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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What Are Mounjaro Flu Symptoms?
When starting Mounjaro (tirzepatide), some people experience symptoms that resemble flu, including nausea, fatigue, and body aches. These symptoms are typically temporary and tend to improve as your body adjusts to the injection. However, it's important to understand what counts as a normal side effect and when you should contact your GP. This guide explains flu-like symptoms associated with Mounjaro in a UK health context, based on clinical trial data and NHS guidance.
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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.
Common Flu-Like Side Effects and What to Expect
Nausea and fatigue are the most frequently reported side effects when starting Mounjaro, affecting more than 1 in 10 people in clinical trials. These symptoms usually begin within the first few days of your injection and may last several days. Other flu-like symptoms can include headaches, dizziness, muscle aches, and a general sense of malaise.
- Nausea typically peaks around day 2–3 after injection and improves by day 5–7
- Fatigue may persist longer and can affect energy levels for up to 10 days
- Symptoms often become milder with each dose escalation as your body adapts
- Most people find these side effects manageable and temporary, not requiring medical intervention
The exact mechanisms causing these symptoms relate to how tirzepatide affects your digestive system and appetite regulation. These are not signs of infection or serious illness—they are expected physiological responses to the medication.
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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 and NHS Safety Guidance
While mild flu-like symptoms are expected, certain symptoms warrant urgent GP contact. If you develop severe stomach pain spreading to your back, this may indicate pancreatitis and requires immediate medical attention. Equally, if you experience persistent vomiting preventing you from keeping fluids down, contact your GP to prevent dehydration.
All Mounjaro prescriptions in the UK come with patient information leaflets detailing side effects and safety guidance. Your prescriber should have discussed what to expect before your first dose. If symptoms are severe or you're unsure whether they're normal, never hesitate to contact your NHS 111 service or your GP surgery for reassurance.
If you're considering Mounjaro through NHS specialist tier 3 services or private prescription, use our free GLP-1 weight loss calculator to understand potential outcomes alongside other options like Wegovy. You can also track how you feel during treatment using our progress tracker to monitor symptom patterns across doses.
- 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 from Mounjaro normal?
Yes, flu-like symptoms including nausea, fatigue, and mild body aches are common and expected when starting Mounjaro. These are not signs of infection. They typically appear within the first few days and improve within 5–7 days as your body adjusts. More than 1 in 10 people experience these symptoms during clinical trials. They usually become milder with subsequent doses.
How long do Mounjaro flu symptoms last?
Most flu-like symptoms peak around days 2–3 after injection and resolve by day 5–7. Fatigue may persist slightly longer. Symptoms typically improve with each dose escalation as your body acclimates to the medication. If symptoms persist beyond 10 days or worsen, contact your GP for advice.
What can I do to manage Mounjaro side effects?
Stay hydrated by drinking water regularly throughout the day. Eat small, frequent meals and avoid fatty or heavy foods, which can worsen nausea. Rest and avoid strenuous exercise for the first 24–48 hours after injection. Over-the-counter pain relief like paracetamol may help with headaches or muscle aches. Inform your prescriber if symptoms significantly impact your daily life.
When should I contact my GP about Mounjaro symptoms?
Contact your GP immediately if you experience severe stomach pain spreading to your back (possible pancreatitis), persistent vomiting, signs of serious allergic reaction, or gallbladder symptoms. Contact NHS 111 if you're unsure or need urgent advice outside surgery hours. Mild nausea and fatigue alone do not require medical attention.
Is Mounjaro the right option for me if I'm concerned about side effects?
Your prescriber will assess whether Mounjaro is suitable during clinical assessment. Discuss your concerns about side effects—many are temporary and manageable. Alternative weight loss options exist, including Wegovy (semaglutide), which has a different dose titration schedule. Your NHS GP or private prescriber can help compare options based on your individual health needs.
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.