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 — What's the Difference?
If your GP has discussed weight loss medication with you, you may have heard about Mounjaro and Wegovy. Both are prescription-only injections used for weight management in UK adults, but they work differently. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Understanding how they differ — in mechanism, dosing, clinical trial results, and side effects — can help you and your GP decide which might be suitable for you. Our GLP-1 weight loss calculator can help you estimate potential outcomes with either medication.
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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 — Mechanism Differences
Mounjaro (tirzepatide) activates two natural hormone pathways in the body: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help regulate appetite, reduce cravings, and support feelings of fullness more effectively. Wegovy (semaglutide) activates only the GLP-1 pathway, which also reduces appetite and promotes satiety but through a single mechanism.
In clinical trials, the SURMOUNT study showed Mounjaro participants achieved average weight loss of 15–21% of body weight (depending on dose), whilst the STEP trial showed Wegovy participants lost approximately 15% on average. Both medications work alongside a reduced-calorie diet and increased physical activity — they are not standalone treatments. Individual results vary significantly, and neither medication assures weight loss. Your GP can discuss which mechanism might suit your individual health profile.
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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
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Dosing, Trial Data, and Clinical Suitability
Mounjaro starts at 2.5 mg weekly and titrates up to a maximum of 15 mg, whilst Wegovy starts at 0.25 mg and reaches a maximum of 2.4 mg. Both are once-weekly injections given as pre-filled pens. Side effects are similar for both — nausea, diarrhoea, and constipation are most common, though these typically improve with time. Serious side effects are rare but require immediate medical attention.
Eligibility for either medication is the same: UK adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, cardiovascular disease, or sleep apnoea). Your GP will assess your individual suitability, medical history, and current medications before prescribing. Both Mounjaro and Wegovy are prescription-only and require ongoing clinical assessment. Use our BMI calculator to check your current BMI and discuss options with your GP.
- 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.
What's the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways, whilst Wegovy is a GLP-1 agonist only. This dual mechanism may contribute to slightly higher weight loss in clinical trials, though individual results vary. Both require prescription and clinical assessment from your GP.
Which one will my GP prescribe?
Your GP will consider your individual health profile, medical history, previous treatments, and preferences. Both are licensed in the UK for weight management. Some GPs may have experience with one more than the other. NHS access is typically through specialist tier 3 weight management services; private prescribing is available without NHS referral through online clinical assessments.
Are the side effects the same for both?
Side effects are very similar: nausea, diarrhoea, constipation, and stomach pain are most common. Both medications can cause injection site reactions. Serious side effects like pancreatitis are rare. Your GP will discuss which medication's side effect profile suits you best based on your health history.
How much weight will I lose on either medication?
Clinical trials show average weight loss of 15–21% with Mounjaro and approximately 15% with Wegovy, but individual results vary significantly. Weight loss depends on dose, adherence, diet, physical activity, and individual metabolism. Your GP can discuss realistic expectations for your situation.
Can I switch between Mounjaro and Wegovy?
Switching between medications is possible but must be managed by your GP or prescriber. There is no official established conversion chart — approximate equivalences exist based on clinical practice, but any switch requires a prescription adjustment and ongoing clinical monitoring.
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.