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 Kwikpen Safely
The Mounjaro Kwikpen is a pre-filled pen device that delivers tirzepatide, a dual GIP and GLP-1 receptor agonist used for weight management in UK adults. This guide walks you through safe and correct use of your Kwikpen, from storage to injection technique, to help you get the most from your prescription. Always follow your prescriber's 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.
Step-by-Step Injection Guide
Before you inject: Remove your Kwikpen from the refrigerator 15–30 minutes before use to allow it to reach room temperature. Check the medication inside by looking at the medication window — it should be clear and colourless. Never use if it appears cloudy, discoloured, or contains particles.
Preparing the injection: Wash your hands thoroughly. Remove the outer needle cap and check the dose window shows your prescribed dose (e.g. 2.5 mg, 5 mg, 10 mg, 15 mg). Attach the needle by screwing it on firmly until it clicks. Remove the outer needle cap and small inner cap.
Choosing your injection site: You can inject into your stomach (abdomen), upper thigh, or upper arm. Rotate injection sites each week to prevent lipohypertrophy (thickening of tissue). If injecting into your arm, get assistance from another person.
Injecting safely: Pinch a fold of skin at your chosen site. Insert the needle at a 90-degree angle straight into the skin fold. Press the injection button firmly until you hear a click and see the dose counter return to zero. Hold the button pressed for 3 seconds, then withdraw the needle.
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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.
Storage and Disposal Guidance
Unopened pens: Store at 2°C to 8°C in a refrigerator in their original carton. Do not freeze — if frozen accidentally, discard and do not use. Unopened pens can be used until the expiry date printed on the carton.
After first use: After your first injection, you can store your pen at room temperature (below 30°C) for up to 30 days. Keep it away from heat and direct sunlight. Always keep the pen with its cap on when not in use.
Disposal: Dispose of used needles in a sharps bin — never in household waste. Your pharmacy or GP surgery can provide a sharps bin and advise on safe disposal. Dispose of the entire pen 30 days after first use, even if medication remains.
- 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 I miss a Mounjaro injection?
If you remember within 4 days of the missed dose, inject as soon as you remember, then continue your normal weekly schedule. If more than 4 days have passed, skip the missed dose and take your next injection on your normally scheduled day. Never take a double dose to make up for a missed one.
Can I inject Mounjaro myself, or do I need help?
You can inject yourself into your stomach or thigh. If injecting into your upper arm, you'll need assistance from another person. Most UK patients self-inject into the stomach or thigh without difficulty. Your healthcare provider or pharmacist can demonstrate the technique.
How do I know if my Mounjaro injection worked?
You won't feel the injection working immediately. Mounjaro's effects (reduced appetite, feeling fuller sooner) typically become noticeable within the first few weeks. Track your weight loss using our free progress tracker to monitor results over time.
What happens if I accidentally freeze my Mounjaro pen?
Do not use a frozen Kwikpen — discard it safely and contact your pharmacy or prescriber for a replacement. Freezing damages the medication and makes it unsafe to inject.
Is Mounjaro the same as Wegovy?
No. Mounjaro contains tirzepatide (a dual GIP and GLP-1 agonist), while Wegovy contains semaglutide (GLP-1 agonist only). Both are prescription-only weight management injections licensed in the UK, but they work through different mechanisms. Your prescriber will assess which is most 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.