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 Hives vs Wegovy: What's the Difference?
Both Mounjaro and Wegovy are prescription-only weight loss injections licensed in the UK, but hives and other skin reactions can occur with both medications. Hives (urticaria) are an uncommon but recognised side effect of GLP-1 and GIP/GLP-1 receptor agonists. This guide compares the clinical evidence, mechanisms, and what prescribers consider when assessing your individual suitability for either medication. Understanding how these injections work differently may help you have an informed conversation with your GP or prescriber about which option might be most appropriate for you.
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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.
Clinical Trial Data and Side Effect Profiles
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) have undergone extensive clinical trials in the UK. In the SURMOUNT trials for Mounjaro, hives were reported as uncommon (occurring in fewer than 1 in 100 participants), while Wegovy's STEP trials similarly documented hives as an infrequent allergic or skin reaction. However, the data cannot be directly compared because the trials were conducted separately with different participant populations and study designs.
Key mechanism difference: Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways. Wegovy is a GLP-1 agonist only (single pathway). This dual mechanism may influence how individual bodies respond to the medication, though clinical trial data does not show Mounjaro causes hives more frequently than Wegovy overall. Any allergic or skin response depends on your personal immune system and medical history.
- SURMOUNT-1 trial: Mounjaro (tirzepatide) 15 mg — average clinically studied weight loss
- STEP-1 trial: Wegovy (semaglutide) 2.4 mg — average clinically studied weight loss
- Both trials included diverse UK and international populations
- Hives were classified as uncommon in both trial datasets
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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.
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Individual Suitability and Prescriber Assessment
Whether you are eligible for Mounjaro or Wegovy depends on your BMI, health conditions, and personal medical history — assessed through a clinical consultation. If you have a history of allergies, previous skin reactions to medications, or specific immune conditions, your prescriber needs to know this. Neither medication is suitable if you have had a serious allergic reaction to tirzepatide or semaglutide respectively. Your GP or licensed prescriber will review your full health profile and decide which option carries the lowest risk for you personally.
Use our free GLP-1 weight loss calculator to explore estimated outcomes at different doses. Both medications require careful dose titration (gradual increase), so if a reaction like hives develops, your prescriber can adjust your dose or switch to an alternative approach. Always report any new skin reactions, rashes, or signs of allergic response immediately to your healthcare provider.
- 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 Mounjaro cause hives more than Wegovy?
Clinical trial data shows hives were uncommon with both Mounjaro and Wegovy. The dual mechanism of Mounjaro (GIP + GLP-1) versus Wegovy's single GLP-1 mechanism doesn't appear to increase hives risk based on published trial data. Individual responses vary — some people tolerate one medication better than the other, which is why prescribers assess your personal medical history before recommending either medication.
What should I do if I develop hives while taking Mounjaro or Wegovy?
Contact your GP or the prescriber who issued your prescription immediately. Do not stop taking the medication without guidance. Hives may be manageable with dose adjustment, temporary pause, or a switch to the alternative medication. Seek urgent medical attention if hives are severe, spreading rapidly, or accompanied by breathing difficulty or swelling of the face or throat.
Are hives an allergic reaction or a side effect?
Hives can be either. They may represent a mild allergic reaction to the medication itself, or they can occur as part of the body's response to the injection. Most cases of hives reported in trials were mild and resolved without stopping the medication. However, if you have a history of severe allergies or allergic reactions to injectable medications, inform your prescriber before starting either Mounjaro or Wegovy.
Who should avoid Mounjaro or Wegovy due to allergy risk?
You should not take Mounjaro if you have had a previous serious allergic reaction to tirzepatide. You should not take Wegovy if you have had a previous serious allergic reaction to semaglutide. If you have a history of anaphylaxis, severe urticaria (hives), or other significant allergic reactions to injectable medications, discuss this with your prescriber. They may recommend alternative weight management approaches.
How do I get prescribed Mounjaro or Wegovy in the UK?
Both are prescription-only medicines available through NHS specialist weight management services (via GP referral) or private online consultations with UK-licensed prescribers. A clinical assessment is required before prescription to check your eligibility (BMI and health conditions), medical history, and allergies. Your prescriber will help you choose between them based on your individual circumstances, including any allergy or skin reaction history.
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