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 for Weight Loss on the NHS
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription weight loss injections available in the UK, but they work differently and have different clinical trial results. This guide compares the two medications side-by-side to help you understand how they differ — though your prescriber will decide which is most suitable for your individual health. Both are licensed for adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Read on to see the clinical evidence and mechanism differences between these two weekly weight loss injection options.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
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 your body. This dual mechanism may help regulate appetite more effectively and support feeling fuller after smaller meals. GIP (glucose-dependent insulinotropic polypeptide) is a second hormone that plays a role in energy balance and glucose control.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. GLP-1 works by reducing appetite and slowing how quickly food leaves your stomach, but it does not activate the GIP pathway like Mounjaro does.
In clinical trials, this difference mattered: the SURMOUNT-1 trial showed Mounjaro users lost an average of 20.9% of body weight at the maximum dose (15 mg weekly), whilst the STEP-1 trial showed Wegovy users lost approximately 14.9% at its maximum dose (2.4 mg weekly). Individual results vary significantly based on diet, activity, genetics, and adherence.
- Mounjaro: dual GIP+GLP-1 mechanism, once-weekly injection, doses up to 15 mg
- Wegovy: single GLP-1 mechanism, once-weekly injection, doses up to 2.4 mg
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How It Works
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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
NHS Access and Choosing Between Them
Access to both medications on the NHS is through specialist tier 3 or 4 weight management services — your GP cannot prescribe them directly. Your prescriber will assess your individual health, medical history, and any other conditions (such as type 2 diabetes or heart disease) to decide which medication is more suitable for you.
Both medications are prescription-only and require a clinical assessment. If one is ineffective or causes side effects, your prescriber may switch you to the other. Factors your prescriber considers include: your previous weight loss attempts, other medications you take, contraindications, and your health goals. Neither medication is "better" — the best choice is the one your prescriber determines is safest and most effective for your specific circumstances. Use our free GLP-1 weight loss calculator to estimate potential weight loss at different doses to discuss with your healthcare team.
- 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.
Is Mounjaro more effective than Wegovy for weight loss?
Clinical trials suggest Mounjaro may lead to slightly greater average weight loss (20.9% vs 14.9% body weight), but individual results vary widely. Effectiveness depends on your adherence, diet, activity level, genetics, and how your body responds to the medication. Your prescriber will choose based on your individual health profile, not just average trial data.
Can I switch from Wegovy to Mounjaro on the NHS?
Yes, if your prescriber determines a switch is appropriate. This might happen if Wegovy is ineffective, causes side effects, or if your health circumstances change. Any switch between medications must be managed by your prescriber — never change medications without medical guidance.
What's the main difference between Mounjaro and Wegovy mechanisms?
Mounjaro activates both GIP and GLP-1 hormone pathways (dual action), whilst Wegovy activates only the GLP-1 pathway. The dual mechanism in Mounjaro may contribute to slightly higher average weight loss in trials, though both medications reduce appetite and support feeling fuller after meals.
How long does it take to see weight loss with either medication?
Weight loss typically begins within the first 2–4 weeks, but most significant loss occurs after 8–12 weeks as your dose increases and your body adjusts. Both medications work best alongside a reduced-calorie diet and increased physical activity — medication alone is not sufficient for sustainable weight loss.
Which medication is available on the NHS more readily?
Availability varies by region and ICB (Integrated Care Board). Some areas may have Mounjaro in stock whilst others prioritise Wegovy, or vice versa. Ask your GP or specialist weight management service which medications are currently available in your area — supply can change frequently.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.