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 — UK Guide
Some people experience flu like symptoms when starting Mounjaro (tirzepatide), though these are not among the most commonly reported side effects. Symptoms such as fatigue, body aches, and general malaise can occasionally occur as your body adjusts to this prescription-only GLP-1 and GIP receptor agonist injection. Understanding what's normal, how long symptoms typically last, and when to contact your GP will help you manage your treatment confidently.
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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.
Are Flu Like Symptoms Common With Mounjaro?
Flu like symptoms are not listed as very common or common side effects in Mounjaro's prescribing information. However, some people do report experiencing general malaise, fatigue, body aches, and a sense of feeling unwell during the initial weeks of treatment or dose increases. These symptoms are more likely to occur during the dose titration phase — the first 12–20 weeks when you're gradually increasing from 2.5 mg up to your maintenance dose. Most people experience the more frequent side effects instead: nausea, diarrhoea, constipation, and stomach discomfort. If you're experiencing genuine flu like symptoms (high temperature, severe body aches, or respiratory symptoms), it's important to distinguish these from general adjustment symptoms and contact your GP to rule out an actual viral infection, especially if symptoms persist beyond 48 hours or worsen.
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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 Flu Like Symptoms on Mounjaro
If you're experiencing flu like sensations early in your Mounjaro treatment, several strategies may help. Rest and hydration are your first line — ensure you're drinking plenty of water, as dehydration can worsen fatigue and general discomfort. Most adjustment symptoms improve within a few days as your body adapts. Some people find that remaining on their current dose for a full week before escalating helps reduce symptom severity. Keep a simple diary of when symptoms occur and how long they last — this information is valuable to share with your prescriber at your next review. If symptoms are severe, persistent beyond a week, or accompanied by other concerning signs like fever or shortness of breath, contact your GP immediately rather than waiting for your next scheduled review. Your prescriber may adjust your dose schedule or provide additional support.
- 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 it normal to feel fluey when starting Mounjaro?
Feeling generally unwell or fatigued during the first few weeks of Mounjaro is not uncommon as your body adjusts to the medication, though it's not a formally listed common side effect. Nausea, constipation, and diarrhoea are far more frequently reported. Flu like feelings typically resolve within a few days. If symptoms persist beyond one week, worsen, or you develop a fever, contact your GP to rule out an actual viral infection.
How long do flu like symptoms last on Mounjaro?
Adjustment symptoms during Mounjaro titration typically resolve within 3–7 days as your body acclimates to the medication. If you're experiencing persistent flu like symptoms beyond this timeframe, particularly if accompanied by fever or significant body aches, contact your GP. Genuine illness requires different management than medication adjustment side effects.
Should I stop taking Mounjaro if I have flu like symptoms?
Do not stop taking Mounjaro without speaking to your prescriber, as this could affect your treatment plan. However, if you develop a genuine viral illness (confirmed fever, respiratory symptoms), inform your GP or prescriber immediately. They may recommend pausing a dose or providing additional guidance. Minor adjustment discomfort alone is not a reason to stop — most symptoms resolve naturally.
What's the difference between Mounjaro side effects and actual flu?
Actual flu typically includes a high temperature (usually above 38°C), sudden onset, significant body aches, and respiratory symptoms. Mounjaro adjustment symptoms are usually milder — fatigue, general achiness, mild malaise — without fever. If you're unsure, take your temperature and contact your GP. It's always safer to rule out infection with your healthcare provider.
Can I use our GLP-1 calculator to track symptoms alongside weight loss?
Our free GLP-1 weight loss calculator helps you project your weight loss progress on Mounjaro across different doses. While it doesn't track symptoms specifically, recording your dose adjustments alongside any side effects helps you understand patterns. You can use our free progress tracker to log both your weight and how you felt each week during titration.
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.