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 Spots on Face vs Wegovy: What You Need to Know
Facial skin changes, including spots or rashes, have been reported by some people taking GLP-1 and GIP/GLP-1 receptor agonist medications like Mounjaro and Wegovy. This comparison guide explains what clinical evidence shows about skin side effects with both tirzepatide (Mounjaro) and semaglutide (Wegovy), how they work differently, and how to discuss concerns with your GP. Both are prescription-only medications used alongside lifestyle changes for weight management in UK adults with a BMI of 30 or above, or 27 and above with weight-related health conditions.
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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: Mechanism and Clinical Trial Context
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways to regulate appetite and support weight management. Clinical trials (SURMOUNT-1) showed average weight loss of 15–21% at maximum dose. Wegovy (semaglutide) is a GLP-1 agonist only — it activates one hormone pathway. Clinical trials (STEP-1) showed average weight loss of approximately 15% at maximum dose (2.4 mg).
Regarding facial skin changes: both medications can cause injection site reactions (redness, bruising, pain) which are very common. Reports of acne, rashes, or spots are listed as less common side effects in UK prescribing information. Individual skin responses vary significantly and may be influenced by genetics, diet, hydration, and stress — not solely by the medication itself.
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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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Eligibility, Prescribing, and Individual Suitability
Both Mounjaro and Wegovy are prescription-only medicines in the UK requiring clinical assessment before prescription. Access through NHS specialist weight management services or private online prescribing following evaluation. Your prescriber will assess your medical history, current medications, and skin conditions to determine which medication (if either) is appropriate for you.
If you develop facial spots or skin changes after starting either medication, inform your GP or prescriber immediately. They may suggest skincare adjustments, dose modification, or a switch to an alternative approach. Never stop taking a prescription medication without medical guidance. Use our free GLP-1 weight loss calculator to explore potential outcomes, and speak with your prescriber about individual side effect concerns.
- 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 Mounjaro more likely to cause facial spots than Wegovy?
Clinical trial data does not show Mounjaro causes facial spots more frequently than Wegovy. Both can cause injection site reactions and less commonly, acne or rashes. Individual skin responses vary based on genetics, hydration, diet, and stress factors — not solely medication type. Your prescriber can discuss your personal risk factors.
What should I do if I develop spots after starting Mounjaro or Wegovy?
Contact your GP or prescriber immediately. Do not stop taking the medication without medical guidance. They may recommend skincare changes, hydration improvements, dose adjustment, or switching medications. Skin side effects often improve with time, but professional assessment is essential.
Are injection site reactions the same as facial spots?
No. Injection site reactions occur at the injection location (stomach, thigh, or arm) and are very common — redness, bruising, or itching. Facial spots or acne are separate and less common systemic side effects. Only inject at the sites recommended (not the face).
How does the GIP mechanism in Mounjaro affect skin health?
The additional GIP receptor activation in Mounjaro (compared to Wegovy's GLP-1 only) affects appetite and glucose control, not directly skin health. No clinical evidence shows GIP activation causes more skin problems. Both medications work on appetite pathways, which may indirectly affect skin through diet changes.
Can I use skincare products while taking Mounjaro or Wegovy?
Yes. Continue your normal skincare routine and stay well hydrated — dehydration can worsen acne. If you develop new skin concerns, discuss with your GP or dermatologist. They can advise on suitable products alongside your weight management medication. Always inform healthcare professionals of all medications you're taking.
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