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 to Do If You're Allergic to Mounjaro
If you've had an allergic reaction to Mounjaro (tirzepatide), you're not without options. Several alternative prescription weight loss medications are available in the UK through both NHS specialist services and private prescribing routes. This guide explains what alternatives exist, how to access them, and what you can expect in terms of cost and eligibility. Always discuss any allergic reaction with your GP or prescriber to ensure safe alternatives are identified for your individual circumstances.
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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.
Alternative Weight Loss Medications Available in the UK
If Mounjaro is unsuitable due to allergy, Wegovy (semaglutide) is the primary alternative GLP-1 receptor agonist licensed for weight management in the UK. It works through a single GLP-1 mechanism rather than Mounjaro's dual GIP and GLP-1 pathway, meaning it may suit you if you've had an allergic reaction to tirzepatide specifically.
Saxenda (liraglutide) is another GLP-1 option, delivered as a once-daily injection rather than weekly. Both medications have similar eligibility criteria to Mounjaro — a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease.
Your prescriber will assess whether you may have cross-reactivity with other GLP-1 medications. Some allergic reactions to Mounjaro may be to the tirzepatide itself, whilst others may be to inactive ingredients — this distinction affects which alternatives are safe.
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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 Alternatives — NHS vs Private Routes and Costs
NHS access: Referral to a tier 3 specialist weight management service is required. Availability varies significantly by region — some areas have waiting lists of several months. Once assessed, Wegovy or Saxenda may be prescribed at no cost if approved by your local integrated care board.
Private access: Available through UK-licensed prescribers via online consultation, typically costing £129–£280 per month depending on dose. This route bypasses NHS waiting lists entirely. A prescriber will conduct a clinical assessment, discuss your allergy history, and issue a prescription dispensed by a GPhC-registered pharmacy.
Use our free GLP-1 weight loss calculator to explore potential weight loss projections with alternative medications, and our BMI calculator to confirm your eligibility threshold.
- 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 I be allergic to Wegovy if I'm allergic to Mounjaro?
It depends on what caused your allergic reaction. If you reacted to tirzepatide itself, you're unlikely to react to semaglutide (Wegovy) as it's a different medication. However, if your reaction was to a shared inactive ingredient, cross-reactivity is possible. Always inform your prescriber of your allergy history before starting any alternative. They may conduct an allergy assessment or patch test if needed.
What if I'm allergic to all GLP-1 medications?
Other options exist outside GLP-1 medications, though fewer are currently licensed in the UK for weight management. These might include orlistat (lipase inhibitor) or referral for behavioural interventions through NHS tier 3 services. In some cases, bariatric surgery may be considered. Your GP can discuss non-GLP-1 alternatives based on your health profile and allergy status.
How much does Wegovy cost privately in the UK?
Private Wegovy typically costs £129–£280 per month depending on the dose. Starting doses are cheaper than maintenance doses (0.25 mg is lower cost than 2.4 mg). Costs vary slightly between online prescribing services. Most private providers dispense the full 4-week supply at once, though some offer fortnightly options. Compare services for your specific dose before committing.
Will the NHS prescribe an alternative if I'm allergic to Mounjaro?
Yes, if you're referred to a tier 3 specialist service and diagnosed with a genuine allergy, they can prescribe an alternative such as Wegovy or Saxenda. However, NHS availability depends on your region — some areas have limited stock or specialist capacity. Waiting lists can be long. Private prescribing offers faster access if cost is not a barrier.
How long does an online private consultation take?
Online consultations typically take 15–30 minutes and are conducted by a UK-licensed GP or nurse prescriber. You'll be asked about your allergy history, medical conditions, current medications, and weight loss goals. If approved, a prescription is issued on the same day or within 24 hours and sent to a pharmacy for dispensing. The whole process from booking to receiving medication usually takes 3–7 days.
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.