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 Side Effects: Rash & Skin Reactions
Mounjaro (tirzepatide) can occasionally cause skin rashes and injection site reactions as side effects. This guide explains what rashes associated with Mounjaro look like, when they occur, how to access the medication in the UK through NHS or private prescription routes, and what costs you can expect. If you're considering tirzepatide for weight management, understanding both the potential side effects and the practical access process will help you make an informed decision with your healthcare provider.
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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 Rash & Skin Side Effects Explained
Skin rashes and injection site reactions are among the less common side effects of Mounjaro, but they can occur in some users. Injection site reactions — including redness, bruising, itching, or swelling at the injection location — are the most frequently reported skin-related effect and may affect up to 1 in 10 people. These typically appear within hours or days of injection and usually resolve within a few days without treatment.
A generalised rash elsewhere on the body is less common but can occur as part of an allergic or sensitivity reaction. Serious allergic reactions involving facial swelling, throat tightness, or difficulty breathing are rare but require immediate medical attention. Most minor rashes can be managed with basic skincare — keeping the area clean and dry, avoiding tight clothing over the injection site, and rotating injection locations each week to prevent repeated irritation. If a rash persists beyond a week, worsens, or is accompanied by other symptoms, contact your GP or the prescriber immediately.
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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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Accessing Mounjaro in the UK: NHS vs Private
In the UK, Mounjaro is available through two routes: NHS specialist services or private prescription. NHS access is through tier 3 or tier 4 specialist weight management services following a GP referral, but availability varies significantly by region and waiting times can be lengthy. Private access is faster — you can request an online consultation with a UK-licensed prescriber, who will conduct a clinical assessment (including health history and current medications) before issuing a prescription if appropriate.
Private Mounjaro typically costs between £150–£280 per month depending on dose and provider. Starting doses (2.5 mg) are usually at the lower end; maintenance doses (10–15 mg) at the higher end. Prices may vary between providers, so comparing options is worthwhile. The prescription is then dispensed by a registered UK pharmacy. Use our GLP-1 weight loss calculator to explore potential outcomes, and check your eligibility with our BMI calculator before consulting a prescriber.
- 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 does a Mounjaro injection site rash look like?
Injection site reactions typically appear as redness, mild swelling, bruising, or itching at the injection location. They usually develop within hours of injection and fade within a few days. A generalised rash elsewhere on the body is less common. Any rash that worsens, spreads significantly, or is accompanied by swelling of the face or throat requires immediate medical attention.
How much does Mounjaro cost in the UK on private prescription?
Private Mounjaro typically costs £150–£280 per month depending on dose. Lower starting doses (2.5 mg) are usually cheaper; higher maintenance doses (10–15 mg) cost more. Prices vary between private providers, so it's worth comparing. NHS access is free if prescribed through specialist services, though waiting times can be long.
What's the difference between NHS and private Mounjaro access in the UK?
NHS access requires a GP referral to a specialist tier 3/4 weight management service, but availability and waiting times vary by region. Private access is faster — you can have an online consultation with a UK-licensed prescriber within days. NHS is free; private costs £150–£280 monthly depending on dose.
Can I switch from Mounjaro to Wegovy if I develop a rash?
If you develop a significant allergic rash to Mounjaro, your prescriber may recommend switching to Wegovy (semaglutide), which is a different medication. However, any switch must be managed by your healthcare provider. A minor injection site rash can often be managed by rotating injection sites rather than switching medications.
How do I know if I'm eligible for Mounjaro on the NHS in the UK?
NHS eligibility typically requires a BMI of 30 kg/m² or above, or 27–29.9 with a weight-related health condition (type 2 diabetes, high blood pressure, sleep apnoea, or cardiovascular disease). South Asian and Black African/Caribbean adults may have lower thresholds. Your GP can assess your eligibility and refer you to specialist services if appropriate.
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