other approved drug replicates. It is sold under two brand names: Mounjaro (for type 2 diabetes) and Zepbound (for chronic weight management and obstructive sleep apnoea).

Tirzepatide was first approved by the FDA on May 13, 2022 under the Mounjaro brand for type 2 diabetes. Zepbound followed in November 2023 for weight management, and then received an additional approval in December 2024 for obstructive sleep apnoea in adults with obesity – making it the first medication ever approved specifically for OSA. Available in doses of 2.5 mg to 15 mg administered once weekly by subcutaneous injection, tirzepatide represents a new class of medicine that Eli Lilly has termed a “twincretin.”

The bottom line: Tirzepatide is the first and only dual GIP/GLP-1 receptor agonist approved by the FDA, with indications spanning type 2 diabetes, chronic weight management, and obstructive sleep apnoea.

Tirzepatide activates both the GIP receptor and the GLP-1 receptor, but it does not treat them equally. Research published in JCI Insight in 2020 established that tirzepatide is an “imbalanced and biased” dual agonist – it engages the GIP receptor more strongly than the GLP-1 receptor, and it signals differently through the GLP-1 receptor compared to native GLP-1.

GLP-1 Receptor Pathway

Like semaglutide, tirzepatide activates GLP-1 receptors to stimulate glucose-dependent insulin secretion, suppress glucagon, delay gastric emptying, and reduce appetite through central nervous system signalling. However, tirzepatide shows a distinct signalling profile at this receptor: it favours cAMP generation over beta-arrestin recruitment. This biased agonism means tirzepatide causes less GLP-1 receptor internalisation (the process where the receptor is pulled inside the cell and temporarily deactivated). Research in primary pancreatic islets found that beta-arrestin-1 limits the insulin response to native GLP-1 but does not limit the response to tirzepatide, suggesting this bias may enhance insulin secretion.

GIP Receptor Pathway

The GIP receptor component is what distinguishes tirzepatide from all other approved GLP-1 medications. At the GIP receptor, tirzepatide mimics the actions of native GIP, which include improving insulin sensitivity and influencing fat metabolism. In preclinical models, GIP signalling decreases food intake and increases energy expenditure. When combined with GLP-1 receptor activation, GIP appears to amplify the appetite-suppressing and metabolic effects beyond what GLP-1 agonism achieves alone. Mechanistic analyses from Phase 2b data confirmed that tirzepatide increased multiple insulin sensitivity biomarkers compared to the selective GLP-1 agonist dulaglutide, consistent with a unique insulin-sensitising action arising from dual receptor activation.

The bottom line: Tirzepatide’s dual GIP/GLP-1 mechanism with biased signalling at the GLP-1 receptor creates a pharmacological profile distinct from any single-target GLP-1 agonist, accessing complementary metabolic pathways that may account for its superior clinical outcomes.

Tirzepatide’s clinical evidence base spans multiple large Phase 3 trial programmes across different indications. All evidence below is from completed human clinical trials.

SURPASS Programme (Type 2 Diabetes)

The SURPASS programme established tirzepatide as the most effective injectable for glycaemic control. In the pivotal SURPASS-2 trial (n=1,879), tirzepatide was directly compared head-to-head against semaglutide 1 mg in adults with type 2 diabetes on metformin. All three tirzepatide doses achieved superior A1C and body weight reductions compared to semaglutide over 40 weeks. The highest tirzepatide dose (15 mg) reduced A1C by 2.46% and body weight by 12.4 kg (13.1%), versus semaglutide’s 1.86% A1C reduction and 6.2 kg (6.7%) weight loss. Additionally, 51% of patients on tirzepatide 15 mg achieved an A1C below 5.7% – the non-diabetic threshold (Phase 3 human RCT, published in NEJM).

SURMOUNT Programme (Weight Management)

The SURMOUNT programme produced the strongest weight loss results ever seen from a non-surgical intervention. In SURMOUNT-1 (n=2,539), adults with obesity or overweight without diabetes achieved mean weight reductions of 16.0% (5 mg), 21.4% (10 mg), and 22.5% (15 mg) at 72 weeks, compared to 2.4% with placebo. At the highest dose, participants lost an average of 52 pounds (24 kg). 96% of those on the 10 mg and 15 mg doses achieved at least 5% weight loss (Phase 3 human RCT, published in NEJM).

Long-term data from the 176-week SURMOUNT-1 extension in adults with prediabetes confirmed weight loss was sustained over 3.4 years: 15.4%, 19.9%, and 22.9% reductions for the 5, 10, and 15 mg doses respectively. At 176 weeks, 63% of patients on the highest dose had lost at least 20% of their body weight.

The SURMOUNT-5 trial provided the first head-to-head obesity comparison against semaglutide 2.4 mg. Tirzepatide achieved 20.2% mean weight loss at 72 weeks versus 13.7% for semaglutide – a 47% relative improvement. 31.6% of tirzepatide patients lost at least 25% of body weight compared to 16.1% on semaglutide (Phase 3 human RCT, results reported December 2024).

SUMMIT Trial (Heart Failure)

The SUMMIT trial (n=731) was the first to test whether any medication could improve major heart failure outcomes in patients with heart failure with preserved ejection fraction (HFpEF) and obesity. Presented at AHA 2024 and published in the NEJM, tirzepatide reduced the composite of cardiovascular death or worsening heart failure events by 38% (HR 0.62, 95% CI 0.41-0.95) over a median follow-up of 2 years. A secondary imaging analysis showed tirzepatide decreased left ventricular mass by 11 grams and paracardiac adipose tissue by 45 ml – structural changes that may explain the clinical benefit (Phase 3 human RCT, published in NEJM). Lead investigator Milton Packer (Baylor University Medical Center) described HFpEF as “the most common phenotype of heart failure” and obesity as “the driver of this phenotype.”

SURMOUNT-OSA (Obstructive Sleep Apnoea)

Tirzepatide became the first medication ever approved to treat obstructive sleep apnoea when the FDA granted approval in December 2024 under the Zepbound brand. The SURMOUNT-OSA trial demonstrated significant reductions in the Apnoea-Hypopnoea Index alongside body weight reductions of 12-21% in patients with moderate to severe OSA and obesity (Phase 3 human RCT).

Trial Programme	Indication	Key Result	Evidence Level
SURPASS-2 (n=1,879)	T2D vs semaglutide	Superior A1C (-2.46%) and weight (-13.1%) vs semaglutide	Phase 3 RCT (NEJM)
SURMOUNT-1 (n=2,539)	Obesity/overweight	22.5% mean weight loss at 72 weeks (15 mg)	Phase 3 RCT (NEJM)
SURMOUNT-5 (head-to-head)	Weight loss vs semaglutide 2.4 mg	20.2% vs 13.7% (47% more relative weight loss)	Phase 3 RCT (2024)
SURMOUNT-1 extension (176 weeks)	Long-term durability	22.9% sustained at 3.4 years (15 mg, prediabetes)	Phase 3 extension data
SUMMIT (n=731)	Heart failure (HFpEF)	38% reduction in CV death/worsening HF events	Phase 3 RCT (NEJM)
SURMOUNT-OSA	Obstructive sleep apnoea	Significant AHI reduction; first-ever OSA drug approval	Phase 3 RCT
SURMOUNT-3 (n=579)	Post-lifestyle intervention	Additional -18.4% weight loss after initial -5% from lifestyle	Phase 3 RCT (Nature Medicine)

The bottom line: Tirzepatide has the strongest weight loss data of any approved medication, demonstrating superiority over semaglutide in head-to-head trials for both diabetes and obesity, with additional first-in-class evidence for heart failure and obstructive sleep apnoea.

Tirzepatide is fully approved in the US, UK, EU, and Australia. Like semaglutide, the compounding pathway is being shut down in the US following shortage resolution.

Country	Regulator	Branded Status	Compounded Status
United States	FDA	Approved (Mounjaro, Zepbound)	503B proposed exclusion (Apr 2026); shortage resolved Dec 2024
United Kingdom	MHRA	Approved (Mounjaro, Sep 2022); NICE TA924 for weight management (Nov 2024)	N/A
Australia	TGA	Approved (ARTG-listed, Mounjaro for T2D and weight management)	Schedule 4; NOT PBS-listed (private Rx ~$350-400/month)
EU	EMA	Approved (Mounjaro)	N/A

US Compounding Status

Tirzepatide was added to the FDA drug shortage list in 2022 alongside semaglutide. The tirzepatide shortage was resolved in December 2024, triggering a 90-day wind-down deadline (to March 19, 2025) for 503B outsourcing facilities. On April 30, 2026, the FDA proposed excluding tirzepatide from the 503B Bulks List entirely, alongside semaglutide and liraglutide. The FDA received more than 320 adverse event reports linked to compounded tirzepatide as of early 2025, many involving dosing errors. Eli Lilly, like Novo Nordisk, is actively pursuing litigation against compounders and telehealth distributors. Public comment on the proposed exclusion closes June 29, 2026.

Branded list pricing for Zepbound through Eli Lilly’s direct-to-consumer programme is approximately $1,086/month for lower doses, while compounded versions were running $200-400/month during the shortage period. The cost differential has been the primary driver of the compounding market and the intense legal battles now surrounding it.

For how the US compounding framework works, see our Category 1 vs Category 2 explainer. For the broader US regulatory landscape, see our guide to US peptide legality.

UK Access

Mounjaro received MHRA approval in September 2022, approximately 4-5 months after the FDA approval. NICE approved tirzepatide for type 2 diabetes in adults with BMI of 35 or higher, establishing it within the NHS pathway. In November 2024, NICE issued technology appraisal TA924 approving tirzepatide specifically for obesity management on the NHS, subject to eligibility criteria similar to those applied to Wegovy. Private prescribing through online services has been available in the UK alongside the NHS pathway, though both have been subject to supply constraints. For more on UK peptide regulation, see our UK peptide legality guide.

Australia Access

Mounjaro is TGA-approved and ARTG-listed in Australia for both type 2 diabetes and weight management. However, unlike semaglutide (Ozempic), tirzepatide is not listed on the Pharmaceutical Benefits Scheme (PBS), meaning patients must pay full private prescription costs of approximately $350-400 AUD per month. A PBAC re-entry submission was considered in November 2024 but a previous application had been rejected on cost-effectiveness grounds, and the timeline for PBS listing remains uncertain. For more on Australian peptide law, see our Australian peptide law guide.

The bottom line: Branded tirzepatide is approved in all major markets, but compounded access in the US is being permanently closed, and Australian PBS subsidisation remains uncertain – creating significant affordability barriers for many patients.

Tirzepatide’s safety profile is characterised across the SURPASS, SURMOUNT, and SUMMIT trial programmes. Its adverse event pattern is similar to the GLP-1 receptor agonist class overall, with gastrointestinal effects predominating.

Common Side Effects (Clinical Trial Data)

The most commonly reported adverse events across the SURMOUNT and SURPASS programmes were gastrointestinal: nausea (17-22% depending on dose), diarrhoea (11-16%), vomiting, and constipation. These were generally mild to moderate in severity and most pronounced during the dose-escalation phase. Adverse events led to treatment discontinuation in 4.3-7.1% of participants in SURMOUNT-1. In the SURPASS-2 head-to-head, gastrointestinal adverse event rates were broadly comparable between tirzepatide and semaglutide across all doses.

Serious Safety Concerns

• Thyroid C-cell tumours (boxed warning): In a 2-year carcinogenicity study, tirzepatide caused dose-dependent thyroid C-cell tumours in rats at clinically relevant exposures. Whether this risk applies to humans is unknown. Tirzepatide is contraindicated in patients with a personal or family history of medullary thyroid carcinoma or MEN 2.
• Acute pancreatitis: In pooled SURPASS data, 14 events were confirmed in 13 tirzepatide-treated patients (0.23 per 100 patient-years) versus 3 events in comparator-treated patients (0.11 per 100 patient-years). In the SURMOUNT weight management pool, confirmed pancreatitis occurred in 0.2% of Zepbound-treated patients.
• Gallbladder disease: Cholelithiasis and cholecystitis were reported in approximately 1.1% of tirzepatide users in clinical trials versus 0.5-0.9% with placebo. The difference was not statistically significant in pooled analyses and is likely related to rapid weight loss rather than a direct drug effect.
• Diabetic retinopathy: As with semaglutide, rapid glucose improvement may temporarily worsen existing diabetic retinopathy.
• Hypersensitivity: Injection site reactions (redness, pain, swelling) reported in approximately 3-5% of patients. Serious allergic reactions including anaphylaxis have been reported rarely.
• Pulmonary aspiration: Both the MHRA and TGA have issued safety updates warning of increased aspiration risk during general anaesthesia due to delayed gastric emptying – a class-wide concern for all GLP-1 and GIP/GLP-1 medications.

Importantly, tirzepatide has not shown liver toxicity in studies of more than 5,000 patients, and therapy is often associated with improvements in serum aminotransferase levels – likely as a result of weight loss and improved metabolic health.

The bottom line: Tirzepatide’s safety profile is similar to the broader GLP-1 class, with gastrointestinal effects as the primary concern and the same thyroid C-cell tumour boxed warning that applies to semaglutide; its GI side effect rates are broadly comparable to semaglutide at equivalent efficacy timepoints.

Tirzepatide sits in a rapidly evolving landscape of incretin-based therapies:

• Semaglutide – Novo Nordisk’s selective GLP-1 receptor agonist (Ozempic/Wegovy/Rybelsus). The most widely prescribed GLP-1 medication with the broadest indication set, though tirzepatide has demonstrated superior efficacy in head-to-head comparisons. Semaglutide has the advantage of an oral formulation.
• Retatrutide – Eli Lilly’s investigational triple agonist (GIP/GLP-1/glucagon). Phase 2 data showed up to 24.2% weight loss at 48 weeks, and Phase 3 TRIUMPH trials are ongoing. If approved, it would represent the next evolutionary step beyond tirzepatide’s dual mechanism. See our tirzepatide vs retatrutide comparison.
• Orforglipron – Eli Lilly’s investigational oral non-peptide GLP-1 agonist. Currently in Phase 3 trials, it could provide an oral alternative with potentially lower manufacturing costs and easier storage than injectable peptides.

Date	Brand	Indication
May 2022	Mounjaro	Type 2 diabetes in adults (NDA 215866)
Nov 2023	Zepbound	Chronic weight management in adults with obesity/overweight
Dec 2024	Zepbound	Obstructive sleep apnoea in adults with obesity (first-ever OSA drug)
Feb 2026	Mounjaro	Expanded to paediatric patients 10+ years with T2D