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 Rash vs Wegovy — Side Effects Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK, and skin reactions including rash are possible side effects with either medication. This guide compares the rash risk, the mechanisms behind each injection, and what clinical trial data shows about dermatological side effects. If you're considering either medication, understanding how they differ can help inform your discussion with your prescriber about suitability for your individual health profile.
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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 — Rash and Skin Reactions
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways simultaneously. In the SURMOUNT-1 clinical trial, skin-related adverse events (including rash, dermatitis, and other reactions) were reported in approximately 2–4% of participants across all tirzepatide doses tested with the KwikPen device.
Wegovy is a GLP-1 receptor agonist only, activating a single hormone pathway. In the STEP-1 clinical trial with semaglutide, skin reactions including rash occurred in approximately 1–2% of participants on the 2.4 mg maintenance dose.
- Mounjaro SURMOUNT trial: rash/dermatitis reported at similar rates across 5 mg, 10 mg, and 15 mg doses
- Wegovy STEP trial: rash incidence did not significantly increase with dose escalation
- Both rates are considered relatively uncommon compared to gastrointestinal side effects (nausea, diarrhoea)
Neither medication was associated with severe or life-threatening skin reactions in clinical trials, though individual responses vary considerably.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Why Rash Occurs and Individual Suitability
Rash or skin irritation with GLP-1 and GIP/GLP-1 medications may occur due to individual hypersensitivity to the medication itself, injection site inflammation, or underlying skin conditions being triggered by hormonal changes. Some people experience mild, localised reactions at the injection site, whilst others may develop generalised rash.
Which medication is more suitable depends on your individual health history, not the medication mechanism alone. Your prescriber will assess your eligibility using NHS criteria — a BMI of 30 or above, or 27 or above with weight-related health conditions — and your personal tolerance profile. Both Mounjaro and Wegovy require prescription and clinical assessment. If you have a history of dermatological sensitivity or severe allergies, inform your prescriber, who can determine whether either medication is appropriate for you or recommend monitoring strategies.
- 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.
Does Mounjaro cause rash more than Wegovy?
Clinical trial data suggests both medications have low but similar rash incidence (approximately 2–4% for Mounjaro, 1–2% for Wegovy). Differences are not dramatic. Individual susceptibility varies greatly and depends on personal factors, not just the medication. Your prescriber can assess your individual risk.
What does a Mounjaro rash look like?
Reported rashes vary from mild, localised red bumps at the injection site to generalised urticarial rash (hives) elsewhere on the body. Some people experience itching or inflammation. Any persistent or worsening rash should be reported to your GP or prescriber for assessment.
Is rash a reason to avoid Mounjaro or Wegovy?
No. Rash is uncommon with both medications and not necessarily a reason to avoid them entirely. If you have a history of severe skin reactions or allergies, discuss this with your prescriber before starting either medication. They can help assess your individual risk and recommend alternatives if needed.
Can you switch from Mounjaro to Wegovy if you get a rash?
Possibly, but switching between medications requires prescriber guidance. If you develop rash on one medication, your prescriber may recommend stopping temporarily, investigating the cause, or trying an alternative. Do not switch without clinical advice.
Where can I find more information about Mounjaro side effects in the UK?
The NHS provides guidance on tirzepatide eligibility and effects. Your GP or specialist can discuss all known side effects from the medication information leaflet. Use our free GLP-1 weight loss calculator to explore potential outcomes and discuss with a prescriber whether either medication is suitable for your health goals.
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