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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Mounjaro Instructions — How to Use Your KwikPen Safely
Mounjaro (tirzepatide) is a prescription-only, once-weekly injectable medicine for weight management in UK adults. This guide provides clear, practical instructions on how to use your Mounjaro KwikPen, including storage, injection technique, dose schedules, and what to do if you miss a dose. Always follow the specific instructions provided by your prescriber, as individual treatment plans vary. If you're unsure about any step, contact your healthcare provider or pharmacist before injecting.
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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 to Store Your Mounjaro KwikPen
Before first use (unopened pens): Store in a refrigerator at 2°C to 8°C. Keep the pen in its original carton to protect from light. Do not freeze — if your pen has been frozen, do not use it. Mounjaro can be stored safely in the fridge until the expiry date printed on the carton.
After first use (opened pen): You can store your pen at room temperature below 30°C for up to 30 days. Keep it away from direct heat and sunlight. After 30 days, dispose of the pen safely — ask your pharmacist or GP about needle disposal if you're unsure. Do not refrigerate after first use.
When travelling: If you need to travel, use an insulated cool bag with ice packs to keep unopened pens between 2°C and 8°C. Once opened, the pen can be kept at room temperature if necessary, but keep it away from extreme heat.
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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.
Step-by-Step Mounjaro Injection Instructions
Before injecting: Wash your hands thoroughly. Remove your pen from the fridge 15–30 minutes before use to allow it to reach room temperature — this reduces discomfort. Check that the liquid inside is clear and colourless. Do not use if it appears cloudy or contains particles.
Choose your injection site: You can inject into your stomach (abdomen), upper thigh, or upper arm. Rotate your injection site each week to avoid lipodystrophy (skin changes). Pinch the skin gently and insert the needle at a 90-degree angle. Hold the pen in place for 3 seconds after the injection to ensure the full dose is delivered. You do not need to rub the injection site afterwards.
After injection: Place the used needle in a sharps bin. Store your pen safely away from children and pets.
- 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 dose should I start on?
Your prescriber will determine your starting dose based on your individual health. Most people start on 2.5 mg once weekly. You will stay on each dose for at least 4 weeks before increasing to the next level. Doses escalate gradually: 2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg. Do not increase your dose without guidance from your prescriber.
Can I inject Mounjaro myself at home?
Yes — Mounjaro is designed for self-injection at home once you have received training from your healthcare provider or pharmacist. They will show you the correct injection technique before you start. If you are uncomfortable injecting yourself, ask whether a family member or carer can help, or discuss alternative options with your GP.
What do I do if I miss a dose?
If you remember within 4 days of your missed dose, inject as soon as you remember and continue your normal schedule the following week. If more than 4 days have passed, skip that dose and inject on your next scheduled day. Never inject two doses within 72 hours (3 days). Contact your prescriber if you're unsure.
How do I know if my Mounjaro pen is working correctly?
After injection, you may see a small amount of liquid at the needle tip — this is normal. You should feel a slight click or resistance as you inject. If the pen does not seem to work or you have concerns, do not use it. Contact your pharmacist or prescriber immediately for advice.
Can I use Mounjaro if I'm taking other medications?
Mounjaro may affect how your body absorbs some oral medications, particularly oral contraceptives — you may need additional contraception for 4 weeks after starting and after each dose increase. Always inform your prescriber of all medications you take. They will assess any interactions and adjust your treatment plan if needed.
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.