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
Can Mounjaro Cause Flu-Like Symptoms?
Mounjaro (tirzepatide) can cause several side effects as your body adjusts to the medication, but true flu-like symptoms are not commonly reported in clinical trials. However, some people experience nausea, fatigue, and general malaise during the first few weeks, which might feel similar to mild illness. Understanding what to expect and when to contact your GP is important for safe use of this prescription-only weight management injection in the UK.
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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 Side Effects Does Mounjaro Actually Cause?
Clinical trials showed that Mounjaro causes gastrointestinal side effects most frequently. The most common side effects affecting more than 1 in 10 people include nausea, diarrhoea, vomiting, and constipation. These typically occur during dose titration when starting or increasing your Mounjaro dose.
Up to 1 in 10 people also experience fatigue (tiredness), dizziness, indigestion, and injection site reactions. Fatigue can feel like general malaise or low energy, which might be mistaken for flu-like symptoms. Importantly, true flu symptoms such as fever, muscle aches, cough, or sore throat are not listed as common side effects of Mounjaro in UK prescribing information. If you develop these symptoms, they are more likely due to an actual viral infection rather than the medication itself.
Most gastrointestinal side effects improve within the first 2–4 weeks as your body adjusts. Your GP can advise you on managing side effects during this adaptation period.
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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
Although flu-like symptoms are not a typical Mounjaro side effect, contact your GP or NHS 111 immediately if you develop severe, persistent stomach pain spreading to your back, as this could indicate pancreatitis. Also seek medical attention for serious allergic reactions (swelling of face, lips, or throat), significant changes in heart rate, or vision changes.
If you develop genuine flu symptoms (fever, muscle aches, persistent cough), these are likely unrelated to your Mounjaro injections and should be reported to your GP as normal. Keep your prescriber informed of all new symptoms when you attend follow-up appointments. Using our GLP-1 weight loss calculator can help you track expected progress while monitoring how you feel on your current 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.
Is fever a side effect of Mounjaro?
No, fever is not a reported side effect of Mounjaro. If you develop a fever while taking Mounjaro, it is likely due to a separate infection such as a cold or flu virus, not the medication itself. Contact your GP if you have a high fever or persistent symptoms.
Can Mounjaro cause muscle aches or body aches?
Muscle aches and body aches are not listed as common side effects of Mounjaro in UK clinical data. Fatigue and general tiredness are reported, but true muscle or body pain would suggest a different cause. Discuss any new pain with your GP.
How long do Mounjaro side effects last?
Most common side effects like nausea and diarrhoea typically improve within 2–4 weeks as your body adjusts to the medication. If side effects persist beyond this period or worsen, contact your GP for guidance on managing them or adjusting your dose.
Should I stop Mounjaro if I feel unwell?
Do not stop Mounjaro without speaking to your prescriber first. If you feel unwell, contact your GP or the NHS service that prescribed your medication. They can assess whether your symptoms are medication-related and advise whether to continue, pause, or adjust your treatment.
What's the difference between Mounjaro and Wegovy side effects?
Both are GLP-1-based medications with similar common side effects (nausea, diarrhoea, constipation). Mounjaro is a dual GIP/GLP-1 agonist while Wegovy (semaglutide) is a GLP-1 agonist only. Side effect profiles are comparable, though individual responses vary. Your GP can discuss which is more suitable for you.
Ready to take control of your weight loss journey?
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