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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Can Mounjaro Cause Flu Like Symptoms?
Flu-like symptoms are not commonly reported as a direct side effect of Mounjaro (tirzepatide) in clinical trials, though some people do experience fatigue, dizziness, or general malaise when starting the medication. These symptoms are typically mild and usually resolve as your body adjusts to the injection. If you're considering Mounjaro as a prescription weight loss treatment in the UK, understanding the full range of potential side effects — and how they compare to alternatives like Wegovy — can help you make an informed decision with your healthcare provider.
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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: Side Effect Comparison
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Both are prescription-only weight management injections available in the UK following clinical assessment.
- Mounjaro common side effects: Nausea, diarrhoea, vomiting, constipation, and stomach pain are most frequently reported. Fatigue and dizziness occur in some users but are not classified as flu-like symptoms in the SURMOUNT clinical trials.
- Wegovy common side effects: Similarly, nausea, diarrhoea, vomiting, and constipation are the primary reported effects. Headache, fatigue, and dizziness are also documented.
- Key difference: Neither medication lists flu-like illness as a typical side effect. Any fatigue or general malaise is usually gastrointestinal-related and tends to improve as your body adjusts to the tirzepatide or semaglutide dose.
Clinical trial data (SURMOUNT for Mounjaro, STEP for Wegovy) does not identify flu-like symptoms as a recognised adverse effect. Most side effects are mild to moderate and peak in the first few weeks of treatment.
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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.
When to Seek Medical Advice
If you develop symptoms that concern you — such as severe fatigue, fever, chills, or persistent body aches — contact your GP or the prescriber who issued your prescription. These could indicate an infection unrelated to the medication itself.
- Injection-related issues: Redness, bruising, or pain at the injection site are common and usually resolve in 1–2 days. Rotating your injection site each week helps minimise this.
- When fatigue is expected: Mild tiredness or low energy in the first 2–4 weeks of starting Mounjaro or increasing your dose is normal as your body adapts. This typically improves as you continue treatment.
- Individual variation: Everyone responds differently to tirzepatide. Use our free GLP-1 weight loss calculator to understand projected outcomes, and discuss any concerns with your prescriber before adjusting your dose.
- 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?
No, flu-like symptoms are not listed as a common or expected side effect of Mounjaro in clinical trials. Some users report fatigue or mild malaise in early weeks, which is usually gastrointestinal in origin and resolves as the body adjusts. If you develop fever, chills, or severe symptoms, contact your GP — this may indicate a separate infection.
Which is better for side effects — Mounjaro or Wegovy?
Both have similar common side effects: nausea, diarrhoea, vomiting, and stomach pain. Individual tolerance varies significantly. Mounjaro uses a dual GIP+GLP-1 mechanism whilst Wegovy is GLP-1 only, but clinical data shows comparable safety profiles. Your prescriber will assess which is most suitable for you after a clinical assessment.
How long do Mounjaro side effects last?
Most side effects, including nausea and fatigue, peak in the first 1–4 weeks and typically improve as your body adjusts. Gastrointestinal effects often reduce further as your dose stabilises. If side effects persist beyond 4–6 weeks, discuss this with your prescriber — dose adjustment may help.
Can I mistake a viral illness for Mounjaro side effects?
Yes — viral infections (including actual flu) can occur independently of Mounjaro treatment. If you develop fever, chills, body aches, sore throat, or cough, seek advice from your GP. These are not typical Mounjaro effects and may require medical attention regardless of whether you're taking tirzepatide.
Is fatigue a normal Mounjaro side effect?
Mild fatigue or low energy can occur when starting Mounjaro or increasing your dose, usually due to nausea, reduced appetite, or electrolyte changes. This is different from flu-like illness and normally resolves within weeks. If tiredness is severe or persistent, contact your prescriber to rule out other causes.
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.