Quick comparison summary
- Mounjaro (tirzepatide): Dual GIP/GLP-1 agonist by Eli Lilly — average weight loss ~20–22% at maximum dose
- Wegovy (semaglutide 2.4 mg): GLP-1 agonist by Novo Nordisk — average weight loss ~15–17% at maximum dose
- NHS availability: Both available through specialist weight management services; Mounjaro also accessible via some GPs for type 2 diabetes
- Private cost: Mounjaro £180–£350/month; Wegovy £180–£300/month
- Cardiovascular data: Wegovy has proven CV benefit (SELECT trial); Mounjaro CV outcome trial (SURPASS-CVOT) ongoing
Introduction: two leading weight loss medications
Mounjaro and Wegovy represent the two most effective injectable weight loss medications currently available in the United Kingdom. Both are prescribed for chronic weight management and have transformed how obesity is treated medically. However, they differ in mechanism, clinical evidence, NHS access pathways and cost.
This guide provides a thorough, evidence-based comparison to help patients and prescribers understand the differences. All clinical data referenced here is drawn from published peer-reviewed trials, NICE technology appraisals and MHRA-approved prescribing information.
How they work: mechanism of action
Wegovy (semaglutide)
Semaglutide is a GLP-1 receptor agonist. It mimics the naturally occurring incretin hormone GLP-1 to reduce appetite, slow gastric emptying and improve insulin sensitivity. Ozempic contains the same active ingredient at a lower dose, licensed for type 2 diabetes rather than weight management.
Mounjaro (tirzepatide)
Tirzepatide is a dual GIP/GLP-1 receptor agonist — the first medication in its class. By activating both the glucose-dependent insulinotropic polypeptide (GIP) receptor and the GLP-1 receptor simultaneously, Mounjaro works through complementary pathways. GIP receptor activation enhances fat oxidation, improves insulin sensitivity and may increase energy expenditure through mechanisms distinct from GLP-1 alone.
Key distinction: Wegovy acts on one receptor pathway (GLP-1). Mounjaro acts on two (GIP + GLP-1). This dual mechanism is thought to explain the greater average weight loss observed in tirzepatide trials.
Head-to-head comparison table
| Feature | Mounjaro (tirzepatide) | Wegovy (semaglutide) |
|---|---|---|
| Manufacturer | Eli Lilly | Novo Nordisk |
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Administration | Once-weekly subcutaneous injection | Once-weekly subcutaneous injection |
| Starting dose | 2.5 mg | 0.25 mg |
| Maximum dose | 15 mg | 2.4 mg |
| Dose levels | 6 (2.5, 5, 7.5, 10, 12.5, 15 mg) | 5 escalation steps to 2.4 mg maintenance |
| Average weight loss (max dose) | ~20.9% (SURMOUNT-1, 72 weeks) | ~15.2% (STEP 1, 68 weeks) |
| Participants losing ≥20% | ~57% (SURMOUNT-1) | ~32% (STEP 1) |
| HbA1c reduction (T2D) | Up to 2.4% | Up to 1.8% |
| Cardiovascular outcomes | Trial ongoing (SURPASS-CVOT) | Proven 20% CV risk reduction (SELECT) |
| NICE weight management approval | Yes | Yes (+ CV risk reduction April 2026) |
| NHS access route | Specialist weight management services; GPs for T2D | Specialist weight management services |
| Private cost (approx.) | £180–£350/month | £180–£300/month |
Clinical trial evidence compared
SURMOUNT trials (Mounjaro)
The SURMOUNT programme is the primary evidence base for tirzepatide in weight management:
- SURMOUNT-1: 2,539 adults with obesity (no diabetes). Tirzepatide 15 mg produced mean weight loss of 20.9% at 72 weeks versus 3.1% with placebo. Approximately one-third lost more than 25% of body weight.
- SURMOUNT-2: 938 adults with type 2 diabetes and obesity. Tirzepatide 15 mg produced mean weight loss of 14.7% at 72 weeks.
- SURMOUNT-3: Assessed tirzepatide after an initial 12-week intensive lifestyle intervention. Participants achieved total weight loss of approximately 26.6% from original baseline.
- SURMOUNT-4: Withdrawal study demonstrating weight regain after discontinuation, reinforcing the chronic nature of treatment.
STEP trials (Wegovy)
The STEP programme established semaglutide 2.4 mg for weight management:
- STEP 1: 1,961 adults with obesity (no diabetes). Semaglutide 2.4 mg produced mean weight loss of 14.9% at 68 weeks versus 2.4% with placebo.
- STEP 2: 1,210 adults with type 2 diabetes. Mean weight loss of 9.6% at 68 weeks.
- STEP 3: Combined with intensive behavioural therapy. Mean weight loss of 16.0%.
- STEP 5: Two-year data. Weight loss of 15.2% was sustained at 104 weeks.
SELECT trial (cardiovascular outcomes)
The SELECT trial is a key differentiator for Wegovy. This 17,604-participant study demonstrated that semaglutide 2.4 mg reduced major adverse cardiovascular events by 20% in adults with overweight or obesity and established cardiovascular disease, independent of diabetes status. In April 2026, NICE approved Wegovy for cardiovascular risk reduction, making an additional 1.2 million patients eligible.
Why this matters: Mounjaro does not yet have completed cardiovascular outcomes data. The SURPASS-CVOT trial is ongoing. Until those results are published, Wegovy remains the only GLP-1 medication with proven cardiovascular benefit for weight management.
Side effects compared
Both medications share similar gastrointestinal side effect profiles because they both activate the GLP-1 receptor. The most commonly reported adverse events are nausea, diarrhoea, vomiting and constipation, typically occurring during dose escalation.
| Side effect | Mounjaro incidence | Wegovy incidence |
|---|---|---|
| Nausea | 24–33% | 44% |
| Diarrhoea | 18–23% | 30% |
| Vomiting | 9–13% | 24% |
| Constipation | 12–17% | 24% |
| Injection site reactions | 3–7% | 3–5% |
| Treatment discontinuation due to AEs | ~4–7% | ~7% |
Overall, clinical trial data suggests that tirzepatide may be slightly better tolerated than semaglutide 2.4 mg, with lower rates of nausea and vomiting. The six-step dose escalation for Mounjaro (compared with five for Wegovy) may contribute to a more gradual adjustment. For detailed guidance on managing side effects, see our GLP-1 side effects guide.
NHS availability in the UK
Wegovy on the NHS
- Available through specialist weight management services (Tier 3) since 2023
- Eligible patients: BMI ≥35, or BMI ≥30 with at least one weight-related comorbidity
- Must be prescribed alongside a structured behavioural programme
- April 2026: NICE approved for cardiovascular risk reduction (1.2 million additional eligible patients)
- Adjusted BMI thresholds for certain ethnic groups (typically 2.5 points lower)
Mounjaro on the NHS
- Available for type 2 diabetes through GPs and secondary care
- NICE-recommended for weight management through specialist services
- Available via some GPs since June 2025 under expanded NICE guidance
- QOF 2026/27 includes indicators recognising GLP-1 and dual agonist prescribing
For more details on accessing Wegovy through the NHS, see our complete NHS Wegovy guide. For Mounjaro NHS eligibility, see our Mounjaro NHS 2026 guide.
Private prescription costs
Both medications are available through UK online clinics and private pharmacies. Costs vary by provider and dose level:
- Mounjaro: Approximately £180 (2.5 mg) to £350 (15 mg) per month
- Wegovy: Approximately £180 (0.25 mg) to £300 (2.4 mg) per month
Providers include Second Nature, Numan, Voy, MyJuniper and various online pharmacies. See our UK weight loss clinics comparison for a detailed provider guide.
Who should take which?
The choice between Mounjaro and Wegovy depends on individual clinical circumstances. There is no single correct answer, and this decision should always be made with a prescriber.
Mounjaro may be more suitable if:
- You have type 2 diabetes and need both glycaemic control and weight management
- Maximum weight loss is your primary goal
- You have previously tried semaglutide with inadequate results
- You prefer a wider range of maintenance dose options
- You tolerate GI side effects poorly (tirzepatide may have lower nausea rates)
Wegovy may be more suitable if:
- You have established cardiovascular disease and want proven CV risk reduction
- Your prescriber recommends it based on the SELECT trial evidence
- You are already established on semaglutide and responding well
- You prefer the more extensive long-term safety data available for semaglutide
- Your NHS service primarily offers semaglutide-based options
Important: Switching between medications should only be done under medical supervision. If you are considering a change, discuss this with your prescriber who can advise on appropriate washout periods and dose-conversion strategies.
Frequently asked questions
Is Mounjaro more effective than Wegovy?
In separate clinical trials, tirzepatide produced greater average weight loss than semaglutide (approximately 20.9% versus 15.2% at maximum doses). However, no direct head-to-head trial comparing Mounjaro 15 mg with Wegovy 2.4 mg has been completed. Individual results vary significantly, and the best medication depends on your specific clinical needs.
Can I switch from Wegovy to Mounjaro?
Switching is possible under medical supervision. Your prescriber will determine the appropriate starting dose of Mounjaro based on your current semaglutide dose and clinical response. There is no standardised conversion protocol, so this requires individualised clinical judgement.
Which has fewer side effects?
Trial data suggests slightly lower rates of nausea and vomiting with Mounjaro compared with Wegovy, though both medications share similar gastrointestinal side effect profiles. Individual tolerance varies considerably. The gradual dose escalation with both medications helps mitigate side effects.
Will the NHS prescribe either one?
Both are available on the NHS for eligible patients through specialist weight management services. Availability can vary by Integrated Care Board (ICB) and local commissioning arrangements. Your GP can refer you to your local specialist weight management service to determine eligibility.