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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Taking Mounjaro vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections available in the UK, but they work differently and have different dose ranges. If you're considering taking either medication, understanding how they compare can help you and your GP or prescriber decide which might be suitable for you. Both require a clinical assessment before prescription. This guide breaks down the key differences to help you make an informed decision.
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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
The fundamental difference between Mounjaro and Wegovy lies in their mechanism of action. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in the body simultaneously. This dual action may help regulate appetite, reduce food cravings, and support satiety more intensively than single-mechanism medications.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. Both medications slow gastric emptying and signal fullness to the brain, but Mounjaro's additional GIP activation may provide a different weight loss profile.
In clinical trials, Mounjaro demonstrated average weight loss of approximately 15–21% of body weight (SURMOUNT-1 trial), whilst Wegovy showed approximately 15% average weight loss (STEP-1 trial). However, individual results vary significantly, and trial results were achieved alongside reduced-calorie diet and increased physical activity. Use our GLP-1 weight loss calculator above to explore projected outcomes at different doses.
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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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Doses, Side Effects, and Individual Suitability
Mounjaro dosing starts at 2.5 mg weekly and escalates through 5 mg, 7.5 mg, 10 mg, 12.5 mg, to a maximum of 15 mg weekly. Wegovy dosing begins at 0.25 mg and escalates through 0.5 mg, 1 mg, 1.7 mg, to a maximum of 2.4 mg weekly.
Both medications share similar common side effects including nausea, diarrhoea, vomiting, and constipation—typically occurring during dose escalation and often improving over time. Serious side effects are rare but require urgent attention (severe stomach pain, serious allergic reaction, gallbladder problems).
Which medication suits you depends on individual factors including your health history, BMI, weight-related conditions, and how your body responds. A UK-licensed prescriber will assess your eligibility and suitability following a clinical consultation. Both are prescription-only medicines requiring ongoing monitoring.
- 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 show Mounjaro achieved slightly higher average weight loss (15–21%) compared to Wegovy (15%), but individual results vary significantly. Some people respond better to one medication than the other. Your prescriber will assess which is most suitable for your circumstances.
Can I switch from Wegovy to Mounjaro if it's not working?
Switching between weight loss injections should only be done under prescriber guidance. There is no official conversion chart—any switch requires a new clinical assessment and dose titration. Discuss with your prescriber if you feel a medication isn't working for you.
Which injection has fewer side effects—Mounjaro or Wegovy?
Both share similar common side effects (nausea, diarrhoea, constipation). Incidence and severity vary between individuals, not systematically between the medications. Side effects often improve during dose escalation. Your prescriber can discuss which medication may suit your tolerability profile.
Do I need a GP referral to get Mounjaro or Wegovy in the UK?
NHS access requires referral to a specialist tier 3 weight management service through your GP—access varies by region and waiting lists can be lengthy. Private prescribing is available through online consultations with UK-licensed prescribers, without NHS referral. Both routes require a clinical assessment.
How long does it take to see weight loss results with these injections?
Weight loss typically begins within 2–4 weeks but varies between individuals. Both medications work alongside dietary changes and increased physical activity—medication alone is not effective. Track your progress using our free progress tracker to monitor changes over time.
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.