Mounjaro and Wegovy are the two most talked-about weight loss medications in the UK right now. Both are weekly injections, both produce significant weight loss, and both are available privately and (with restrictions) through the NHS. But they work in slightly different ways, have different clinical evidence behind them, and differ in cost and availability.
This guide breaks down everything you need to know to understand how they compare — based on clinical trial data, UK regulatory guidance, and real-world considerations.
How they work: the fundamental difference
Both medications work by mimicking gut hormones that regulate appetite and blood sugar, but they target different receptors.
Wegovy contains semaglutide, a GLP-1 receptor agonist. It mimics one hormone — GLP-1 (glucagon-like peptide-1) — which signals to your brain that you're full, slows gastric emptying, and improves insulin response.1
Mounjaro contains tirzepatide, a dual GIP/GLP-1 receptor agonist. It mimics two hormones — both GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). GIP plays an additional role in insulin secretion and fat metabolism. This dual action is thought to explain why Mounjaro tends to produce slightly more weight loss in clinical trials.2
Weight loss: what the trials show
The head-to-head trial (SURMOUNT-5)
Published in the New England Journal of Medicine in May 2025, the SURMOUNT-5 trial was the first direct comparison of tirzepatide and semaglutide for weight loss. It enrolled 751 adults with obesity (without Type 2 diabetes) across 32 sites in the US and Puerto Rico.3
The results were clear: participants on tirzepatide lost an average of 20.2% of their body weight over 72 weeks, compared to 13.7% on semaglutide — a statistically significant difference. In absolute terms, that's about 22.8kg lost on tirzepatide versus 15.0kg on semaglutide. Tirzepatide also produced greater reductions in waist circumference (18.4cm vs 13.0cm).3
Participants on tirzepatide were also more likely to achieve meaningful weight loss milestones: 31.6% achieved at least 25% body weight reduction, compared to 16.1% on semaglutide.4
Important context: the Wegovy 7.2mg dose
There's a critical piece of context that many comparisons miss. The SURMOUNT-5 trial compared tirzepatide against semaglutide at its 2.4mg dose — which was the maximum available at the time. However, in January 2026, the MHRA approved a higher 7.2mg dose of Wegovy, making the UK the first country in the world to approve it.5
The STEP UP trial showed that Wegovy at 7.2mg produces an average weight loss of 20.7% over 72 weeks — significantly closing the gap with Mounjaro's 22.5% (from SURMOUNT-1) and 20.2% (from SURMOUNT-5).6
In practical terms, at their maximum licensed doses, both medications now deliver approximately 20–22% average weight loss. The gap is much smaller than the SURMOUNT-5 headline numbers suggest, because that trial used the older, lower dose of Wegovy.
Individual results vary
Clinical trial averages are useful for comparison, but individual responses vary significantly. Some people respond better to semaglutide, others to tirzepatide. Factors including genetics, starting weight, diet, exercise, and how well you tolerate each medication all play a role. A medication that produces 20% average weight loss in a trial might produce 10% in one person and 30% in another.
Side effects compared
Both medications share similar gastrointestinal side effects — this is inherent to how GLP-1 medications work (slowing digestion inevitably affects the gut). The most common side effects for both are nausea, diarrhoea, constipation, vomiting, and stomach pain.12
In the SURMOUNT-5 head-to-head trial, gastrointestinal adverse events causing treatment discontinuation were actually lower with tirzepatide (2.7%) than with semaglutide (5.6%). Serious adverse events occurred in 4.8% of tirzepatide users and 3.5% of semaglutide users.3
There are a few differences worth noting:
- Nausea rates: Wegovy tends to cause nausea in a higher proportion of patients (~40–44%) compared to Mounjaro (~20–25%), based on their respective trial programmes
- Contraception: Mounjaro may reduce the effectiveness of oral contraceptive pills and oral HRT. The MHRA recommends using barrier contraception for 4 weeks after starting and after each dose increase. This warning does not apply to Wegovy2
- Dysaesthesia: The higher 7.2mg dose of Wegovy has been associated with a new side effect — dysaesthesia (tingling or pins-and-needles sensations) — which was not seen at lower doses5
Both carry rare risks of pancreatitis (approximately 1 in 100) and gallbladder problems. For a full breakdown, see our side effects comparison tool.
Cardiovascular evidence
This is one area where Wegovy currently has a clear advantage. The SELECT trial, published in the New England Journal of Medicine in 2023, demonstrated that semaglutide 2.4mg reduced major adverse cardiovascular events (heart attack, stroke, and cardiovascular death) by 20% in people with obesity and established cardiovascular disease — over a follow-up period of nearly 3 years.7
This was the first time any obesity medication had demonstrated a direct cardiovascular mortality benefit in non-diabetic patients, and it led to NICE recommending Wegovy for cardiovascular risk reduction in April 2026.
Mounjaro does not yet have equivalent cardiovascular outcomes data. Eli Lilly's cardiovascular outcomes trial (SURPASS-CVOT) is still ongoing. Post-hoc analysis of SURMOUNT-5 suggests tirzepatide may provide greater predicted 10-year cardiovascular risk reduction than semaglutide, but predicted risk reduction from a weight loss trial is not the same as proven cardiovascular outcomes from a dedicated trial.8
For people with existing heart disease, this is a meaningful distinction. Wegovy currently has the stronger evidence base for cardiovascular protection.
Dosing and practical differences
Both are once-weekly subcutaneous injections using pre-filled pens. The injection process is virtually identical — choose a site (abdomen, thigh, or upper arm), clean the area, inject, done.
Mounjaro dosing: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg, increasing every 4 weeks2
Wegovy dosing: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg → 7.2mg, increasing every 4 weeks1
Both reach their maintenance dose in approximately 20 weeks, though you can stay on a lower dose if it's working well and you're tolerating it. Your prescriber will advise based on your response.
Cost in the UK
Private prescription costs are broadly similar, though Mounjaro is typically slightly cheaper:
- Mounjaro: £159–£359/month (varies by dose and provider)
- Wegovy: £99–£299/month (varies by dose and provider)
Mounjaro saw a price increase in September 2025, which prompted some patients to consider switching to Wegovy. Costs can also vary significantly between providers, so it's worth comparing — see our UK providers comparison guide.
On the NHS, both are free (standard prescription charge of £9.90 in England applies), but access is limited. For more detail, see our guides to Mounjaro on the NHS and Wegovy on the NHS.
NHS availability
Mounjaro: NICE approved tirzepatide for obesity in December 2024 (TA1026). NHS England is rolling it out in phases — Cohort 1 (BMI ≥40 with 4+ comorbidities) started from June 2025, with Cohort 2 (BMI ≥35 with 2+ comorbidities) expected from June 2026. Only approximately 220,000 people are expected to access it on the NHS by 2028.9
Wegovy: NICE approved semaglutide for obesity in March 2023 (TA875). It's available through Tier 3 specialist weight management services, but access remains limited with significant waiting times in most areas.10
The reality is that most people in the UK currently access either medication through private prescriptions, which is why cost and provider comparison matters.
Who should choose which?
There's no universal "better" option — the right choice depends on your individual circumstances. Here's a general framework:
Mounjaro might be the better fit if:
- Maximising weight loss is your primary goal
- You also have Type 2 diabetes (Mounjaro is licensed for both)
- You've tried semaglutide and didn't tolerate it well (tirzepatide may have lower GI discontinuation rates)
- You don't rely on oral contraception or oral HRT (or are willing to use additional barrier methods)
Wegovy might be the better fit if:
- You have existing cardiovascular disease and want proven heart protection (SELECT trial data)
- You use oral contraception or oral HRT and want to avoid the interaction concern
- Cost is a factor and you find a provider offering Wegovy at a competitive price
- You're already on semaglutide (e.g. Ozempic for diabetes) and your prescriber recommends transitioning to Wegovy for weight management
Ultimately, this is a decision to make with your prescriber, who can consider your full medical history, current medications, and treatment goals. Both medications are clinically proven, MHRA-approved, and backed by robust trial data. Either one, combined with dietary changes and physical activity, can produce meaningful, sustained weight loss.
Related articles
ComparisonWegovy vs Ozempic: What's the Difference?
Same ingredient, different licence — why one is for weight loss and the other isn't.
ResultsWhat Happens When You Stop Taking GLP-1 Medication?
Weight regain rates, the Oxford BMJ study, and what it means for long-term treatment.
GuideNHS vs Private GLP-1 Prescriptions
The reality of access, cost differences, and what to look for in a private provider.