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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Monjaro vs Wegovy — Which Weight Loss Injection Is Right for You?
Both Monjaro and Wegovy are prescription-only weight loss injections licensed in the UK, but they work differently and produce different results. Monjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 receptor agonist only. Understanding the differences between these two medications can help you and your prescriber decide which might be more suitable for your individual circumstances. This guide compares the key aspects of both injections based on clinical trial data and UK medical guidance.
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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 Monjaro and Wegovy Work — Clinical Trial Results
Monjaro (tirzepatide) activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help regulate appetite, reduce food cravings, and support feeling fuller after meals. In the SURMOUNT-1 clinical trial, participants on the maximum 15 mg dose lost an average of 20.9% of their body weight over 72 weeks alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) activates only the GLP-1 pathway. In the STEP-1 clinical trial, participants on the 2.4 mg dose lost an average of 14.9% of their body weight over 68 weeks with the same lifestyle support. Both medications are once-weekly injections, though Monjaro uses a higher-capacity GIP receptor mechanism, which some evidence suggests may produce greater average weight loss.
Our GLP-1 weight loss calculator can help you explore projected outcomes at different doses for context.
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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 UK Eligibility — What's the Same and Different
Both medications are prescription-only and require a clinical assessment before being prescribed. You're eligible if you have a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes or hypertension. Monjaro doses range from 2.5 mg to 15 mg weekly; Wegovy doses range from 0.25 mg to 2.4 mg weekly. Common side effects for both include nausea, diarrhoea, vomiting, and constipation—though these often reduce over time. The main difference: Monjaro may require longer to escalate through doses (minimum 4 weeks per dose), whilst Wegovy typically escalates more quickly. Your GP or prescriber will assess which suits your health profile and circumstances best.
- 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 Monjaro more effective than Wegovy?
Clinical trial data shows Monjaro produced slightly higher average weight loss (20.9% vs 14.9% on maximum doses). However, individual results vary significantly, and "more effective" depends on your personal response. Both are effective medications—the best choice is the one your prescriber assesses as most suitable for your health profile and circumstances.
Can I switch from Wegovy to Monjaro or vice versa?
Switching between medications is possible but must be managed by your prescriber. There is no official dose conversion chart—approximate equivalences exist (e.g., Wegovy 2.4 mg ≈ Monjaro 15 mg), but any switch requires medical supervision and reassessment. Your prescriber will advise based on your response and health needs.
Are side effects different between Monjaro and Wegovy?
Both cause similar side effects (nausea, diarrhoea, constipation, stomach pain) because they work on similar appetite-regulating pathways. Side effect severity and duration vary between individuals. Monjaro's dual mechanism doesn't make it inherently "worse" or "better" for tolerability—it's very individual.
How do I know which one is right for me?
You cannot choose yourself—only a prescriber can assess eligibility and recommend which medication suits your health profile. Factors include your BMI, existing health conditions, medication interactions, and personal medical history. Book a consultation with a GP or licensed prescriber for personalised advice.
What's the difference between GLP-1 and GIP receptors?
GLP-1 regulates appetite and blood sugar. GIP also regulates appetite and may support fat utilisation. Monjaro activates both; Wegovy activates GLP-1 only. This dual mechanism is why Monjaro shows slightly higher average weight loss in trials, though both are effective for weight management.
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.