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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What Is In Mounjaro Injection?
Mounjaro is a prescription-only weekly weight loss injection containing tirzepatide as its active ingredient. Unlike Wegovy, which uses a single hormone pathway, Mounjaro works as a dual GIP and GLP-1 receptor agonist—meaning it activates two natural hormone systems to help regulate appetite and support weight management. If you're considering this weekly weight loss injection for UK weight loss, understanding what's actually inside the KwikPen and how it differs from other options is essential for making an informed decision with your GP or prescriber.
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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.
Tirzepatide vs Semaglutide: Active Ingredients Explained
Mounjaro contains tirzepatide as its active pharmaceutical ingredient, while Wegovy contains semaglutide. Both are GLP-1 receptor agonists, but tirzepatide adds a second mechanism: it also activates GIP receptors. This dual action—GIP plus GLP-1—is what distinguishes Mounjaro from other weight loss injections on the market.
In the SURMOUNT-1 clinical trial, tirzepatide at the maximum dose produced average weight loss of approximately 20.9% of body weight over 72 weeks, alongside dietary changes and increased physical activity. Semaglutide (Wegovy) showed average weight loss of around 14.9% in the STEP-1 trial under similar conditions. Both medications slow gastric emptying and reduce appetite signals, but the dual mechanism may explain the higher average weight loss seen with Mounjaro, though individual results vary significantly.
Your prescriber will assess which active ingredient is most suitable for your individual health profile and medical history. Both require a clinical assessment before prescription.
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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.
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Mounjaro KwikPen: Inactive Ingredients & Formulation
Each Mounjaro KwikPen contains tirzepatide dissolved in a sterile solution with inactive ingredients including sodium phosphate dibasic dihydrate, sodium phosphate monobasic dihydrate, sodium chloride, and water for injection. The pen delivers a once-weekly subcutaneous injection available in five doses: 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg. Each prefilled pen contains four doses (one month's supply) and must be stored in a refrigerator at 2–8°C until use.
The KwikPen design allows for easy self-injection into the stomach, thigh, or upper arm. After opening, pens can be kept at room temperature below 30°C for up to 30 days. Always check the expiry date and never use a frozen pen. If you have questions about ingredients or allergies, discuss these with your prescriber before starting treatment.
- 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 tirzepatide the same as semaglutide?
No. Both are GLP-1 receptor agonists, but tirzepatide (Mounjaro) is a dual GIP and GLP-1 agonist, while semaglutide (Wegovy) acts only on GLP-1 receptors. This dual mechanism may explain why Mounjaro showed higher average weight loss in clinical trials, though individual results vary. Both require prescription and clinical assessment.
What makes Mounjaro different from other weight loss injections?
Mounjaro's dual GIP and GLP-1 mechanism sets it apart. GIP receptors help regulate glucose and fat metabolism alongside GLP-1's appetite and satiety effects. This combination may provide stronger appetite suppression than single-mechanism injections like Wegovy or Saxenda. Your GP can advise which is most suitable for you.
Does Mounjaro contain any allergens?
Mounjaro's inactive ingredients are generally well tolerated, but if you have known allergies to any component, inform your prescriber before starting. The medication itself does not contain common allergens like gluten, dairy, or nuts. Always read the patient information leaflet with your prescription.
How is Mounjaro different from Ozempic or Saxenda?
Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist and is licensed for weight management. Ozempic (semaglutide) is licensed for type 2 diabetes at lower doses. Saxenda (liraglutide) is a daily GLP-1 injection. Mounjaro is once-weekly, which may improve adherence. All are prescription-only and require clinical assessment.
Can I use our GLP-1 weight loss calculator to estimate results with Mounjaro?
Yes. Our free GLP-1 weight loss calculator can help you understand potential weight loss ranges at different Mounjaro doses based on clinical trial data. Remember that individual results vary significantly depending on diet, activity, and personal metabolism. Use it alongside your GP's guidance.
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.