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
What Is the Active Ingredient in Mounjaro?
Tirzepatide is the active ingredient in Mounjaro, a prescription-only weight management injection licensed in the UK. It's a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways to help regulate appetite and support weight loss. If you're considering Mounjaro, understanding how tirzepatide works and how to access it in the UK is essential. Our GLP-1 weight loss calculator can help you estimate potential results at different doses.
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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 Tirzepatide Works and Accessing Mounjaro in the UK
Tirzepatide works by mimicking two natural gut hormones: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action is what makes Mounjaro unique compared to other weight loss injections like Wegovy, which targets GLP-1 alone. When you take tirzepatide, it helps reduce your appetite, supports you feeling fuller after smaller meals, and slows how quickly food leaves your stomach. In the SURMOUNT-1 clinical trial, adults using tirzepatide alongside a reduced-calorie diet and increased physical activity lost an average of 15–21% of their body weight over 72 weeks — though individual results vary significantly.
In the UK, tirzepatide comes as a Mounjaro KwikPen — a pre-filled injection device you use once weekly. Doses range from 2.5 mg (starting dose) to 15 mg (maximum). Your prescriber will gradually increase your dose over several weeks to find the level that works best for you — this process is called dose titration. Access in the UK is available through two main routes: NHS specialist weight management services or private online prescription.
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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.
NHS vs Private Access: Costs and How to Get Started
NHS access: NICE has approved tirzepatide for use within NHS specialist tier 3 weight management services. However, availability varies significantly by region, and waiting lists can be very long in many areas. You'll need a referral from your GP to a specialist service.
Private access: Tirzepatide is available through private online prescribing following a clinical assessment with a UK-licensed prescriber. Costs typically range from £129–£320 per month depending on your dose and provider. Private access doesn't require an NHS referral. Most people start at the 2.5 mg dose and progress through the titration schedule. Use our BMI calculator above to check your eligibility, or consult with a prescriber about whether tirzepatide is right for you.
- 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 Mounjaro?
Yes. Tirzepatide is the active pharmaceutical ingredient, and Mounjaro is the brand name under which it's marketed in the UK. All Mounjaro injections contain tirzepatide in doses ranging from 2.5 mg to 15 mg. When someone mentions Mounjaro, they're referring to tirzepatide.
How does tirzepatide differ from semaglutide (Wegovy)?
Tirzepatide (Mounjaro) is a dual GIP and GLP-1 receptor agonist, while semaglutide (Wegovy) is a GLP-1-only agonist. Tirzepatide activates two hormone pathways instead of one, which may contribute to greater average weight loss in clinical trials — though results vary between individuals.
What is the Mounjaro KwikPen?
The KwikPen is the pre-filled injection device for Mounjaro. Each pen contains tirzepatide at a specific dose and delivers one week's treatment. The pen is easy to use and comes with clear instructions. You'll receive a new pen each week, and each contains four weeks of medication.
How long does it take to reach the maximum tirzepatide dose?
Dose titration typically takes 20 weeks to reach the maximum 15 mg dose. You start at 2.5 mg and increase by 2.5 mg every 4 weeks (minimum). Some people find an effective dose before reaching 15 mg and may stay at that level if results are satisfactory.
Can I access tirzepatide (Mounjaro) without waiting for the NHS?
Yes. Private online prescribing is available through UK-licensed prescribers who can assess your eligibility and issue a prescription if appropriate. This route doesn't require an NHS referral and can be faster than NHS specialist services, though costs apply.
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.