The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Mounjaro vs Wegovy — UK Cost, Eligibility & Mechanism Comparison
Mounjaro and Wegovy are both prescription-only weight loss injections available in the UK, but they work differently and have distinct cost profiles. This guide explains the key differences between tirzepatide (Mounjaro) and semaglutide (Wegovy), including how their mechanisms differ, UK eligibility criteria, and what you should know before speaking to your GP or a licensed prescriber. Both medications require clinical assessment and are used alongside lifestyle changes for sustainable weight management.
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What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
How Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in the body. This dual mechanism may provide stronger appetite suppression and support for feeling fuller after smaller meals. Clinical trials (SURMOUNT-1) showed an average weight loss of approximately 15–21% of body weight over 72 weeks at the maximum dose of 15 mg.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. The STEP-1 clinical trial demonstrated an average weight loss of approximately 14.9% of body weight at the maximum dose of 2.4 mg over 68 weeks. Both medications slow gastric emptying and reduce appetite, but Mounjaro's dual action may result in slightly greater weight loss on average—though individual results vary significantly.
- Mounjaro: GIP + GLP-1 agonist; maximum dose 15 mg/week
- Wegovy: GLP-1 agonist only; maximum dose 2.4 mg/week
- Both: once-weekly injections; prescription-only; require clinical assessment
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Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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UK Eligibility, Cost & Practical Considerations
Both Mounjaro and Wegovy require a BMI of 30 or above, or 27–29.9 with at least one weight-related health condition (type 2 diabetes, hypertension, cardiovascular disease), to be eligible. NHS access is through specialist tier 3 weight management services, though waiting lists vary significantly by region. Private prescribing is available online through GPhC-registered pharmacies following a clinical assessment. Private costs range from approximately £129–£320 per month depending on dose and provider. Your GP or a licensed prescriber will assess which medication (if any) is suitable for your individual health profile, considering side effects, medical history, and personal circumstances. Both require ongoing use to maintain weight loss.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
What is the main difference between Mounjaro and Wegovy?
Mounjaro contains tirzepatide and activates both GIP and GLP-1 pathways (dual mechanism). Wegovy contains semaglutide and activates only the GLP-1 pathway (single mechanism). This dual action in Mounjaro may result in slightly stronger appetite suppression, though both are effective. Clinical trial data showed Mounjaro averaged 15–clinically studied weight loss versus Wegovy's clinically studied weight loss, but individual results vary significantly.
Which medication is cheaper in the UK — Mounjaro or Wegovy?
Private prescription costs for both medications are similar, typically ranging from £129–£320 per month depending on dose and provider. NHS access is limited to specialist weight management services and varies by region; waiting times can be lengthy. Costs are not publicly set differently between the two. Your prescriber will discuss costs and availability with you.
Can I switch from Wegovy to Mounjaro on the NHS?
Switching between medications on the NHS would need to be managed through your specialist weight management service. Approximate dose equivalences exist (0.5 mg Wegovy ≈ 5 mg Mounjaro; 2.4 mg Wegovy ≈ 15 mg Mounjaro), but any switch requires prescriber guidance and must be done safely over time. Discuss this directly with your NHS specialist team.
How do I know which injection is right for me — Mounjaro or Wegovy?
A licensed prescriber will assess your medical history, current medications, side effect tolerance, and health goals to determine suitability. Both require clinical assessment. Factors include BMI, comorbidities, whether you're taking other medications (especially oral contraceptives or diabetes medicines), and your preference for once-weekly versus dosing frequency. Your GP or a private prescriber can guide this decision.
Do Mounjaro and Wegovy cause the same side effects?
Both commonly cause nausea, vomiting, diarrhoea, and constipation—especially when starting or increasing doses. Side effect profiles are very similar because both are GLP-1 pathway activators. Some individuals may tolerate one better than the other. Serious side effects (pancreatitis, gallbladder problems) are rare but possible with both. Always inform your prescriber of any side effects.
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