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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Can Mounjaro Cause a Rash?
Yes, a rash can occur with Mounjaro (tirzepatide), though it is not among the most common side effects reported in clinical trials. Rashes are typically injection site reactions rather than full-body reactions. Both Mounjaro and Wegovy (semaglutide) can cause skin reactions at the injection site, but the frequency and severity vary between individuals. Understanding the difference between these two prescription weight loss injections may help you discuss which option is more suitable for you with your prescriber during your clinical assessment.
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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 vs Wegovy: Rash and Injection Site Reactions
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist injected once weekly. In clinical trials, injection site reactions including redness, bruising, pain, and sometimes rash-like reactions occur but are uncommon. These are localised reactions at the injection site rather than generalised rashes. Wegovy (semaglutide) is a GLP-1 receptor agonist (single mechanism) also injected once weekly, with similar injection site reaction rates.
Key differences in the SURMOUNT and STEP clinical trial data show both medications have broadly comparable side effect profiles at the injection site. The dual mechanism of Mounjaro (GIP + GLP-1) versus Wegovy's single GLP-1 pathway does not significantly change skin reaction risk. Individual tolerance varies greatly—some people experience no reactions, while others develop mild redness or temporary irritation.
- Mounjaro: Injection site reactions in clinical trials (uncommon)
- Wegovy: Similar injection site reaction frequency
- Both: Reactions typically resolve within hours to days
- Risk factors: Injection technique, site rotation, skin sensitivity
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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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When to Seek Medical Advice and Prescriber Guidance
A mild rash or redness at the Mounjaro injection site that fades within a few hours is usually not serious. However, if a rash spreads beyond the injection site, persists for more than 24 hours, or is accompanied by swelling, itching, or warmth, contact your GP or prescriber promptly. Serious allergic reactions to tirzepatide or semaglutide are rare but require urgent medical attention.
Both Mounjaro (tirzepatide) and Wegovy require a UK prescription and clinical assessment before starting. Your prescriber will review your medical history, allergies, and skin sensitivity during this assessment. If you have a history of skin reactions or sensitive skin, mention this when discussing your weight loss options. Proper Mounjaro KwikPen injection technique and rotating injection sites each week significantly reduces the risk of local reactions. Your prescriber or nurse can show you the correct technique during your consultation.
- 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.
Is a rash a common side effect of Mounjaro?
No, rash is not listed as a common side effect in UK clinical trial data. Injection site reactions like mild redness or bruising occur but are uncommon and usually temporary. Serious rashes are rare. Always report any unusual or persistent skin reactions to your prescriber.
What's the difference between Mounjaro and Wegovy side effects?
Both medications cause similar injection site reactions at comparable rates. Mounjaro is a dual GIP+GLP-1 agonist while Wegovy is GLP-1 only, but this mechanism difference does not significantly affect rash risk. Gastrointestinal side effects (nausea, diarrhoea) may differ slightly between individuals.
How long does a Mounjaro injection site rash last?
Mild injection site reactions typically resolve within a few hours to one day. If redness, swelling, or itching persists beyond 24 hours or spreads, contact your GP or prescriber. Proper injection technique and site rotation help prevent reactions.
Can I continue Mounjaro if I develop a rash?
Mild, temporary injection site reactions do not usually require stopping Mounjaro. However, if you experience a spreading rash, severe itching, or signs of allergic reaction, contact your prescriber immediately. They will assess whether to continue, pause, or switch treatment.
How can I reduce the risk of a Mounjaro injection site rash?
Rotate your injection site weekly (stomach, thigh, upper arm), use proper injection technique as shown by your nurse, allow the KwikPen to reach room temperature before injecting, and avoid injecting into areas of irritated or damaged skin. Discuss skin sensitivity with your prescriber during your clinical assessment.
Ready to take control of your weight loss journey?
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