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 to Expect
Mounjaro (tirzepatide) is a prescription-only GLP-1 and GIP receptor agonist injection used for weight management in UK adults. Some people experience flu-like symptoms when starting treatment or during dose titration, though these are not actually flu. Understanding what's normal versus what requires medical attention will help you manage your treatment confidently. This guide explains the difference between common adjustment symptoms and genuine illness, 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.
Are Flu-Like Symptoms Normal on Mounjaro?
Flu-like symptoms are not listed as a direct side effect of Mounjaro in clinical trials or the prescribing information. However, some people report experiencing symptoms that feel similar to flu when starting treatment. These typically include fatigue, aches, headache, or general malaise—which can resemble early flu signs.
What's likely happening: These symptoms may be related to nausea, vomiting, or gastrointestinal adjustment rather than actual flu. When your digestive system is adapting to Mounjaro, dehydration can develop, which causes fatigue and body aches. Additionally, the adjustment period itself—alongside reduced food intake—can leave you feeling generally unwell.
The good news: These symptoms typically improve within a few days to a week as your body adjusts. They are most common in the first 2–4 weeks of treatment or when increasing to a higher dose. Common adjustment symptoms include nausea, vomiting, constipation, diarrhoea, and abdominal discomfort—not actual influenza.
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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.
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When to Contact Your GP — Red Flags
While mild adjustment symptoms are normal, contact your GP immediately if you experience: severe or persistent flu-like fever (over 38.5°C); symptoms lasting more than 7–10 days without improvement; difficulty breathing or chest pain; severe dehydration (dizziness, confusion, no urination for 8+ hours); or severe abdominal pain spreading to your back.
These could indicate actual flu, infection, pancreatitis, or another medical condition requiring urgent assessment. Never assume symptoms are just Mounjaro adjustment—always err on the side of caution and contact your GP. They can assess whether you need medical attention or whether your symptoms are expected adjustment effects. Keep your prescriber informed of all symptoms so they can guide your treatment safely.
- 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.
Can Mounjaro actually cause flu?
No. Mounjaro does not cause influenza. However, some people report flu-like symptoms (fatigue, aches, malaise) during the adjustment period, usually due to nausea, reduced food intake, or dehydration rather than actual flu infection. True flu symptoms warrant a GP visit to rule out genuine illness.
How long do flu-like symptoms last on Mounjaro?
If you're experiencing adjustment symptoms, they typically improve within a few days to one week as your body adapts. Symptoms are most common in the first 2–4 weeks or after a dose increase. If symptoms persist beyond 7–10 days, contact your GP for medical advice.
What's the difference between Mounjaro adjustment and actual flu?
Mounjaro adjustment causes nausea, fatigue, and mild aches. Real flu usually includes high fever (38.5°C+), cough, sore throat, and rapid symptom onset. If you have fever, respiratory symptoms, or sudden severe illness, contact your GP—don't assume it's Mounjaro-related.
Should I stop Mounjaro if I have flu-like symptoms?
Do not stop Mounjaro without advice from your prescriber. Contact your GP or healthcare team first. If symptoms are mild adjustment effects, continuing treatment (as advised) usually helps them resolve. If you have genuine flu or infection, your GP will advise whether to pause treatment.
Can I prevent flu-like symptoms when starting Mounjaro?
Stay well-hydrated, eat small frequent meals even if you feel nauseous, and rest. Some people find ginger tea or small snacks helpful. Dose titration is gradual to allow your body to adjust. If nausea is severe, your prescriber may adjust your dose schedule. Always follow your prescriber's guidance.
Ready to take control of your weight loss journey?
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