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
Free tracker included with every download this week
What Is Mounjaro Flu and Should You Be Concerned?
"Mounjaro flu" is not an actual illness — it's a colloquial term used to describe temporary flu-like symptoms that some people experience when starting tirzepatide or increasing their dose. These symptoms, which may include fatigue, headache, muscle aches, and general malaise, are usually mild and typically resolve within a few days. Understanding what to expect and when to contact your GP can help you manage this adjustment period confidently.
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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.
Symptoms, Timeline, and When They Usually Resolve
When people refer to "Mounjaro flu," they're describing temporary side effects that may occur during the first few days after an injection or dose increase. Common symptoms include fatigue, headache, muscle or body aches, general malaise, and sometimes mild dizziness. These are not signs of infection — your body is adjusting to the medication.
- Symptoms typically appear within 24–48 hours of injection
- Most resolve within 3–5 days as your body adapts
- Symptoms may recur each time your dose increases (e.g., from 5 mg to 7.5 mg)
- Not everyone experiences these symptoms — many people have minimal side effects
- Severity varies considerably between individuals
This adjustment pattern is different from gastrointestinal side effects like nausea or diarrhoea, which may persist longer. If symptoms are severe or do not improve after 5 days, contact your GP for personalised advice.
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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.
Managing Symptoms and When to Seek Medical Help
If you experience "Mounjaro flu" symptoms, there are practical steps you can take: stay hydrated, rest adequately, take paracetamol for headache or muscle aches if needed, and eat light, nutritious meals. Most symptoms resolve naturally as your body adjusts. Contact your GP if symptoms are severe, persist beyond 5 days, or you develop a genuine fever or respiratory infection.
It's important to distinguish between adjustment symptoms and actual illness. If you develop a cough, high temperature, or genuine flu symptoms, seek medical advice rather than assuming it's a Mounjaro-related adjustment. Your GP can assess whether you have an actual infection requiring treatment and whether to continue or pause your medication.
- 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.
Is Mounjaro flu the same as actual influenza?
No. "Mounjaro flu" is a colloquial term for temporary adjustment symptoms like fatigue and muscle aches that some people experience when starting or increasing tirzepatide. Actual influenza is a viral infection with different symptoms. If you develop a genuine fever, cough, or respiratory symptoms, contact your GP — this is not a normal Mounjaro adjustment.
How long do Mounjaro flu symptoms typically last?
Most temporary adjustment symptoms resolve within 3–5 days as your body adapts to the medication. Some people may experience mild fatigue for up to a week. If symptoms persist beyond 5 days or worsen, contact your GP for guidance.
What can I do to manage Mounjaro flu symptoms at home?
Stay well hydrated, get plenty of rest, and eat light, balanced meals. Paracetamol can help with headache or muscle aches. Most symptoms improve naturally within a few days without intervention. If symptoms are severe, speak to your GP about whether to continue your dose or pause treatment temporarily.
Will I experience Mounjaro flu symptoms each time my dose increases?
Many people may experience temporary adjustment symptoms when their dose increases (e.g., from 5 mg to 7.5 mg). However, not everyone has symptoms, and they tend to be milder with each dose escalation as your body becomes accustomed to tirzepatide.
Should I stop taking Mounjaro if I experience flu-like symptoms?
Do not stop your medication without speaking to your GP. Temporary adjustment symptoms are usually not a reason to discontinue treatment. However, if you develop severe symptoms, actual illness, or have concerns, contact your GP immediately. Your prescriber can advise whether to continue, pause, or adjust your treatment plan.
Ready to take control of your weight loss journey?
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