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 Kwikpen 2.5 mg vs Wegovy — Starting Doses Explained
Mounjaro and Wegovy are both prescription-only weight loss injections available in the UK, but they work differently and start at different doses. This guide compares Mounjaro's starting dose of 2.5 mg (in the Kwikpen device) with Wegovy's 0.25 mg opening dose, explains the clinical trial evidence behind each, and helps you understand why prescribers might recommend one over the other. Both require clinical assessment and are not available on the NHS as routine treatment, though specialist services may provide access. Individual suitability depends entirely on your health profile and prescriber judgment.
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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 Differently — Trial Data and Mechanisms
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two separate hormone pathways in the body. Clinical trial data from SURMOUNT-1 showed that participants taking Mounjaro 2.5 mg as the starting dose lost an average of 15% of their body weight over 72 weeks when combined with diet and exercise. The dose is escalated every 4 weeks if tolerated, reaching up to 15 mg maximum.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single hormone pathway. The STEP-1 clinical trial demonstrated approximately 14.9% average body weight loss at the maximum dose of 2.4 mg after 68 weeks. Wegovy's starting dose is much lower at 0.25 mg, escalating more gradually over 16 weeks to reach 2.4 mg. This slower titration may mean fewer early side effects but extends the time to reach maximum dose. Use our free GLP-1 weight loss calculator to understand how these dose differences translate to potential outcomes for your circumstances.
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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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Which Is Right for You? Clinical Assessment and Individual Suitability
Neither Mounjaro nor Wegovy is superior for everyone — suitability is determined by your individual health profile, medical history, and how you respond to treatment. Both are prescription-only medicines requiring a prescription from a UK-licensed prescriber following a clinical assessment. Your GP or a qualified healthcare professional will consider your BMI, any weight-related health conditions, current medications, and whether you have any contraindications. Some people tolerate the faster titration of Mounjaro better; others prefer Wegovy's slower dose escalation. Cost, availability, and personal preference also matter. Use our free BMI calculator above to check your current status — this helps frame the conversation with your prescriber about whether weight loss medication 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.
Why does Mounjaro start at 2.5 mg but Wegovy starts at just 0.25 mg?
The drugs have different mechanisms and molecular structures. Mounjaro's dual GIP+GLP-1 action is potent at low doses, while Wegovy's single GLP-1 action requires a gentler escalation to minimise early nausea and digestive side effects. Both approaches are evidence-based; the different starting points reflect their pharmacology, not relative strength.
Which injection loses more weight — Mounjaro or Wegovy?
Clinical trials show similar average weight loss: Mounjaro approximately 15–21% of body weight, Wegovy approximately 14.9%. Results vary significantly between individuals. Trial designs differed slightly, making direct comparison difficult. Your prescriber will discuss realistic expectations based on your health profile.
Can I switch from Mounjaro to Wegovy or vice versa?
Switching between medications must be managed by your prescriber — there is no official conversion chart. Approximate equivalences exist based on clinical practice, but any change requires medical supervision to ensure safe transition and ongoing monitoring.
Is Mounjaro available on the NHS in the UK?
NICE has approved tirzepatide for NHS specialist weight management services, but access is limited to tier 3/4 services and varies significantly by region. Waiting lists can be very long. Private prescription remains the fastest route in most areas.
What are the main side effects of Mounjaro's 2.5 mg starting dose?
Common side effects include nausea, diarrhoea, constipation, and stomach pain. These often improve as your body adjusts. Report persistent or severe symptoms to your prescriber. Never stop the injection without medical advice — dose adjustment may help.
Ready to take control of your weight loss journey?
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