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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Rash From Mounjaro — What You Need to Know
A rash can occasionally occur when taking Mounjaro (tirzepatide), though it's not one of the most common side effects. If you develop a skin reaction while using this prescription injection, it's important to understand what it might be, when to seek medical advice, and how to access Mounjaro safely in the UK. This guide explains rash reactions, eligibility for prescription weight management treatment, and the costs involved in accessing Mounjaro through both private and NHS routes.
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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 — Causes and When to Seek Help
Rash or injection site reactions can occur with Mounjaro, typically appearing as redness, itching, or irritation at the injection site or occasionally as a generalised skin reaction elsewhere on the body. Most injection site reactions are mild and resolve within a few days without treatment. However, a serious allergic reaction — presenting as swelling of the face, lips, tongue, or throat — requires urgent medical attention and you should call 999 or go to A&E immediately.
Rashes that are widespread, severe, blistering, or accompanied by fever should be reported to your GP without delay, as these may indicate a more serious reaction. For minor rashes, rotate your injection sites (stomach, thigh, or upper arm) to allow skin to recover, and avoid injecting into irritated areas. If a rash persists beyond a week, worsens, or causes significant discomfort, contact your GP or the prescriber who issued your Mounjaro prescription. Do not stop taking Mounjaro without medical guidance — your prescriber may adjust your dose or recommend switching to an alternative weight management medication such as Wegovy (semaglutide), which has a different injection mechanism.
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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 Mounjaro in the UK — Cost and Routes
NHS route: Mounjaro is available through NHS specialist weight management services (tier 3/4), but access is limited and varies significantly by region. You'll need a GP referral and may face waiting lists of several months. Private route: UK-licensed prescribers offer online consultations (typically £20–£50) followed by prescription to a GPhC-registered pharmacy. Private Mounjaro costs approximately £129–£180 per month at lower doses and £200–£280 at higher doses, depending on your dose titration schedule. Both routes require eligibility assessment: 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. Use our GLP-1 weight loss calculator to estimate potential results at different doses, and our free BMI calculator to check your eligibility 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.
Is a rash a common side effect of Mounjaro?
Rash is not listed as a very common side effect of Mounjaro, though injection site reactions (redness, bruising, pain) occur in some people. A generalised rash is less frequent but possible. Always report persistent or worsening skin reactions to your GP or prescriber.
Can I continue taking Mounjaro if I develop a rash?
Minor injection site rashes often resolve with continued treatment, especially if you rotate injection sites. However, do not continue if the rash is severe, spreading, or accompanied by swelling. Contact your prescriber or GP immediately for guidance — they may adjust your dose or suggest an alternative.
How much does Mounjaro cost privately in the UK?
Private Mounjaro costs approximately £130–£280 per month depending on your dose (2.5mg through 15mg). This includes the prescription and medication dispensed by a GPhC-registered pharmacy. Online consultations typically cost £20–£50 separately. Costs vary between providers.
Can I switch from Mounjaro to Wegovy if I get a rash?
Yes, if a rash becomes problematic on Mounjaro, your prescriber may recommend switching to Wegovy (semaglutide), a GLP-1 agonist with a different mechanism. Both are prescription-only and require clinical assessment. Wegovy costs similarly in the UK private market.
How do I access Mounjaro on the NHS if I have a rash concern?
Ask your GP for a referral to your local tier 3 specialist weight management service. They'll assess you, monitor side effects including rash, and manage your Mounjaro prescription. NHS access is free but availability varies by region and waiting times can be lengthy.
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.