TL;DR: Tirzepatide activates both GIP and GLP-1 receptors, while semaglutide targets GLP-1 alone. The SURMOUNT trials found large average weight reductions in adults with obesity, and SURPASS-2 found better glucose and weight outcomes than semaglutide 1 mg in adults with type 2 diabetes. Those are different populations and doses, so cross-trial comparisons need care. It is a prescription medicine, and the common trial side effects were mainly gastrointestinal.
What Is Tirzepatide?
Tirzepatide is the active substance in Mounjaro, a prescription medicine that activates two hormone receptors: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). The European Medicines Agency (EMA) lists Mounjaro for type 2 diabetes and for weight management in specified adults, alongside diet and physical activity.
While Semaglutide (Ozempic, Wegovy) acts on the GLP-1 receptor, tirzepatide also acts on the GIP receptor. That is a meaningful difference in mechanism, but outcomes still need to be read from the relevant human trial rather than inferred from the number of receptors involved.
Key point: Tirzepatide is not simply a stronger GLP-1 agonist. It combines GLP-1 and GIP receptor activity. The clinical results come from trials of the medicine; the mechanism alone does not prove how much each receptor contributed.
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.
Combined GLP-1 and GIP agonism is one proposed explanation for tirzepatide's clinical results. The trials test the finished medicine, however, and do not isolate exactly how much of the outcome came from GIP receptor activity.
GIP receptors are also highly expressed in adipose tissue. Researchers believe their activation may affect how fat cells store and mobilise lipids, although that proposed mechanism does not by itself prove the cause of observed body-composition changes.
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.
| SURMOUNT-1 trial group | Mean Body Weight Change at 72 Weeks | ≥5% Weight Loss | ≥20% Weight Loss |
|---|---|---|---|
| Placebo | −2.4% | 35% | 3% |
| SURMOUNT-1 trial: 5 mg group | −15.0% | 85% | 30% |
| SURMOUNT-1 trial: 10 mg group | −19.5% | 89% | 45% |
| SURMOUNT-1 trial: 15 mg group | −22.5% | 91% | 57% |
The SURMOUNT-1 trial reported a 22.5% mean body-weight reduction in its 15 mg study group over 72 weeks. Separately, STEP-1 reported approximately 14.9% with Semaglutide 2.4 mg over 68 weeks. These were different trials, not a head-to-head comparison, so the gap between the headline figures should not be treated as a direct treatment effect.
SURMOUNT-2 (Type 2 Diabetes Population)
SURMOUNT-2 (Garvey et al., The Lancet 2023) used the same 72-week study design but enrolled 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.
| SURMOUNT-2 trial group | Mean Body Weight Change at 72 Weeks | ≥5% Weight Loss |
|---|---|---|
| Placebo | −2.5% | 29% |
| SURMOUNT-2 trial: 10 mg group | −13.4% | 79% |
| SURMOUNT-2 trial: 15 mg group | −15.7% | 83% |
SURMOUNT-2 reported a 15.7% mean reduction in its Tirzepatide 15 mg study group. STEP-2 separately reported approximately 9.6% with Semaglutide 2.4 mg in adults with type 2 diabetes. Different trials can differ in participants and design, so those figures do not establish a direct advantage on their own.
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.
| Treatment | HbA1c Reduction | Body Weight Change | Superior to Sema 1mg? |
|---|---|---|---|
| SURPASS-2 trial: Semaglutide 1 mg | −1.86% | −5.7 kg | Comparator |
| SURPASS-2 trial: Tirzepatide 5 mg | −2.01% | −7.6 kg | Yes (both endpoints) |
| SURPASS-2 trial: Tirzepatide 10 mg | −2.24% | −9.3 kg | Yes (both endpoints) |
| SURPASS-2 trial: Tirzepatide 15 mg | −2.46% | −11.2 kg | Yes (both endpoints) |
All three Tirzepatide doses were statistically superior to Semaglutide 1 mg for glycaemic control (HbA1c) and body-weight reduction. The comparator was Semaglutide 1 mg, not the 2.4 mg Wegovy obesity dose. This trial therefore cannot answer a maximum-dose obesity comparison on its own.
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.
| Compound | Mechanism | Best Weight Loss (Phase 3) | EU authorisation (checked 20 Sep 2026) | Brand Name |
|---|---|---|---|---|
| Semaglutide | GLP-1 agonist | ~14.9% (STEP-1, 68 wks, non-T2D) | Ozempic (T2D) ✓ — Wegovy (obesity) ✓ (limited availability) | Ozempic / Wegovy |
| Tirzepatide | GLP-1 + GIP dual agonist | 22.5% (SURMOUNT-1, 72 wks, non-T2D) | Mounjaro: type 2 diabetes and adult weight management ✓ | Mounjaro |
| Retatrutide | GLP-1 + GIP + glucagon triple agonist | 24.2% (Phase 2 only, 12 mg, 48 wks) | Not approved — Phase 3 (TRIUMPH) ongoing | None yet |
Important context: Trial populations, durations, and dose-escalation designs differ between programmes. These figures are not directly interchangeable. The fact that Retatrutide's Phase 2 peak matches Tirzepatide's Phase 3 peak in non-diabetic populations is notable but should not be treated as a settled comparison — Phase 3 Retatrutide data (the TRIUMPH series) is needed before firm conclusions can be drawn.
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
Can GIP explain the side-effect pattern?
Not reliably from the available clinical evidence. Tirzepatide and semaglutide differ in mechanism, but that does not establish that one will cause less nausea for an individual. The useful comparison is the adverse-event data from the relevant trial or official medicine information, followed by an individual risk discussion with a prescriber or pharmacist.
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) — current EU status
Mounjaro has an EU marketing authorisation for type 2 diabetes and for weight management in specified adults, alongside diet and physical activity. It is prescription-only. This corrects the previous version of this page, which incorrectly described the EU weight-management indication as pending.
EU authorisation does not tell you whether a medicine is currently in stock, reimbursed or appropriate for you in Ireland. Check the HPRA medicines register and ask a pharmacist or prescriber about current access and individual suitability. For the wider distinction between authorised medicines and research products, use the peptide evidence and Irish-authorisation overview.
Found an outdated source or something unclear? Email keith@nononsensefitness.ie. I can clarify what the page says or correct the page, but I cannot provide personal dosing, cycle or medical advice.
Do not confuse authorisation with a recommendation. The EMA page explains the authorised uses and links to the official product information. A qualified healthcare professional must assess contraindications, interactions and individual risk.
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.
GIP receptors are present in adipose tissue, so researchers are studying whether that pathway contributes to changes in lipid storage and release. That is a mechanistic question, not proof that an individual will lose more fat or respond in a particular way.
Mechanism is useful for explaining why tirzepatide is different; it is not enough to predict an individual's side effects or to declare it better tolerated than semaglutide. Those questions need clinical evidence and individual assessment, not a mechanistic guess.
Sport status: Tirzepatide is not listed as prohibited in the WADA 2026 List. Its markers are included in the 2026 Monitoring Program; monitoring does not mean prohibition. Check current anti-doping advice for the exact medicine and circumstances.
Frequently Asked Questions
What is Tirzepatide?
Tirzepatide is the active substance in Mounjaro, a prescription medicine that activates both GLP-1 and GIP receptors. Mounjaro is authorised in the EU for type 2 diabetes and for weight management in specified adults, alongside diet and physical activity. It is not a peptide in the research-compound sense.
Is Mounjaro available in Ireland?
Mounjaro has an EU marketing authorisation for type 2 diabetes and for weight management in specified adults. It is prescription-only. Authorisation does not guarantee current Irish supply or reimbursement, so check the HPRA medicines register and ask a pharmacist or prescriber about access.
How does Tirzepatide compare to Ozempic?
In adults with type 2 diabetes, the SURPASS-2 head-to-head trial found greater HbA1c and body-weight reductions with the studied Tirzepatide groups than with Semaglutide 1 mg. SURMOUNT-1 and STEP-1 used different obesity-study populations and designs, so their results should not be treated as a direct comparison.
What did the SURMOUNT-1 trial show?
SURMOUNT-1 was a 72-week randomised controlled trial in 2,539 adults with obesity (no type 2 diabetes). At the 15 mg dose, participants achieved a mean body weight reduction of 22.5% versus 2.4% for placebo. Over 91% achieved at least 5% weight loss, and 57% achieved at least 20% weight loss at the highest dose. These were the highest figures reported in any Phase 3 weight loss drug trial at the time of publication.
What are the main side effects of Tirzepatide?
The EMA medicine overview lists nausea, vomiting, abdominal pain, constipation and diarrhoea among the most common side effects; low blood glucose is also listed when Mounjaro is used with a sulphonylurea or insulin. Discuss individual risks and restrictions with a prescriber or pharmacist.
Research Updates
Related Reading
- Semaglutide Ireland: Ozempic, Wegovy & Clinical Trial Data Explained
- Retatrutide Ireland: Phase 2 Trial Data & Triple Agonist Mechanism