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 on Stomach vs Wegovy — What's the Difference?
Injection site reactions, including rashes on the stomach, can occur with both Mounjaro and Wegovy. While neither medication assures a rash will develop, understanding the differences between these two GLP-1 and dual GIP/GLP-1 medications can help you make an informed decision. Both are prescription-only weight management injections available in the UK following clinical assessment. This guide compares their side effect profiles, mechanism of action, and what to expect if a rash develops at your injection site.
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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.
How Mounjaro and Wegovy Differ in Side Effect Profiles
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it works on two hormone pathways rather than one. In clinical trials (SURMOUNT-1), common injection site reactions were reported in approximately 5–7% of participants, including redness, bruising, pain, and in some cases, rash. Wegovy (semaglutide) is a GLP-1-only receptor agonist. The STEP-1 trial reported similar injection site reaction rates, with redness and bruising being most common, though rashes were less frequently reported than with Mounjaro. Both medications are injected once weekly under the skin (subcutaneously), typically in the stomach, thigh, or upper arm. The dual mechanism of Mounjaro may contribute to slightly higher rates of localised reactions, though individual responses vary significantly. Neither medication's rash is typically serious, but persistent or spreading rashes warrant medical attention. Your prescriber can advise whether to continue, pause, or switch to the alternative injection.
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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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Which Medication Might Suit You — Individual Factors
The choice between Mounjaro and Wegovy depends on your personal health profile, not just side effect risk. Both require a UK prescription following clinical assessment of your BMI, weight-related health conditions, and medical history. If you develop a stomach rash on either medication, your GP or prescriber can help you manage it — rotating injection sites, using a new pen, or considering a switch to the alternative. Our free GLP-1 weight loss calculator can help you understand projected outcomes at different doses of either medication, while the BMI calculator on this page confirms your eligibility based on UK NHS thresholds. Some people tolerate one medication better than the other, which is why prescriber support and follow-up are essential. Report any persistent rashes, spreading reactions, or signs of infection (warmth, swelling, pus) to your GP immediately.
- 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?
A Mounjaro rash at the injection site typically appears as redness, small raised bumps, or itching within hours to days after injection. It may resemble a mild allergic reaction localised to the stomach, thigh, or arm where you injected. Most resolve within 1–2 days. If the rash spreads beyond the injection site, persists beyond 3 days, or shows signs of infection, contact your GP.
Is a rash more common with Mounjaro or Wegovy?
Clinical trials show similar injection site reaction rates for both medications (around 5–7%). Rashes specifically are slightly more frequently reported with Mounjaro, possibly because of its dual GIP/GLP-1 mechanism. However, individual responses vary considerably — some people experience no rash with either medication, whilst others develop one.
Can I switch from Mounjaro to Wegovy if I develop a rash?
Yes. If you develop a persistent or bothersome rash on Mounjaro, your prescriber may recommend switching to Wegovy (or vice versa). Always discuss this with your GP or the prescriber managing your treatment — they can assess whether the rash is truly medication-related or caused by injection technique, site sensitivity, or another factor.
How should I inject Mounjaro to avoid a rash?
Rotate injection sites weekly — don't inject in the same spot twice in a row. Use a new pen each week. Ensure your skin is clean and dry before injecting. If you develop a rash after injecting in the stomach, try your thigh or upper arm next. Allow at least a finger-width of space between injection sites.
When should I contact my GP about a Mounjaro stomach rash?
Contact your GP if the rash persists beyond 3 days, spreads beyond the injection site, becomes painful or warm to touch, shows signs of infection (pus, increased swelling), or if you develop an allergic reaction (facial swelling, difficulty breathing). For minor redness that resolves within 24 hours, monitor at home and mention it at your next review.
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