Tirzepatide (Mounjaro) Ireland: Dual Agonist Mechanism, SURMOUNT Trial Data & Comparison to Ozempic

What Is Tirzepatide?

Tirzepatide is a once-weekly injectable medicine developed by Eli Lilly. It belongs to a class sometimes called dual incretin agonists because it activates two hormone receptors simultaneously: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). Most people in Ireland will have encountered it under the brand name Mounjaro, which received European Medicines Agency (EMA) approval for the treatment of type 2 diabetes in adults.

While Semaglutide (Ozempic, Wegovy) acts on only the GLP-1 receptor, Tirzepatide's addition of GIP agonism creates a different and, according to clinical trial data, more potent metabolic signal. The SURMOUNT and SURPASS trial programmes are the largest evidence base for understanding what that difference looks like in practice.

The Dual Agonist Mechanism: GLP-1 + GIP

Both GLP-1 and GIP are incretin hormones — they are released from the gut after eating and signal to the pancreas to release insulin in a glucose-dependent manner. Beyond insulin secretion, each receptor has distinct downstream effects that are relevant to body weight and metabolic health.

The synergistic relationship between GLP-1 and GIP agonism is not simply additive. Research suggests that GIP receptor activation potentiates the GLP-1 signal, meaning the combined effect is greater than either receptor pathway acting alone. This is the mechanistic basis for the superior weight loss figures seen in Tirzepatide trials versus GLP-1 monotherapy.

Additionally, GIP receptors are highly expressed in adipose (fat) tissue. Activation at these receptors is believed to influence how fat cells store and mobilise lipids, contributing to the body composition changes observed beyond what appetite suppression alone would predict.

Clinical Trial Data: The SURMOUNT Programme

The SURMOUNT programme is the Phase 3 obesity trial series for Tirzepatide. Participants were adults with obesity (BMI ≥30, or ≥27 with at least one weight-related comorbidity) and all trials used weekly subcutaneous injection with standard dose escalation to the target dose. The two most clinically significant results are SURMOUNT-1 and SURMOUNT-2.

SURMOUNT-1 (Non-Diabetic Obese Population)

SURMOUNT-1 (Jastreboff et al., NEJM 2022) enrolled 2,539 adults without type 2 diabetes over 72 weeks. This is the headline trial — it examined what Tirzepatide does in an obesity context without the confounding variable of diabetes management.

The 22.5% mean body weight reduction at the 15 mg dose over 72 weeks was, at the time of publication, the highest ever reported in a Phase 3 randomised controlled trial for a weight-loss medicine. For context, Semaglutide 2.4 mg (Wegovy) produced approximately 14.9% reduction in the STEP-1 trial over 68 weeks in a comparable population. The magnitude of the difference is clinically meaningful.

SURMOUNT-2 (Type 2 Diabetes Population)

SURMOUNT-2 (Garvey et al., The Lancet 2023) ran the same 72-week protocol but in adults who did have type 2 diabetes. The presence of diabetes blunts the absolute weight loss potential for GLP-1 class agents. Even so, results were substantially above what had been seen for earlier agents in this population.

The 15.7% reduction in SURMOUNT-2 compares favourably to Semaglutide data in the same T2D population. STEP-2 (Semaglutide 2.4 mg in T2D) produced approximately 9.6% weight loss at 68 weeks — a substantial gap that aligns with the mechanistic advantages of the dual agonist approach.

The SURPASS Programme: Tirzepatide vs Semaglutide Head-to-Head

The SURPASS programme was Tirzepatide's Phase 3 diabetes trial series. SURPASS-2 is the most directly relevant to the Tirzepatide versus Semaglutide question because it was a head-to-head randomised controlled trial — the two compounds were compared directly in the same study population rather than across separate trials.

SURPASS-2 (Frías et al., NEJM 2021) enrolled 1,879 adults with type 2 diabetes inadequately controlled on Metformin. Participants were randomised to Tirzepatide 5 mg, 10 mg, or 15 mg versus Semaglutide 1 mg (the standard T2D dose of Ozempic), all once weekly for 40 weeks.

All three doses of Tirzepatide were statistically superior to Semaglutide 1 mg for both glycaemic control (HbA1c) and body weight reduction. It is worth noting that the comparator here was Semaglutide 1 mg (the T2D dose), not 2.4 mg (the Wegovy obesity dose). A direct head-to-head at the maximum doses of both drugs in an obesity population does not yet exist as a published Phase 3 trial, though SURPASS-2 data combined with STEP and SURMOUNT data provides a strong inference of Tirzepatide's advantage.

Tirzepatide vs Semaglutide vs Retatrutide: Where They Stand

Irish adults researching weight management options are increasingly encountering all three of these agents. The table below provides a cross-trial comparison. Direct head-to-head data exists only for Tirzepatide vs Semaglutide (SURPASS-2, T2D population). Retatrutide data comes from Phase 2 only and is not yet directly comparable at the same scale or duration.

Side Effects: What the Trial Data Shows

The side effect profile of Tirzepatide is consistent with the broader GLP-1 drug class. The most commonly reported adverse events in SURMOUNT and SURPASS trials were gastrointestinal. These were predominantly mild to moderate in severity and most common during the dose escalation phase as the body adjusts to increasing drug concentrations.

Most Frequently Reported (SURMOUNT-1 and SURMOUNT-2)

• Nausea — most common, typically peaks during escalation and subsides with stable dosing
• Diarrhoea — second most common; similar frequency pattern to nausea
• Vomiting — less common than nausea; more prevalent at higher doses
• Decreased appetite — expected pharmacological effect; reported as an adverse event in some participants
• Constipation — reported less frequently; linked to slowed gastric motility

GIP and Nausea: What the Evidence Suggests

One of the hypotheses around Tirzepatide's GIP component is that it may moderate GLP-1-associated nausea. In preclinical models, GIP receptor activation in the gut appears to counteract some of the emetic effects of GLP-1 signalling. Some analyses of SURPASS-2 trial data noted slightly lower discontinuation rates due to gastrointestinal events for Tirzepatide versus Semaglutide 1 mg, though the populations and dose escalation schedules were not identical.

This is a meaningful area of ongoing research but should not be overstated. Individual responses vary considerably, and trial data does not support a categorical claim that Tirzepatide is definitively better tolerated than Semaglutide across all users.

Other Notable Safety Observations

• Gallbladder disease — as with all GLP-1 class agents, rapid weight loss can increase gallstone risk; cholelithiasis was reported at higher rates than placebo in some trial arms
• Hypoglycaemia — low risk in non-diabetic populations; higher risk when combined with insulin or insulin secretagogues in T2D patients
• Heart rate — modest increase in resting heart rate is a class effect; monitoring is standard clinical practice
• Pancreatitis — reported rarely; carries a class-level precautionary notice in prescribing information

Availability in Ireland: What You Actually Need to Know

Mounjaro (Tirzepatide) — Type 2 Diabetes

Mounjaro received EMA marketing authorisation and is available in Ireland as a prescription medicine for adults with type 2 diabetes as an adjunct to diet and exercise to improve glycaemic control. It is available in the 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg autoinjector pen presentations. Prescribing is within the discretion of the treating physician and, as with Ozempic, supply constraints have affected access at various points.

Zepbound (Tirzepatide for Obesity) — EU Status

Zepbound is the obesity-specific branding for Tirzepatide at the same doses. It received FDA approval in November 2023 in the United States for adults with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one weight-related condition. As of mid-2026, the EMA review process was ongoing — Zepbound had not yet received EU marketing authorisation for the obesity indication.

This means that in Ireland, as in the rest of the EU, Tirzepatide's approved indication remains type 2 diabetes. Physicians may exercise clinical judgement within the applicable prescribing framework, but Tirzepatide is not formally approved for obesity management in the EU as a standalone indication at the time of this writing.

Why the GIP Addition Matters: A Clearer Explanation

If you've read about Ozempic or Wegovy, you already know the GLP-1 story: it slows digestion, reduces appetite, and tells the brain you're full. That mechanism alone is powerful enough to produce 15% weight loss over 68 weeks. So why does adding GIP push that number to 22.5%?

The short answer is that GLP-1 and GIP appear to work through complementary pathways that reinforce each other. GIP receptors are expressed in areas of the brain — including the hypothalamus — that regulate energy balance, and animal model research has demonstrated that combined GLP-1/GIP receptor activation produces greater reductions in food intake and body weight than either agonist alone.

At the fat cell level, GIP receptor activation influences how adipocytes (fat cells) handle lipid storage and release. This is distinct from the appetite suppression driven by GLP-1 and represents a separate metabolic lever that Tirzepatide pulls simultaneously. Some researchers describe this as the difference between only reducing "calories in" versus also improving how the body handles the energy it already has stored.

There is also the nausea question. GLP-1 receptor activation in the gut is partly responsible for the nausea side effects of Semaglutide. GIP agonism appears to counteract some of this signal — meaning the GIP component in Tirzepatide may allow higher effective GLP-1 stimulation with somewhat better tolerability than an equivalent GLP-1-only agent. This is an area of active research and should not be treated as a definitive clinical claim, but it is a plausible mechanistic explanation for the tolerability data observed in SURPASS-2.

Frequently Asked Questions

Research Updates

Related Reading

• Semaglutide — Beyond the Headlines
• Retatrutide Ireland: Phase 2 Trial Data & Triple Agonist Mechanism
• Tirzepatide Research Guide — Deeper Dive