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 Like Symptoms — What You Need to Know
Flu-like symptoms are not typically listed as common side effects of Mounjaro (tirzepatide), but some people report feeling unwell during the first few days after injection. This guide explains what symptoms may occur, why they happen, and when to contact your GP. Most side effects are temporary and improve within a few days as your body adjusts to the medication. Understanding what's normal helps you use Mounjaro safely and effectively as part of your weight management journey.
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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 Side Effects vs Flu-Like Symptoms
Mounjaro's most common side effects are digestive: nausea, diarrhoea, constipation, vomiting, and stomach pain. These affect more than 1 in 10 people. Flu-like symptoms such as fever, muscle aches, chills, or general malaise are not listed as typical Mounjaro side effects in clinical trials or product information.
However, some people report feeling unwell or tired during the first week after starting or increasing their dose. This could be related to:
- Nausea and stomach upset causing general malaise
- Dehydration from diarrhoea or vomiting
- Your body adjusting to a new medication
- Coincidental viral infection (flu or cold) unrelated to Mounjaro
These symptoms typically resolve within 3–7 days. If symptoms persist beyond one week or worsen, contact your GP for 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.
When to Contact Your GP About Mounjaro Symptoms
Seek urgent medical attention if you experience severe stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, or throat), severe vomiting preventing you from keeping fluids down, or signs of serious infection such as high fever or confusion.
Contact your GP if symptoms persist longer than expected, if you develop new symptoms you're unsure about, or if you're concerned about how Mounjaro is affecting you. Your GP can assess whether your symptoms are related to Mounjaro or another cause, and advise whether to continue treatment, adjust your dose, or try an alternative weight management option.
- 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.
Are flu-like symptoms a normal side effect of Mounjaro?
Flu-like symptoms such as fever, chills, or muscle aches are not listed as common Mounjaro side effects. Most people experience digestive symptoms instead. If you develop what feels like flu, it may be coincidental viral infection, dehydration, or general malaise from nausea. Contact your GP if symptoms persist beyond a week.
How long do Mounjaro side effects typically last?
Most Mounjaro side effects improve within 3–7 days as your body adjusts. Nausea and digestive symptoms are most common in the first week after injection. Side effects may be worse when you start treatment or increase your dose. Your body often adapts over time.
What should I do if I feel unwell after a Mounjaro injection?
Stay hydrated, eat small meals if you can tolerate them, and rest. Most symptoms resolve within a few days. Contact your GP if you develop severe pain, persistent vomiting, high fever, difficulty breathing, or symptoms lasting longer than one week. Never stop taking Mounjaro without medical advice.
Could my flu-like symptoms be something other than Mounjaro?
Yes. You may have caught a viral infection like flu or a cold coincidentally. Dehydration from diarrhoea can also cause feeling unwell. If symptoms match a typical illness pattern (fever, cough, body aches), it's likely not Mounjaro itself. Your GP can help determine the cause.
Is Mounjaro safe if I develop flu symptoms?
Developing flu-like symptoms doesn't mean Mounjaro is unsafe for you. However, if you suspect you have actual flu (confirmed positive test or typical flu pattern), contact your healthcare provider. They may advise on managing both Mounjaro and the viral infection safely together.
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.