Wegovy vs Mounjaro: what the real-world data shows in 2026.

1. The molecules: GLP-1 mono-agonist vs GLP-1/GIP dual agonist

Both medications belong to the broader family of incretin-based therapies, but their pharmacology is not identical.

Semaglutide is a long-acting analogue of human glucagon-like peptide-1 (GLP-1). It binds the GLP-1 receptor (expressed in pancreas, gut, central nervous system and cardiovascular system), stimulating glucose-dependent insulin secretion, suppressing inappropriate glucagon release, slowing gastric emptying and reducing appetite via central pathways. Administered subcutaneously once weekly. Marketed as Wegovy for adult obesity (titrated to 2.4 mg weekly) and Ozempic for type 2 diabetes (up to 2 mg weekly).

Tirzepatide is a dual agonist of the GLP-1 receptor and the GIP (glucose-dependent insulinotropic polypeptide) receptor. The dual mechanism appears to translate, in clinical trials, into greater weight loss at a comparable tolerability profile. Administered subcutaneously once weekly. Marketed as Mounjaro for type 2 diabetes (up to 15 mg weekly) and, in the United States and selected other markets, as Zepbound for adult obesity (up to 15 mg weekly). In a few European markets (notably the UK from 2025 via the NHS phased rollout), tirzepatide has obtained funded access for selected adults with obesity; in most others, including France and Germany, it remains self-pay.

The clinical question is rarely "which receptor pharmacology is better in the abstract"; it is "which medication is the right starting point for this patient, in this country, with this access situation".

2. The pivotal trials side by side: STEP-1 and SURMOUNT-1

The headline numbers from the randomised trials are widely quoted, sometimes loosely. Here they are with the right comparators.

STEP-1 (Wilding 2021) randomised 1,961 adults with overweight or obesity, without diabetes, to weekly semaglutide 2.4 mg or placebo, alongside lifestyle intervention, for 68 weeks. Mean change in body weight was −14.9% with semaglutide vs −2.4% with placebo. About 86% of participants on semaglutide achieved at least 5% weight loss, 69% achieved at least 10%, and 50% achieved at least 15%.¹

SURMOUNT-1 (Jastreboff 2022) randomised 2,539 adults with obesity (or overweight with at least one related condition), without diabetes, to weekly tirzepatide 5 mg, 10 mg or 15 mg, or placebo, for 72 weeks. Mean change in body weight at 15 mg was −20.9% with tirzepatide vs −3.1% with placebo. About 91% of participants on tirzepatide 15 mg achieved at least 5% weight loss, 71% at least 15%, and 57% achieved at least 20% (treatment-regimen estimand, consistent with the cited −20.9%).²

Three caveats are essential when reading these numbers:

• The trial populations are not identical. SURMOUNT-1 included a subset of participants with one related condition; baseline BMI distributions differed slightly. Cross-trial comparisons are suggestive, not definitive.
• Trial conditions are not real life. Trial participants get structured lifestyle support, free medication, supply guarantees, regular contact and active monitoring. Real-world weight loss is consistently lower.
• The mean hides the distribution. Both trials show a substantial tail of low responders and a substantial tail of high responders. Two patients on the same dose of the same drug can have radically different outcomes.

3. The first head-to-head: SURMOUNT-5 (2025)

Until 2025, the comparison between semaglutide and tirzepatide rested on cross-trial inference. SURMOUNT-5, published in the New England Journal of Medicine in May 2025, was the first prospective, randomised, head-to-head trial in adults with obesity without type 2 diabetes.³

The trial randomised 751 adults to maximum tolerated doses of either tirzepatide (10 or 15 mg weekly) or semaglutide (1.7 or 2.4 mg weekly), in an open-label design, for 72 weeks. Key results:

• Mean change in body weight: −20.2% with tirzepatide vs −13.7% with semaglutide (p<0.001).
• Mean reduction in waist circumference: −18.4 cm vs −13.0 cm in favour of tirzepatide (p<0.001).
• Roughly 32% of tirzepatide participants achieved at least 25% weight loss, vs about 16% with semaglutide.
• Treatment discontinuation due to gastrointestinal events was somewhat higher with semaglutide (5.6%) than tirzepatide (2.7%).

The trial confirms what cross-trial inference suggested: at maximum tolerated dose, tirzepatide produces greater mean weight loss than semaglutide, with broadly comparable tolerability. It does not answer several other clinically important questions: cardiovascular outcomes, long-term persistence beyond 72 weeks, comparative effectiveness in subgroups (older adults, post-bariatric patients, patients with severe gastroparesis), or comparative impact on lean mass.

4. Real-world persistence: who actually stays on

Trial data describe what is biologically possible. Real-world data describe what actually happens. In the largest US claims analysis to date (Rodriguez et al., JAMA Network Open 2025, n>125,000), about 65% of adults without diabetes who started a dual-labelled GLP-1 for weight discontinued within 12 months, vs about 47% of those with type 2 diabetes; a meaningful fraction stopped within 6 months.⁶ Persistence is consistently better when the indication is type 2 diabetes than obesity alone. Discontinuation in real life is rarely about weight loss not happening; it is more often about side effects, supply interruption, cost, insurance changes, life events, or the absence of structured support between consultations (see why half of GLP-1 patients stop in the first year).

For a patient choosing between two molecules, the practical implication is that the molecule you can stay on, in your real life, will outperform the molecule with the higher trial mean that you stop after 4 months.

5. Tolerability: nausea, diarrhoea and the dose-step reality

Both molecules share the GLP-1 receptor mechanism that drives most gastrointestinal side effects. The aggregate burden is broadly similar, with some published differences in pattern. Across pivotal trials and real-world cohorts:

• Nausea is the most common side effect of both, peaking around dose escalations. Some real-world data and the SURMOUNT-5 discontinuation pattern suggest semaglutide may produce slightly more frequent or persistent nausea than tirzepatide at maximum dose.
• Diarrhoea is common with both molecules; in pivotal trials it was reported slightly more often with semaglutide 2.4 mg (around 30%) than with tirzepatide 15 mg (around 23%), with overlap in real-world cohorts.
• Constipation is common with both, frequently under-reported.
• Severe but rare events (pancreatitis, gallbladder disease, bowel obstruction) have been documented with GLP-1 receptor agonists at low absolute frequencies. A widely cited pharmacovigilance analysis quantified these signals across the class.⁵

"Tolerability" is not just about which molecule generates which symptom on average. It is about how a particular body responds at a particular dose. A patient who tolerates semaglutide poorly at 1 mg may tolerate tirzepatide well at 7.5 mg, and vice versa. For a structured way to bring this into the consultation, see GLP-1 side effects, month by month.

6. The cardiovascular evidence asymmetry: SELECT and what is still pending

For people with obesity, weight loss is rarely an end in itself. The deeper goal is to reduce cardiometabolic risk. The cardiovascular outcome data for the two molecules are at very different stages of maturity.

For semaglutide, the SELECT trial (Lincoff 2023) randomised 17,604 adults with overweight or obesity and pre-existing cardiovascular disease, without diabetes, to weekly semaglutide 2.4 mg or placebo, for a mean follow-up of about 40 months. Major adverse cardiovascular events were reduced by 20% (HR 0.80, 95% CI 0.72 to 0.90, p<0.001).⁴ This is the first time a weight-targeted pharmacological therapy has shown a clear cardiovascular benefit in a population with obesity but without diabetes.

For tirzepatide, the dedicated cardiovascular outcomes trial in adults with obesity (SURMOUNT-MMO) is ongoing as of 2026, with results expected later in the decade. Until then, the cardiovascular benefit of tirzepatide in obesity is biologically plausible and supported by metabolic surrogates, but not yet proven by a hard-endpoint trial.

This asymmetry matters: for a patient with established cardiovascular disease who needs an AOM today, SELECT-aligned semaglutide is currently the only AOM with a proven hard cardiovascular benefit in this population. For a patient without established cardiovascular disease for whom maximum weight reduction is the priority, tirzepatide has the strongest weight-loss evidence; the cardiovascular reasoning rests on metabolic surrogates.

The 2026 GCC-CSO/FORCE French position paper translates this evidence into phenotype-level guidance: semaglutide 2.4 mg first-line in obesity without type 2 diabetes plus secondary cardiovascular prevention (Avis n°7), in painful knee osteoarthritis (Avis n°12), and in MASH (Avis n°9); tirzepatide 15 mg first-line in moderate-to-severe obstructive sleep apnoea (Avis n°10), in HFpEF (Avis n°8), and in obesity with type 2 diabetes plus secondary cardiovascular prevention.⁷

7. Supply, access and cost: by country, in 2026

The clinical literature converges on a relatively clear picture; the access reality varies country by country and changes month by month. As of 2026:

• United Kingdom. Wegovy is available on the NHS via specialist weight management services for a defined eligible population, with treatment capped at 24 months per NICE TA875 guidance. Mounjaro is being rolled out via NHS specialist services and primary care under a phased plan (NICE TA1026). Both are available privately, with persistent supply pressure on Wegovy starter doses.
• France. Wegovy obtained marketing authorisation; reimbursement is being negotiated and access remains primarily private for adults with obesity. Mounjaro is available privately. Reimbursement decisions are evolving.
• United States. Wegovy and Zepbound are both available, with uneven insurance coverage. Out-of-pocket cost remains a major driver of discontinuation. Compounded "GLP-1" preparations have raised safety concerns; manufacturers and the FDA have warned against unregulated alternatives.
• Other markets. The general pattern is that authorisation precedes reimbursement, sometimes by years.

For most patients, the choice between molecules is partly framed before the consultation by what is actually accessible at affordable cost.

8. The clinical reality: the right molecule is the one you can stay on

Two molecules of the same broad family. One has more cardiovascular evidence; the other has more weight-loss magnitude. Both have strong safety records, both produce real, life-changing outcomes for some patients, and both are routinely discontinued by about half of patients in the first year for reasons that have little to do with their pharmacology. What matters in practice:

• Your medical history (cardiovascular disease, diabetes, GI sensitivity, gallbladder disease, pancreatitis, prior bariatric surgery, current medications).
• Your priorities (cardiovascular risk reduction, weight loss magnitude, tolerability ceiling, future pregnancy plans).
• Your access reality (what is reimbursed or affordable, what is reliably in stock, what your physician can prescribe).
• Your support system between consultations (what helps you stay on when side effects bite or the plateau hits). For more, see plateau and rebound.

The goal of this article is not to tilt the conversation toward one molecule. It is to give you a clean, evidence-based vocabulary for the conversation you will have with the physician who knows your file. Bring the questions; do not bring a verdict.

References

1. Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. New England Journal of Medicine 2021;384(11):989-1002. doi:10.1056/NEJMoa2032183. PMID 33567185.
2. Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine 2022;387(3):205-216. doi:10.1056/NEJMoa2206038. PMID 35658024.
3. Aronne LJ, Horn DB, le Roux CW, et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5). New England Journal of Medicine 2025. doi:10.1056/NEJMoa2416394. PMID 40353578.
4. Lincoff AM, Brown-Frandsen K, Colhoun HM, et al. Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes. New England Journal of Medicine 2023;389(24):2221-2232. doi:10.1056/NEJMoa2307563. PMID 37952131.
5. Sodhi M, Rezaeianzadeh R, Kezouh A, Etminan M. Risk of Gastrointestinal Adverse Events Associated With Glucagon-Like Peptide-1 Receptor Agonists for Weight Loss. JAMA 2023;330(18):1795-1797. doi:10.1001/jama.2023.19574. PMID 37796527.
6. Rodriguez PJ, Goodwin Cartwright BM, Gratzl S, et al. Discontinuation and Reinitiation of Dual-Labeled GLP-1 Receptor Agonists Among US Adults With Overweight or Obesity. JAMA Network Open 2025;8(1):e2457349. doi:10.1001/jamanetworkopen.2024.57349. PMID 39888616.
7. Aron-Wisnewsky J, Disse E, et al. Pharmacological treatment of obesity in adults: position statement of the GCC-CSO and FORCE working groups. Medecine des Maladies Metaboliques 2025. doi:10.1016/j.mmm.2025.10.003.