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 — Flu-Like Symptoms Explained
If you're considering Mounjaro or Wegovy for weight management in the UK, flu-like symptoms are a concern many people ask about. Both prescription weight loss injections can cause side effects during the initial weeks, though the patterns differ. This guide compares the mechanisms behind these symptoms and helps you understand what to expect. Always discuss potential side effects with your prescriber before starting treatment.
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Calculate Your BMI Instantly
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.
Clinical Evidence and Symptom Profiles
Mounjaro (tirzepatide) and Wegovy (semaglutide) work differently, which affects their side effect profiles. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates GLP-1 only. Both can cause gastrointestinal symptoms in the first few weeks, though clinical trial data shows different patterns.
In SURMOUNT trials (Mounjaro), nausea and gastrointestinal symptoms were very common, particularly at dose escalation. In STEP trials (Wegovy), similar gastrointestinal side effects occurred but peaked earlier in treatment and often improved with dose stabilisation. Neither medication directly causes influenza or true flu, but some people describe initial side effects as "flu-like" because they involve fatigue, nausea, and general malaise.
Flu-like symptoms, if they occur, are typically temporary and usually resolve within 1–2 weeks as your body adjusts to the injection. Your prescriber can provide personalised guidance on managing these effects. Try our free GLP-1 weight loss calculator to explore potential outcomes with either medication.
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Which Medication May Be Better for You?
There is no universally "better" choice between Mounjaro and Wegovy—suitability depends on your individual health profile, previous medication responses, and prescriber assessment. Some people tolerate one medication better than the other, particularly regarding early side effects.
A UK-licensed prescriber will assess your eligibility based on your BMI, weight-related health conditions, and medical history. Use our BMI calculator above to check your starting point. Both medications require a prescription and clinical assessment, available through NHS specialist weight management services or private online prescribing. If you experience significant flu-like symptoms with either injection, inform your prescriber immediately—they can adjust your dose or approach.
- 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.
Does Mounjaro cause flu-like symptoms more than Wegovy?
Clinical trials show both cause gastrointestinal side effects, but the patterns differ. Mounjaro may have more persistent nausea during dose escalation, whilst Wegovy symptoms often peak earlier and improve with stabilisation. Individual experiences vary significantly.
How long do flu-like symptoms last on Mounjaro or Wegovy?
If they occur, flu-like side effects typically resolve within 1–2 weeks as your body adjusts. Some people experience no flu-like symptoms at all. Always report severe or persistent symptoms to your GP or prescriber.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro is a dual GIP and GLP-1 agonist (two hormone pathways), whilst Wegovy is a GLP-1 agonist only (single pathway). This difference may affect how your body responds to the medication and its side effect profile.
Can I switch from Mounjaro to Wegovy if I get flu-like symptoms?
Switching medications should only be done under prescriber guidance. Some people tolerate one better than the other, but it requires a clinical decision. Your GP or prescriber can advise whether a switch is appropriate for you.
Are these injections available on the NHS in the UK?
Yes—tirzepatide (Mounjaro) and semaglutide (Wegovy) are approved for NHS specialist weight management services. Access varies by region and may involve waiting lists. Private prescribing is also available through UK-licensed online services.
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.