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
How to Use Your Mounjaro Pen — Step-by-Step Guide
Using a Mounjaro pen correctly is straightforward once you understand the basics. Your Mounjaro KwikPen is a pre-filled, single-use injection device that delivers your weekly dose of tirzepatide. This guide explains how to prepare, inject, and store your pen safely. Always follow the instructions provided with your prescription, and contact your prescriber or pharmacist if you have any questions about using your pen.
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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.
How to Inject Your Mounjaro Pen
Before you inject: Check that your pen is the correct dose and that the liquid is clear and colourless. If it looks cloudy or discoloured, do not use it — contact your pharmacy. Remove the pen from the fridge 15 minutes before injection to allow it to reach room temperature; this makes the injection more comfortable.
Choosing an injection site: You can inject into your stomach (abdomen), upper thigh, or upper arm. Rotate your injection site each week to prevent skin irritation. Never inject into areas where the skin is tender, bruised, or scarred.
The injection process: Pinch the skin at your chosen site. Insert the needle at a 90-degree angle and press the injection button until you hear a click. Hold the button down for 3 seconds to ensure you receive the full dose. Withdraw the needle and release the skin. You may see a small amount of blood or clear liquid at the injection site — this is normal.
After injection: Place the used pen in a sharps bin. Do not reuse the pen. If you find injecting difficult, ask a family member or healthcare professional for help.
Clinically BackedWeight Loss
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.
Storage and Care of Your Mounjaro Pen
Before first use: Store unopened pens in the fridge at 2°C to 8°C. Keep them in the original carton to protect from light. Check the expiry date — do not use if expired.
After first use: Once you have used your pen for the first time, you can store it at room temperature (below 30°C) for up to 30 days. After 30 days, dispose of the pen even if there is medication remaining. Never freeze your pen — if frozen, it cannot be used.
Travel tips: If travelling within the UK, keep your pen in an insulated cool bag with ice packs. When flying internationally, carry your Mounjaro in your hand luggage (never checked luggage) with a prescription letter from your GP or prescriber. Always check airline regulations in advance.
- 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 do I do if I miss my weekly Mounjaro injection?
If you remember within 4 days of your scheduled injection, take it as soon as you can 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 two injections close together — maintain at least 72 hours (3 days) between doses. Contact your pharmacist or prescriber if you're unsure.
Can I reuse my Mounjaro KwikPen?
No. Each Mounjaro KwikPen is single-use only and contains enough medication for one weekly injection. After you have used the pen, it must be disposed of in a sharps bin. Do not attempt to reuse it. Each new week requires a fresh pen from your prescription supply.
What injection site pain or redness is normal?
Mild redness, bruising, or slight soreness at the injection site is common and usually resolves within a few hours. Rotating your injection site each week helps prevent irritation. If you experience severe pain, large bruises, persistent redness lasting more than a few days, or signs of infection (warmth, pus, increasing swelling), contact your GP or pharmacist.
How should I dispose of my used Mounjaro pen?
Place your used pen in a sharps bin specifically designed for needles and injection devices. Do not put it in regular household waste or recycling. Most UK pharmacies and GP surgeries accept sharps bins for disposal. Some prescribing services provide sharps bins with your medication delivery. Ask your pharmacist where to dispose of your pen safely.
Is Mounjaro safe to use with other medications I'm taking?
Mounjaro may interact with some medications, particularly oral contraceptives and certain diabetes treatments. If you take oral contraceptives, add a barrier method or switch to a non-oral contraceptive during your first 4 weeks on Mounjaro and for 4 weeks after each dose increase. Always inform your prescriber of all medications you take before starting treatment.
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.