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 Jabs vs Wegovy — Which Weight Loss Injection?
If you're exploring weight loss treatment options in the UK, you've likely heard about both Monjaro jabs and Wegovy. Both are prescription-only weekly weight loss injections approved for UK adults, but they work differently and produce different results. Understanding the key differences between tirzepatide (Monjaro) and semaglutide (Wegovy) will help you and your prescriber decide which option might be suitable for your individual health situation. This guide breaks down the facts.
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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 Differ — Mechanism and Trial Results
Monjaro (tirzepatide) is a dual GIP receptor and GLP-1 receptor agonist — it activates two natural hormone pathways. Wegovy (semaglutide) is a GLP-1 receptor agonist only — it targets one pathway. This dual mechanism is what sets Monjaro apart.
In clinical trials, average weight loss differed:
- Monjaro (SURMOUNT trial): Approximately 15–clinically studied weight loss at 72 weeks on the maximum 15 mg dose
- Wegovy (STEP trial): Approximately clinically studied weight loss at 68 weeks on the maximum 2.4 mg dose
These are clinical trial averages with reduced-calorie diet and increased physical activity. Individual results vary significantly. Some people lose more, some less. Neither medication assures specific weight loss outcomes.
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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 Getting Prescribed — What You Need to Know
Monjaro starts at 2.5 mg weekly, escalating to a maximum of 15 mg. Wegovy starts at 0.25 mg, escalating to 2.4 mg maximum. Both are once-weekly weekly weight loss injections available as pre-filled pens (Monjaro uses a KwikPen). Both have similar side effects — nausea, diarrhoea, constipation, and stomach pain are most common, typically improving over time.
Both require a UK prescription through either NHS specialist weight management services or private online prescribing. Your GP or prescriber will assess your health, BMI, and any weight-related conditions to determine suitability. Use our GLP-1 weight loss calculator to explore potential outcomes, and our BMI calculator to assess your starting point.
Cost, availability, and personal health factors determine which is right for you — not one is universally "better". Monjaro may suit someone looking for a dual mechanism approach; Wegovy may suit someone starting with a single GLP-1 approach or preferring established trial data. Your prescriber will guide you based on your individual situation, medical history, and response preferences.
- 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 or Wegovy more effective for weight loss?
Clinical trials show Monjaro achieved slightly higher average weight loss (15–21% vs 14.9%), but this varies significantly between individuals. Neither is "better" universally — effectiveness depends on your personal response, adherence, diet, and activity levels. Your prescriber will recommend the most suitable option for you.
What's the main difference between Monjaro and Wegovy?
Monjaro is a dual GIP and GLP-1 receptor agonist (activates two hormone pathways), while Wegovy is a GLP-1 receptor agonist only (single pathway). This dual mechanism in Monjaro may explain slightly higher trial weight loss, but individual response varies considerably.
Can I switch from Wegovy to Monjaro or vice versa?
Yes, people switch between these medications, but only under prescriber guidance. There's no official conversion chart — approximate equivalences exist (Monjaro 10 mg ≈ Wegovy 1.7 mg) but switching requires medical oversight to ensure safe transition and appropriate dosing.
How do I get Monjaro or Wegovy on the NHS?
Both are available through NHS tier 3 specialist weight management services. You'll need a GP referral and assessment. NHS availability varies by region and waiting lists can be lengthy. Private online prescribing is also available in the UK without NHS referral.
Do Monjaro and Wegovy have the same side effects?
Yes, both share similar common side effects: nausea, diarrhoea, constipation, and stomach pain. Side effects usually improve as your body adjusts. Rare serious side effects (pancreatitis, gallbladder issues) can occur with both. Always report concerning symptoms to your prescriber.
Ready to take control of your weight loss journey?
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