Both are weekly injections with large randomised trials behind them. But Mounjaro and Wegovy work differently, carry different evidence profiles, and suit different patients. This is an honest, clinical comparison, not a marketing exercise, written to help you understand which medication may be more appropriate for you.
The short answer
Mounjaro produces greater average weight loss in clinical trials. For most patients without significant heart disease, Mounjaro is the more potent option. Wegovy 2.4mg is the only weight-management medicine with a UK licence to reduce the risk of heart attack, stroke and cardiovascular death, for adults with established cardiovascular disease and a BMI of 27 or more.
How they work differently
Wegovy contains semaglutide, a GLP-1 receptor agonist. It suppresses appetite, slows gastric emptying, and improves insulin sensitivity, all by mimicking a single gut hormone.
Mounjaro contains tirzepatide, a dual GLP-1 and GIP receptor agonist. It does everything semaglutide does, plus activates GIP receptors, which promote fat metabolism, regulate energy balance, and enhance the appetite-suppressing effects of GLP-1.
What the clinical evidence shows
SURMOUNT-1 (NEJM 2022, n=2,539, 72 weeks)
In the SURMOUNT-1 trial (New England Journal of Medicine, 2022), adults without diabetes who took tirzepatide alongside diet and activity advice lost an average of 15.0% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% on placebo, counting everyone who started treatment. Among people who stayed on treatment, the averages were 16.0%, 21.4% and 22.5%. Individual results vary. At the highest dose, 36.2% of patients lost 25% or more of their body weight.
STEP-1 (NEJM 2021, n=1,961, 68 weeks)
Average weight loss with semaglutide 2.4mg: 14.9%. Half of all patients lost 15% or more. Placebo group: 2.4%.
SURMOUNT-5, a direct head-to-head trial (NEJM 2025, Aronne et al, n=751, 72 weeks)
This was the first randomised controlled trial to directly compare tirzepatide against semaglutide in adults with obesity but without diabetes. The results were unambiguous:
- Tirzepatide: 20.2% average weight loss
- Semaglutide: 13.7% average weight loss
- Estimated treatment difference: −6.5 percentage points (95% CI −8.1 to −4.9, P<0.001)
- Achievement of ≥25% weight loss: 31.6% (tirzepatide) vs 16.1% (semaglutide)
- GI-related discontinuations: 2.7% (tirzepatide) vs 5.6% (semaglutide)
Conclusion: tirzepatide demonstrated superior weight loss to semaglutide at 72 weeks, the first direct clinical confirmation of what cross-trial comparisons had suggested.
"In SURMOUNT-5 (New England Journal of Medicine, 2025), a 72-week trial comparing the two medicines directly in 751 adults with obesity but not diabetes, people on the highest dose they could tolerate of tirzepatide lost an average of 20.2% of their body weight, compared with 13.7% on semaglutide. Individual results vary."
Clinical trial results: weight loss at 72 weeks
SURMOUNT-5 (NEJM 2025) compared the two medicines directly in 751 adults with obesity but not diabetes. Average weight loss over 72 weeks on the highest dose people could tolerate, counting everyone who started treatment.
Source: SURMOUNT-5, a direct head-to-head trial (NEJM 2025). Individual results vary.
Wegovy's newer 7.2mg dose narrows the gap. In the STEP UP trial (Lancet Diabetes and Endocrinology, 2025), adults with obesity and without diabetes lost an average of 18.7% of their body weight over 72 weeks on 7.2mg, compared with 15.6% on 2.4mg and 3.9% on placebo, counting everyone who started treatment. Among those who stayed on treatment, the averages were 20.7% and 17.5%. Individual results vary.
In SURMOUNT-5 (New England Journal of Medicine, 2025), a 72-week trial comparing the two medicines directly in 751 adults with obesity but not diabetes, people on the highest dose they could tolerate of tirzepatide lost an average of 20.2% of their body weight, compared with 13.7% on semaglutide. Individual results vary.
Cardiovascular protection: Wegovy's unique licence
In the SELECT trial (New England Journal of Medicine, 2023), 17,604 adults aged 45 or over with established cardiovascular disease, a BMI of 27 or more and no diabetes took semaglutide 2.4mg or placebo for an average of about 40 months. A heart attack, stroke or cardiovascular death occurred in 6.5% of people on semaglutide compared with 8.0% on placebo, a 20% lower relative risk. Individual results vary. Wegovy 2.4mg is the only weight-management medicine with a UK licence to reduce the risk of heart attack, stroke and cardiovascular death, for adults with established cardiovascular disease and a BMI of 27 or more.
For patients with established heart disease, your pharmacist will advise which medicine is right for you. Tirzepatide's heart outcome trial in people with obesity without diabetes (SURMOUNT-MMO) is still running. In people with type 2 diabetes and heart disease, the SURPASS-CVOT trial (New England Journal of Medicine, 2025) found that a heart attack, stroke or cardiovascular death occurred in 12.2% of people on tirzepatide and 13.1% on dulaglutide, another diabetes medicine, over a median of four years, showing tirzepatide was no worse. Individual results vary.
Dosing comparison
| Mounjaro | Wegovy | |
|---|---|---|
| Injection frequency | Once weekly | Once weekly |
| Starting dose | 2.5mg | 0.25mg |
| Dose increases | Every 4 weeks | Every 4 weeks |
| Maximum dose | 15mg | 7.2mg |
| Time to maintenance | ~20 weeks | ~16 weeks |
Side effects
Both medications share a similar gastrointestinal side effect profile, nausea, diarrhoea, vomiting, and constipation, predominantly during dose escalation. In SURMOUNT-5 (New England Journal of Medicine, 2025), stomach side effects were the most common with both medicines and were mostly mild to moderate: nausea affected 43.6% of people on tirzepatide and 44.4% on semaglutide. Individual results vary. For both medications, side effects improve with time.
Who suits each medication?
Mounjaro may be more appropriate for
- Patients prioritising maximum weight loss
- Patients without significant cardiovascular comorbidities
- Patients with type 2 diabetes
- Patients who have previously tried Wegovy
Wegovy may be more appropriate for
- Patients with established cardiovascular disease or high CV risk
- Patients where Mounjaro is contraindicated
- Patients wanting the most mature long-term cardiovascular outcome data
Can you switch?
Yes, switching between the two is clinically straightforward. Your pharmacist sets a starting dose on the new medicine from how long you have been treated, your current dose and how well you tolerate it: if you are well established, you usually don't restart at the lowest dose, but you don't jump straight to a dose that feels equivalent either.
"The most important factor is not which drug is theoretically superior in a trial. It's which is right for you, based on your full medical history. That is exactly what our consultation is designed to assess."