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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Does Mounjaro Cause Flu-Like Symptoms?
"Mounjaro flu" is not a recognised medical condition, but some people experience flu-like symptoms when starting tirzepatide (Mounjaro) or increasing their dose. These symptoms are typically mild and temporary. Understanding what to expect helps you distinguish between normal adjustment side effects and signs that require medical attention. This guide explains what these symptoms are, how long they last, and when to contact your GP.
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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.
What Are Flu-Like Symptoms with Mounjaro?
When people refer to "Mounjaro flu", they usually mean temporary flu-like feelings that can occur during the first few weeks or after a dose increase. These may include fatigue, body aches, chills, or mild fever-like sensations. Importantly, Mounjaro does not cause actual influenza — the term is used colloquially because the symptoms feel similar to how someone might feel when fighting off a mild viral illness.
The most common side effects when starting Mounjaro or stepping up your tirzepatide dose are nausea, diarrhoea, and stomach discomfort, not flu-like symptoms. However, fatigue is listed as a common side effect in clinical trials. Some patients report feeling generally unwell or having body aches as their body adjusts to the medication. These symptoms typically resolve within a few days to a week as your system acclimates to the tirzepatide.
- Fatigue or tiredness (common)
- Muscle or body aches (less common)
- Mild headache or dizziness
- General malaise or "not feeling well"
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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 Contact Your GP
Most flu-like feelings with Mounjaro settle down without intervention. However, contact your GP immediately if you experience severe or persistent symptoms, high fever (above 38°C), or severe body aches lasting more than a few days. You should also seek urgent advice if you develop signs of infection such as swollen lymph nodes, sore throat, or cough — these could indicate an actual infection unrelated to Mounjaro.
If you're concerned about whether your symptoms are a normal adjustment reaction or something more serious, your prescriber or GP can assess you. Keep a brief symptom diary when starting Mounjaro — note when symptoms began, how long they lasted, and their severity. This information helps your healthcare team understand how you're tolerating the medication and whether a dose adjustment might help.
- 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.
How long do flu-like symptoms last with Mounjaro?
Most temporary side effects, including fatigue or body aches, typically resolve within 3–7 days of starting or increasing your dose. If symptoms persist beyond two weeks, contact your GP. Everyone responds differently to tirzepatide, so individual timelines vary.
Is "Mounjaro flu" the same as actual flu?
No. Mounjaro does not cause influenza. The term "Mounjaro flu" is informal language people use to describe temporary flu-like feelings. If you develop fever, cough, sore throat, or other signs of actual infection, contact your GP — you may have a separate illness requiring treatment.
Can I take paracetamol for Mounjaro flu-like symptoms?
Yes. Paracetamol (up to 1000mg per dose, maximum 4000mg daily) can help with body aches or mild fever-like sensations. Always follow the label instructions. However, if your symptoms are mainly nausea or stomach discomfort, paracetamol may not help — speak to your pharmacist about what's appropriate for your specific symptoms.
Should I stop taking Mounjaro if I feel flu-like?
Do not stop your Mounjaro without speaking to your prescriber first. Temporary flu-like feelings during the adjustment period do not usually mean the medication is unsuitable. Contact your healthcare provider to discuss your symptoms — they may suggest staying on your current dose a bit longer or adjusting the timing of your next injection.
Is fatigue with Mounjaro different from actual illness?
Mounjaro-related fatigue is usually mild to moderate, improves with rest, and resolves within days to a week. Fatigue from actual infection is often accompanied by fever, body aches, and other symptoms like cough or sore throat. If you're unsure, contact your GP — they can assess whether you need medical attention or if your symptoms are normal adjustment effects from tirzepatide.
Ready to take control of your weight loss journey?
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