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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How Mounjaro and Wegovy Compare for Calorie Deficit Weight Loss
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections available in the UK that work alongside calorie deficits to support weight management. While they're both GLP-1 related medications, they work through different mechanisms and show different results in clinical trials. Understanding how each one functions with diet and exercise can help you discuss which option might suit your circumstances with your GP or prescriber. This guide compares both medications factually so you can have an informed conversation about your 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.
Clinical Trial Results and How They Work with Calorie Restriction
Both medications are designed to work alongside a reduced-calorie diet and increased physical activity — they are not replacements for lifestyle changes. In the SURMOUNT-1 clinical trial, Mounjaro showed average weight loss of approximately 15–21% of body weight over 72 weeks at maximum dose (15 mg), whilst the STEP-1 trial showed Wegovy achieved approximately clinically studied weight loss at maximum dose (2.4 mg) over 68 weeks. Both trials involved participants following reduced-calorie diets, typically around 500 kcal below their daily need. The key difference is mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways to reduce appetite and support satiety, whilst Wegovy is a GLP-1 agonist only, activating a single pathway. This dual action in Mounjaro may explain slightly higher average weight loss, though individual results vary significantly. Neither medication creates weight loss without a calorie deficit — they help make calorie restriction more manageable by reducing hunger and cravings.
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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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Dose Titration, Calorie Needs, and Individual Suitability
Mounjaro requires dose titration starting at 2.5 mg weekly, escalating every 4 weeks up to a maximum of 15 mg, whilst Wegovy starts at 0.25 mg and escalates over 16 weeks to 2.4 mg maximum. Both require a UK-licensed prescriber to assess your eligibility and monitor your progress. Your daily calorie needs depend on your age, activity level, and metabolism — our GLP-1 weight loss calculator can help estimate potential results based on your starting weight and dose. Calorie requirements typically decrease as you lose weight, so your prescriber may adjust your calorie target during treatment. Neither medication is suitable for everyone: both are prescription-only, require clinical assessment, and may not be prescribed during pregnancy or whilst breastfeeding. Your GP or specialist will consider your medical history, other medications, and health conditions when deciding 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.
Does Mounjaro work better than Wegovy for weight loss?
Clinical trials show Mounjaro achieved slightly higher average weight loss (15–21%) compared to Wegovy (15%), but both results were achieved alongside calorie deficit diets. Individual results vary significantly and depend on adherence, dose tolerance, lifestyle changes, and individual metabolism. Neither medication is "better" — suitability depends on your personal health circumstances and prescriber assessment.
Can I take these injections without reducing calories?
No. Both Mounjaro and Wegovy are designed to work alongside a reduced-calorie diet and increased physical activity. Clinical trials showed weight loss when participants combined medication with lifestyle changes. The medication helps make calorie restriction easier by reducing hunger, but a calorie deficit is still necessary for weight loss.
How much of a calorie deficit do I need with Mounjaro or Wegovy?
Most clinical trials used approximately 500 kcal daily deficits, which supports gradual weight loss of around 0.5 kg per week. Your prescriber will discuss appropriate calorie targets based on your current intake, health conditions, and weight loss goals. These recommendations are personalised — not one-size-fits-all.
Will my calorie needs change as I lose weight?
Yes. As you lose weight, your daily calorie needs decrease because a smaller body requires less energy. Your prescriber or dietitian will adjust your calorie target periodically during treatment. This is why it's important to have ongoing medical supervision whilst taking these medications.
Which injection is available on the NHS in the UK?
NICE has approved tirzepatide (Mounjaro) within NHS specialist weight management services, though availability varies significantly by region and local commissioning. Wegovy access on the NHS is more limited. Both are available privately through UK-licensed online prescribers following clinical assessment, independent of NHS referral.
Ready to take control of your weight loss journey?
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