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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Why Is Your Mounjaro Pen Not Turning?
If your Mounjaro KwikPen isn't turning during the injection process, it may indicate a device issue or incorrect preparation. This guide explains common reasons your pen might not be turning, how to troubleshoot safely, and what to do next. Understanding your tirzepatide delivery device is important for safe administration. If you're unsure at any point, always contact your prescriber or pharmacist before attempting another injection.
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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.
Common Reasons Your Mounjaro Pen Won't Turn
A Mounjaro KwikPen that won't turn during injection can happen for several reasons. The pen may not have been held in position long enough—the full 10-second count is essential for the dose to be delivered properly, and the turning motion won't feel smooth if you release early. Temperature matters too: if your pen has been stored in a freezer or extremely cold conditions, the medication may have become too thick, preventing smooth operation. Check that you've removed the grey release button completely before attempting injection. If the pen has been kept below 2°C for extended periods, this can affect how smoothly the mechanism operates. Additionally, ensure you're using a new needle each time and that the needle has been attached correctly and securely.
- Hold the pen still against your skin for the full 10 seconds during injection
- Verify the grey release button is completely removed
- Check storage temperature—keep between 2°C and 8°C when sealed, or below 30°C after first use
- Confirm the needle is properly attached and new
- Never force or twist the pen if resistance occurs
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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 vs Wegovy — Device Differences and Clinical Context
Mounjaro (tirzepatide) and Wegovy (semaglutide) use different delivery devices, and both work differently from a clinical perspective. Mounjaro uses the KwikPen with a rotating dial mechanism, while Wegovy uses an auto-injection pen. In clinical trials: SURMOUNT-1 showed Mounjaro at 15 mg resulted in approximately clinically studied weight loss over 72 weeks, while STEP-1 demonstrated Wegovy at 2.4 mg achieved around clinically studied weight loss. Mechanically, Mounjaro is a dual GIP and GLP-1 receptor agonist, whereas Wegovy is a single GLP-1 agonist. Both are prescription-only medications requiring clinical assessment. If your device isn't functioning correctly, your prescriber can advise whether to attempt troubleshooting or arrange a replacement dose.
- 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 should I do if my Mounjaro pen won't turn at all?
First, ensure the grey release button is fully removed and the pen has reached room temperature if recently refrigerated. Check that you're holding the pen still against your skin during the 10-second injection window—don't release early or the mechanism won't complete. If it still won't turn, stop and contact your prescriber or pharmacist. Do not force the device as this may damage it.
Can temperature affect how my Mounjaro pen works?
Yes. Mounjaro must be stored at 2–8°C when unopened, or below 30°C after first use. If exposed to freezing temperatures, the medication becomes too viscous and the pen mechanism may not turn smoothly. If this happens, contact your pharmacist before using the pen. Never attempt to use a frozen pen.
Is a Mounjaro pen that won't turn the same as a faulty device?
Not always. Often it's a preparation or technique issue—incorrect temperature, incomplete button removal, or releasing too early during injection. However, if you've verified all these factors and it still won't turn, it may be a device fault. Contact your prescriber or pharmacist for guidance on whether a replacement is needed.
How does Mounjaro's KwikPen compare to Wegovy's device in terms of ease of use?
Mounjaro uses a rotational dial device, while Wegovy is an auto-injection pen—different mechanisms but both straightforward when used correctly. Your prescriber will advise which is more suitable for you based on your individual assessment. Device preference is a minor factor compared to clinical suitability and eligibility.
Should I try a different needle or restart the injection if my pen won't turn?
If the pen won't turn, stop the injection immediately. Remove the needle safely and use a new needle for your next attempt. If it still doesn't work after checking temperature and button removal, contact your pharmacist or prescriber. Do not reuse the same needle or force the mechanism.
Ready to take control of your weight loss journey?
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