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
Free tracker included with every download this week
Mounjaro Pens vs Wegovy: How Do They Compare?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weekly weight loss injections licensed in the UK, but they work in different ways. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates only the GLP-1 pathway. This guide compares the two medications factually so you understand the key differences. Individual results vary significantly, and which medication may be suitable for you depends on your personal health profile — a prescriber will assess this during a clinical consultation.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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.
How Mounjaro and Wegovy Work: Mechanism Differences
Mounjaro (tirzepatide) activates two natural hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help regulate appetite more broadly, reduce food cravings, and support feeling fuller after smaller meals. It also slows gastric emptying and helps regulate how the body uses fat.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single hormone pathway. It works by signalling fullness to the brain, reducing appetite, and slowing how quickly food leaves the stomach.
The dual mechanism of Mounjaro may be why some clinical data suggests slightly higher average weight loss, though both medications are effective. The choice between them often depends on individual tolerability and prescriber assessment rather than mechanism alone.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Clinical Trial Data: What the Evidence Shows
Mounjaro (SURMOUNT trial): At 72 weeks on the maximum 15 mg dose, participants lost an average of 20.9% of their body weight alongside a reduced-calorie diet and increased physical activity.
Wegovy (STEP-1 trial): At 68 weeks on the maximum 2.4 mg dose, participants lost an average of 14.9% of their body weight under similar lifestyle conditions.
These are clinical trial averages — individual results vary considerably. Use our free GLP-1 weight loss calculator to explore potential weight loss projections. Both medications require ongoing lifestyle changes; medication alone does not studied results. A prescriber will discuss expected outcomes based on your personal circumstances.
- 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 is the main difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Wegovy (semaglutide) is a GLP-1 agonist only. The dual mechanism may contribute to slightly higher average weight loss in clinical trials, but both are effective. Individual response varies.
Which one causes fewer side effects?
Both medications share common side effects including nausea, diarrhoea, and stomach discomfort — especially during dose escalation. Frequency and severity vary between individuals. There is no definitive evidence that one has significantly fewer side effects than the other. A prescriber assesses tolerability during your treatment.
How often do I inject Mounjaro or Wegovy?
Both are once-weekly injections. Mounjaro doses range from 2.5 mg to 15 mg weekly. Wegovy doses range from 0.25 mg to 2.4 mg weekly. Each medication starts at a low dose and escalates gradually over weeks to minimise side effects.
Can I switch from Mounjaro to Wegovy or vice versa?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion chart — approximate equivalences exist (for example, Mounjaro 5 mg is roughly equivalent to Wegovy 0.5 mg), but a prescriber will determine the safest switching approach based on your individual response.
How do I know which one is right for me?
Neither medication is universally "better" — suitability depends on your health profile, any existing conditions, tolerability to side effects, and prescriber assessment. A clinical consultation will evaluate your BMI, weight-related health conditions, medications, and medical history to determine which option may be appropriate for you.
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.