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 New Rules UK: How It Compares to Wegovy
Mounjaro and Wegovy are both prescription weight loss injections available in the UK, but they work differently and have distinct eligibility criteria under current NHS and private prescribing rules. Mounjaro uses a dual GIP and GLP-1 mechanism, while Wegovy is a GLP-1-only injection. Both are prescription-only medicines requiring clinical assessment. This guide compares how they work, what the clinical trials showed, and how prescribers decide between them based on individual suitability. Use our free BMI calculator and GLP-1 weight loss calculator to check your starting point and explore potential outcomes with each medication.
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Calculate Your BMI Instantly
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.
Clinical Trial Data and Mechanism Differences
Mounjaro (tirzepatide) activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). In the SURMOUNT-1 clinical trial, participants on the maximum 15 mg dose lost an average of 20.9% of body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. Those on 10 mg lost approximately 19.5%.
Wegovy (semaglutide) activates GLP-1 only. In the STEP-1 clinical trial, participants on the maximum 2.4 mg dose lost an average of 14.9% of body weight over 68 weeks under similar lifestyle conditions.
- Mounjaro's dual mechanism may account for greater average weight loss in trials
- Individual results vary significantly — trial averages do not predict personal outcomes
- Both require reduced-calorie diet and physical activity for results
- Mounjaro uses a kwikpen device with doses from 2.5 mg to 15 mg weekly
- Wegovy uses an autoinjector with doses from 0.25 mg to 2.4 mg weekly
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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.
Eligibility, Prescribing Rules, and Individual Suitability
Both Mounjaro and Wegovy follow the same UK eligibility rules: adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, prediabetes, high blood pressure, dyslipidaemia, obstructive sleep apnoea, or cardiovascular disease history). Lower BMI thresholds apply for South Asian, Chinese, Black African, and Black Caribbean adults. NICE has approved both medications for use within NHS specialist weight management services.
Private prescribing is available through registered online clinics following clinical assessment. Prescribers assess factors including tolerability of gastrointestinal side effects, baseline health conditions, concomitant medications (oral contraceptives interact with both), and individual response patterns. Neither medication is "better" universally — suitability depends on your personal health profile, medical history, and prescriber judgement. Your GP or licensed prescriber can assess which, if either, is appropriate for you.
- 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 better than Wegovy for weight loss?
Clinical trials show Mounjaro produced slightly greater average weight loss (15–21%) than Wegovy (15%), but individual results vary significantly. Trial averages do not predict personal outcomes. Suitability depends on your health profile, tolerability, and prescriber assessment — not trial data alone.
What's the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist; Wegovy is GLP-1 only. Mounjaro is injected once weekly using a kwikpen (2.5 mg to 15 mg). Wegovy is also weekly (0.25 mg to 2.4 mg). Both work on appetite and satiety but through slightly different mechanisms.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, switching is possible and may be managed by your prescriber if one medication is not tolerated or less effective. There is no official conversion chart — approximate equivalence is 2.4 mg Wegovy ≈ 15 mg Mounjaro. Any switch must be managed by your prescriber, not self-directed.
Are Mounjaro and Wegovy available on the NHS in the UK?
Both are NICE-approved for NHS use within specialist tier 3 weight management services. NHS access is through referral from your GP and varies by region (ICB) and availability — waiting lists can be lengthy. Private prescribing is also available through registered online clinics.
What are the main side effects of each medication?
Both commonly cause nausea, diarrhoea, vomiting, and stomach pain, particularly during dose escalation. Side effects typically diminish over time. Serious side effects (severe stomach pain, pancreatitis risk, gallbladder problems) are rare but require urgent medical attention. Your prescriber will discuss tolerance risks based on your health.
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.