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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What's the difference between Mounjaro and Wegovy in the UK?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work differently and have different access routes. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. Understanding the key differences helps you have an informed conversation with your GP or prescriber about which medication might be suitable for you. This guide covers the mechanism, doses, clinical trial results, and NHS access for both injections.
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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) activates two natural hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may help regulate appetite and support feeling fuller after smaller meals. Wegovy (semaglutide) activates GLP-1 only, which also reduces appetite and slows gastric emptying. The dual mechanism in Mounjaro may explain why clinical trials showed slightly higher average weight loss, though individual results vary significantly.
- Mounjaro: Dual GIP + GLP-1 agonist; starting dose 2.5 mg, maximum 15 mg once weekly
- Wegovy: GLP-1 agonist only; starting dose 0.25 mg, maximum 2.4 mg once weekly
- Both are once-weekly injections administered via a pre-filled pen device
- Both require a prescription and clinical assessment before being prescribed
Use our GLP-1 weight loss calculator to explore potential weight loss projections at different doses with either medication.
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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.
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NHS eligibility and access pathways in the UK
Both Mounjaro and Wegovy are approved by NICE for use within NHS specialist weight management services (tier 3/4). To access either medication through the NHS, your GP can refer you to your local integrated care board's (ICB) specialist weight management service. However, NHS access is limited and waiting times vary significantly by region—some areas have waiting lists of several months or longer. Private access is available through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber, often with faster availability. Your GP can advise on both routes and assess your individual eligibility based on BMI and health conditions.
Check your personal BMI with our free BMI calculator to understand your starting point and discuss with your healthcare provider.
Clinical trial data shows Mounjaro achieved average weight loss of approximately 15–21% of body weight, while Wegovy achieved approximately 14.9% in the STEP-1 trial. However, these are averages—individual results vary considerably. Both medications work best when combined with a reduced-calorie diet and increased physical activity. Your prescriber will assess which medication is most appropriate based on your health history, current medications, and personal preferences. Neither medication is a substitute for lifestyle changes.
- 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 or Wegovy better for weight loss in the UK?
Both are effective weight loss medications. Clinical trials show Mounjaro achieved slightly higher average weight loss (~15–21%) than Wegovy (~14.9%), but individual results vary significantly. Your prescriber will recommend based on your medical history, other medications, and personal circumstances. Neither is universally "better"—suitability depends on your individual health profile.
Can I get Mounjaro or Wegovy on the NHS in the UK?
Yes, both are NICE-approved for NHS specialist weight management services (tier 3/4). Your GP can refer you to your local service. However, NHS availability and waiting times vary considerably by region—some areas have long waiting lists. Private access through an online prescriber is often faster but involves private cost.
What's the difference between the doses of Mounjaro and Wegovy?
Mounjaro starts at 2.5 mg and goes up to 15 mg once weekly. Wegovy starts at 0.25 mg and goes up to 2.4 mg once weekly. Approximate equivalence: Mounjaro 5 mg ≈ Wegovy 0.5 mg, Mounjaro 10 mg ≈ Wegovy 1.7 mg, Mounjaro 15 mg ≈ Wegovy 2.4 mg. Any switch between medications must be managed by your prescriber—there is no official conversion chart.
Are the side effects of Mounjaro and Wegovy the same?
Side effects are very similar because both activate GLP-1 pathways. Common side effects include nausea, diarrhoea, constipation, and stomach pain—especially when first starting or increasing doses. Most side effects are temporary and reduce over time. Serious side effects (severe stomach pain, gallbladder problems, allergic reactions) are rare but require immediate medical attention.
How long does it take to see weight loss with Mounjaro versus Wegovy?
Weight loss typically begins within 2–4 weeks for both medications, though some people notice appetite reduction sooner. Most significant weight loss occurs between weeks 4 and 16 as doses are gradually increased. Full effects are usually seen after 16–20 weeks at an effective maintenance dose. Track your progress with our free progress tracker.
Ready to take control of your weight loss journey?
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