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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How to Use Your Mounjaro Pen — UK Guidance
Mounjaro (tirzepatide) is administered once weekly using a pre-filled KwikPen injection device. If you've been prescribed Mounjaro through NHS specialist weight management services or a private online prescriber, understanding how to use your pen correctly is essential for safety and effectiveness. This guide covers storage, injection technique, site rotation, dose schedules, and practical tips for managing your Mounjaro treatment in the UK. Always follow your prescriber's specific instructions alongside this information.
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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 KwikPen: Storage, Doses, and Injection Technique
Your Mounjaro KwikPen is a pre-filled, single-use injection device containing a 4-week supply (4 doses). Each pen delivers one weekly dose automatically when you press the injection button.
Storage before first use: Keep unopened pens in the refrigerator at 2–8°C in the original carton. Do not freeze. Pens can be used until the expiry date if refrigerated.
Storage after first use: After your first injection, the pen can be kept at room temperature (below 30°C) for up to 30 days. Dispose of the pen 30 days after first use, even if it still contains medication.
Injection sites: Rotate between your abdomen (stomach), outer thigh, or upper arm. Change the site each week to reduce skin irritation. Avoid areas with bruising, hardness, or redness.
Dose schedule: Inject once per week on the same day each week. If you miss a dose within 4 days, inject as soon as you remember. If more than 4 days have passed, skip that dose and continue your normal schedule.
Clinically BackedWeight Loss
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
NHS Access and Eligibility for Mounjaro in the UK
In the UK, Mounjaro (tirzepatide) is available through NHS tier 3 specialist weight management services following referral from your GP, or through private online prescribing following a clinical assessment. NICE (National Institute for Health and Care Excellence) has approved tirzepatide for use within NHS weight management services.
NHS eligibility: You may be eligible if you have a BMI of 30 or above, or 27–29.9 with weight-related conditions such as type 2 diabetes, high blood pressure, or heart disease. South Asian, Chinese, Black African, or Black Caribbean adults may have lower BMI thresholds. NHS access varies significantly by region and involves waiting lists. Our free BMI calculator can help you check your BMI instantly, and our GLP-1 weight loss calculator can project potential results at different Mounjaro doses alongside lifestyle changes.
---COL_J_CONTINUED---- 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.
How do I inject Mounjaro from the KwikPen?
Remove the pen from the fridge 30 minutes before injection to allow it to warm to room temperature. Clean your injection site with an alcohol wipe. Press the pen firmly against your skin at a 90-degree angle and hold the button down until you hear a click and feel a vibration, which signals the injection is complete. Hold in place for 3 seconds, then remove. Do not rub the injection site.
What should I do if I accidentally freeze my Mounjaro pen?
If your Mounjaro pen has been frozen, do not use it. Freezing damages the medication. Contact your prescriber or pharmacy for a replacement pen. Always store unopened pens in the refrigerator at 2–8°C, not the freezer compartment.
Can I switch between Mounjaro and Wegovy on the NHS?
Switching between Mounjaro (tirzepatide) and Wegovy (semaglutide) is a clinical decision made by your prescriber. Both are GLP-1 medications available through NHS specialist services, but Mounjaro is a dual GIP/GLP-1 agonist, while Wegovy is a single GLP-1 agonist. Your prescriber will advise if a switch is appropriate based on your response and health needs.
What if I miss a weekly Mounjaro injection?
If you miss an injection, inject within 4 days of the missed dose and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and take your next injection on your normal scheduled day. Never take a double dose. Maintain at least 72 hours (3 days) between doses.
Is Mounjaro available through all NHS areas in the UK?
Mounjaro availability varies by region. Access is through NHS tier 3 specialist weight management services, and availability depends on your integrated care board (ICB). Some areas have shorter waiting lists than others. Ask your GP to refer you to your local specialist weight management service to check availability and waiting times in your area.
Ready to take control of your weight loss journey?
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