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
What to Do If You're Allergic to Mounjaro
If you've experienced an allergic reaction to Mounjaro (tirzepatide) or discovered you're allergic before starting, it's important to stop using the medication immediately and speak to your GP or prescriber. Fortunately, there are alternative prescription weight management options available in the UK, and your healthcare team can help you explore safer choices tailored to your health needs.
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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.
Mounjaro Allergies and Alternative Options in the UK
Allergic reactions to Mounjaro can range from mild to severe. Mild reactions may include a rash or itching at the injection site, while serious allergic reactions are rare but require immediate medical attention — symptoms include facial swelling, difficulty breathing, or severe rash. If you suspect an allergy, stop using Mounjaro and contact your GP or call 111 immediately.
Fortunately, alternative weight management medications are available in the UK. If you're allergic to tirzepatide, your prescriber may recommend Wegovy (semaglutide), a GLP-1 receptor agonist with a similar mechanism but different chemical structure. Wegovy is also available through NHS specialist tier 3 weight management services and private prescribing pathways. Another option is Saxenda (liraglutide), a daily GLP-1 injection, though this is less commonly prescribed. Your prescriber will assess whether these alternatives are suitable based on your allergy history and individual health profile.
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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.
NHS Support and Next Steps After Mounjaro Allergy
If you were prescribed Mounjaro through an NHS specialist weight management service, inform your tier 3 team immediately of the allergic reaction. They can assess you for alternative medications, usually at no additional cost through the NHS. If you accessed Mounjaro privately, contact the prescriber or pharmacy where you obtained it — they must document the allergy and can help source an alternative.
While exploring alternatives, continue lifestyle measures that support weight management: use our free BMI calculator to track your progress, and consider our GLP-1 weight loss calculator to understand how different medications might support your goals. Your GP can refer you to NHS weight management services if you're not already registered, ensuring you receive coordinated, safe care.
- 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 should I do immediately if I'm having an allergic reaction to Mounjaro?
Stop using Mounjaro at once. If you experience facial swelling, difficulty breathing, or severe rash, call 999 or go to A&E. For mild reactions like itching or a localized rash, contact your GP or call 111. Always inform your prescriber in writing to document the allergy.
Is it safe to switch from Mounjaro to Wegovy if I'm allergic to Mounjaro?
Possibly, but not automatically. Although Wegovy (semaglutide) is a different medication, you may have cross-sensitivity depending on the type of allergy. Your prescriber must assess your specific reaction carefully before recommending any alternative. A thorough allergy history is essential.
Can I access alternative weight management drugs on the NHS if Mounjaro causes an allergy?
Yes. If prescribed through an NHS specialist weight management service, your tier 3 team will assess alternative options. NHS coverage depends on your eligibility and local ICB policy. Wegovy and Saxenda are both available through NHS specialist services for suitable candidates.
What's the difference between tirzepatide allergy and a Mounjaro KwikPen allergy?
An allergy to tirzepatide (the active ingredient) is separate from a reaction to the KwikPen device or other ingredients. Your prescriber will investigate which component caused the reaction. If it's device-related, alternatives may exist; if it's tirzepatide, you'll need a different medication entirely.
Will my allergy to Mounjaro affect my weight loss goals?
Not necessarily. Your prescriber can explore alternative weight management medications, or you may return to lifestyle-based approaches supported by diet, physical activity, and behavioural strategies. Our free BMI calculator and progress tracker can help you monitor your health journey regardless of medication route.
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.