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 — How These Weight Loss Injections Compare
Both Monjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work differently and show different results in clinical trials. Monjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates GLP-1 only. Understanding how they differ can help you and your prescriber decide which might be suitable for your circumstances. This guide compares the two fairly, using trial data and clinical information relevant to UK adults.
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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 Monjaro and Wegovy Work — Key Differences
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 more effectively by working on two natural signalling systems in the body. 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 GLP-1 only — a single hormone pathway that signals fullness and reduces appetite. In the STEP-1 clinical trial, participants on the maximum 2.4 mg dose lost an average of 14.9% of their body weight over 68 weeks with the same lifestyle support.
Important context: These are clinical trial averages. Individual results vary significantly, and both medications work best alongside reduced-calorie diet and increased physical activity. Use our free GLP-1 weight loss calculator 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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Dosing, Side Effects, and Individual Suitability
Dosing: Monjaro ranges from 2.5 mg to 15 mg weekly; Wegovy ranges from 0.25 mg to 2.4 mg weekly. Both are once-weekly injections. Monjaro typically reaches its maximum dose after 20 weeks; Wegovy after 16 weeks.
Side effects: Both commonly cause nausea, diarrhoea, vomiting, and constipation, especially early in treatment. Individual tolerance varies. Neither is suitable for people who are pregnant, breastfeeding, or under 18.
Which is right for you? A prescriber will assess your BMI, health conditions, and medical history to recommend the most suitable option. Both require a clinical assessment before prescription. Your GP or a licensed prescriber can discuss which may be appropriate based on your individual circumstances.
- 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 trials show Monjaro produced slightly higher average weight loss (20.9% vs 14.9% at maximum doses). However, individual results vary considerably, and trial conditions differ. A prescriber assesses your personal suitability rather than effectiveness alone. Both are effective options for many UK adults.
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 conversion chart — approximate equivalences exist (e.g., Monjaro 10 mg ≈ Wegovy 1.7 mg), but any switch requires clinical review and reassessment.
Which injection costs less on the NHS?
Both are available through NHS specialist weight management services, but access and cost vary by region and individual eligibility. Waiting lists are common. Private prescription costs also vary by dose and provider. Your GP can discuss NHS availability in your area.
What's the difference between GLP-1 and GIP?
GLP-1 (glucagon-like peptide-1) is a natural hormone that signals fullness after eating. GIP (glucose-dependent insulinotropic polypeptide) is another natural hormone that affects appetite and blood sugar. Monjaro activates both; Wegovy activates GLP-1 only. This dual action may explain Monjaro's slightly higher average weight loss in trials.
Are there any reasons a prescriber might recommend one over the other?
Yes. A prescriber considers your medical history, other medications, previous responses to GLP-1 treatment, kidney or heart function, and personal health goals. Some people may not be suitable for one medication but suitable for another. Your prescriber will assess this during a clinical assessment.
Ready to take control of your weight loss journey?
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