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 Mouth Ulcers vs Wegovy: What You Need to Know
Mouth ulcers can occur with weight loss injections, but they are not listed as common side effects in clinical trials for either Mounjaro or Wegovy. Both medications are prescription-only GLP-1-based treatments licensed in the UK for weight management in adults. Understanding how these medications work differently, and how individual side effects vary, can help you and your prescriber decide which option may be more suitable for your circumstances. This guide compares the two treatments based on clinical evidence and mechanism of action.
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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 Mechanism and Trials
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist—meaning it activates two natural hormone pathways. In the SURMOUNT-1 clinical trial, participants on 15 mg Mounjaro lost an average of 20.9% of their body weight over 72 weeks. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single pathway. In the STEP-1 trial, participants on the maximum 2.4 mg dose lost approximately 14.9% body weight over 68 weeks.
Neither clinical trial reported mouth ulcers as a common or very common side effect. Mouth ulcers are not listed on the MHRA product information for either medication. However, individual responses vary significantly. Gastrointestinal symptoms like nausea, diarrhoea, and indigestion are the most frequently reported side effects for both tirzepatide and semaglutide. If you experience mouth ulcers while taking either medication, contact your GP or prescriber immediately for assessment.
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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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Choosing Between the Two: Individual Suitability
Both Mounjaro and Wegovy require a prescription and clinical assessment before starting. Your prescriber will consider your medical history, other medications, BMI, and any existing health conditions to determine which is appropriate for you. Neither medication is "better" for mouth ulcers specifically—both are well-tolerated in most people. Eligibility is based on BMI (30 kg/m² or above, or 27–29.9 kg/m² with weight-related health conditions), and access in the UK is through NHS specialist services or private online prescribing following assessment.
Use our free BMI calculator to check your starting point, and our GLP-1 weight loss calculator to understand potential outcomes at different doses. Your prescriber's guidance will be based on your individual health profile, not on mouth ulcers risk alone.
- 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.
Are mouth ulcers a common side effect of Mounjaro?
No. Mouth ulcers are not listed as a common or very common side effect in Mounjaro clinical trials or MHRA product information. The most frequently reported side effects are gastrointestinal (nausea, diarrhoea, constipation). However, individual experiences vary. If you develop mouth ulcers while on Mounjaro, inform your GP or prescriber.
Do Wegovy and Mounjaro cause mouth ulcers equally?
Neither medication lists mouth ulcers as a typical side effect. Both are GLP-1-based treatments with similar gastrointestinal side effect profiles. If mouth ulcers occur with either medication, they are likely individual reactions rather than a class effect, and require prescriber assessment.
What is the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 agonist (activating two hormone pathways), while Wegovy is a GLP-1 agonist only (single pathway). This difference in mechanism may contribute to different weight loss outcomes—Mounjaro users lost approximately 20.9% body weight on average in trials, versus 14.9% for Wegovy.
Can I switch from Mounjaro to Wegovy if I get mouth ulcers?
Yes, switching is possible if mouth ulcers develop, but this requires prescriber guidance. Your GP or specialist will assess whether the ulcers are medication-related and what the safest next step is. Do not stop or switch medications without professional advice.
How do I access Mounjaro or Wegovy in the UK?
Both are prescription-only and require clinical assessment. Access is available through NHS specialist tier 3 weight management services (referral from GP) or private online prescribing services following an online consultation. Eligibility is based on BMI and health conditions.
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