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 Flu Symptoms vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections used in the UK, and both can cause flu-like symptoms in some users. The key difference lies in their mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates GLP-1 only. This affects not just weight loss potential, but also the type and severity of side effects each medication may trigger. Understanding these differences helps you and your prescriber decide which might suit your individual health needs. Use our free GLP-1 weight loss calculator to explore projected outcomes with either medication.
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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.
Mounjaro vs Wegovy: Flu Symptoms and Clinical Trial Data
Mounjaro (tirzepatide) flu-like symptoms reported in clinical trials (SURMOUNT-1) included nausea, fatigue, and general malaise, particularly during dose escalation weeks. These typically resolve within 3–5 days as the body adjusts. Fatigue was reported by approximately 8–12% of participants, while dizziness and general tiredness affected roughly 5–10%. Because Mounjaro works on two hormone pathways simultaneously, the initial adjustment period can feel more pronounced.
Wegovy (semaglutide) flu-like symptoms in STEP trials showed similar nausea rates but slightly lower reported fatigue. Approximately 6–9% experienced fatigue, and dizziness occurred in around 4–7% of participants. The gradual 5-week titration schedule (starting at 0.25 mg) may allow a gentler adjustment phase compared to Mounjaro's 4-week schedule. Both medications cause similar gastrointestinal upset, but the pattern and timing may differ due to their distinct mechanisms.
- Mounjaro: dual mechanism may cause faster symptom onset but potentially higher initial impact
- Wegovy: single GLP-1 mechanism with slightly gentler escalation schedule
- Both: symptoms typically peak in weeks 1–2 of each dose increase, then subside
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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.
Individual Suitability and Prescriber Assessment
Neither Mounjaro nor Wegovy will cause "flu" in the traditional sense—these are side effects of the medication adjustment, not an infection. Your prescriber will assess which medication suits your individual health profile, including your medical history, other medications, and previous reactions to injections. Some patients tolerate one better than the other; this is why clinical assessment is essential before prescription. If you experience severe or persistent symptoms—such as prolonged vomiting, abdominal pain, or difficulty breathing—contact your GP or NHS 111 immediately.
Both medications require a reduced-calorie diet and increased physical activity alongside the injection to achieve weight loss. Use our free BMI calculator to establish your baseline, then work with your prescriber to monitor progress safely over time.
- 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.
Can Mounjaro cause flu-like symptoms more than Wegovy?
Clinical trials show both medications cause similar nausea and fatigue rates, but Mounjaro's dual mechanism may trigger slightly faster symptom onset during dose escalation. Individual responses vary significantly. Symptoms typically resolve within days as your body adjusts. Your prescriber will monitor this during your treatment.
How long do flu symptoms last on Mounjaro or Wegovy?
Flu-like symptoms (nausea, fatigue, dizziness) usually appear in the first few days of starting or increasing a dose and typically subside within 3–7 days. Most patients adjust well after the first dose increase. If symptoms persist beyond two weeks, contact your GP.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist activating two hormone pathways, while Wegovy (semaglutide) activates GLP-1 only. This dual action in Mounjaro may produce stronger appetite suppression but can also mean a more noticeable adjustment period for some users.
Should I choose Mounjaro or Wegovy based on side effects?
Side effect profiles are similar between the two. Your prescriber will assess your personal health history, other medications, and medical conditions to recommend the most suitable option. Both require clinical assessment—never choose based on side effects alone without professional guidance.
Are these medications safe if I get actual flu whilst taking them?
Yes, but inform your GP that you're taking Mounjaro or Wegovy. These medications don't interact with flu vaccines or antiviral treatments, but your prescriber may pause or adjust dosing if you're seriously unwell. Always inform healthcare professionals of all medications you're taking.
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.