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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Allergic Reactions to Mounjaro — What You Need to Know
Allergic reactions to Mounjaro (tirzepatide) are rare but serious. This guide explains what signs to watch for, how to access Mounjaro safely through UK clinical assessment, and the difference between NHS and private prescription routes. Whether you're considering this GLP-1 weight management injection or already taking it, understanding allergic risk and proper access channels helps you make an informed decision about your health.
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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.
Recognising Allergic Reactions to Mounjaro
Serious allergic reactions to Mounjaro are uncommon, but they require immediate medical attention if they occur. Signs of a genuine allergic reaction include: swelling of the face, lips, tongue, or throat; difficulty breathing or shortness of breath; severe rash or hives covering large areas of your body; and dizziness or fainting. These symptoms typically develop within minutes to hours of injection.
- Mild reactions: Local injection site redness, itching, or small bruising at the injection point — these are common and usually resolve within hours without treatment
- Serious reactions: Anaphylaxis (swelling, breathing difficulty, rapid heart rate) — call 999 immediately if this occurs
- Delayed reactions: Rare rash or itching developing days after injection — contact your GP or pharmacist
If you have a history of allergies to semaglutide, other GLP-1 medications, or any component of the Mounjaro KwikPen device, inform your prescriber before starting treatment. A clinical assessment ensures this medication is safe for you.
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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.
Accessing Mounjaro Safely: NHS vs Private Routes in the UK
In the UK, Mounjaro is a prescription-only medicine requiring a clinical assessment from a licensed prescriber before you can access it. NHS access comes through specialist tier 3 weight management services — your GP can refer you, though waiting lists vary significantly by region and can span months or years. Private access is available through GPhC-registered online consultations; you complete a health questionnaire, a UK-licensed prescriber reviews your eligibility, and if suitable, issues a prescription dispensed by a pharmacy.
- Both routes require assessment of your medical history, current medications, and allergies
- Online consultations typically cost £20–£50; medication costs depend on your dose
- You'll receive written information about signs of allergic reaction and emergency procedures
Use our free BMI calculator above to check your eligibility based on your current weight and height before booking a consultation.
Whichever access route you choose, your prescriber will discuss dose titration (gradual increase from 2.5 mg to your effective dose), potential side effects including allergic reactions, and what to do if problems arise. Keep your GP informed of any private prescription treatment so they can monitor your health. If you experience an allergic reaction after receiving Mounjaro, seek immediate medical attention — call your GP surgery, NHS 111, or 999 if symptoms are severe. Report any serious adverse reactions to the MHRA via the Yellow Card scheme so regulatory data supports safe use for future patients.
- 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.
What's the difference between a Mounjaro side effect and an allergic reaction?
Side effects (nausea, diarrhoea, stomach pain) are common and typically improve over time as your body adjusts. Allergic reactions involve immune system response — swelling, breathing difficulty, or rash. Side effects occur in many users; allergic reactions are rare but serious and require immediate medical attention.
Can I access Mounjaro quickly through the NHS in the UK?
NHS access is through specialist weight management services (tier 3), which involves GP referral and clinical assessment. Waiting times vary significantly by region — some areas have waits of several months to over a year. Private online prescription routes are faster, typically involving a consultation within days.
How much does Mounjaro cost privately in the UK?
Private costs vary by dose and provider. Consultations typically cost £20–£50, and medication costs range from approximately £129–£320 per month depending on your dose level. Always confirm the full cost (consultation plus medication) before proceeding with a private provider.
What should I do if I think I'm having an allergic reaction to Mounjaro?
If you experience swelling of face/lips/tongue, breathing difficulty, or severe rash, call 999 or go to A&E immediately. For less urgent symptoms (mild rash, itching) developing after injection, contact your GP or pharmacist during working hours. Always inform them you've had Mounjaro and describe symptoms clearly.
Will my GP know if I get Mounjaro privately?
Not automatically — private prescriptions are separate from NHS records. However, it's important to tell your GP about any prescription medications you're taking, including privately prescribed Mounjaro, so they can monitor your health and avoid prescribing conflicting medicines.
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.