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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What Is an Injection Pen for Weight Management?
An injection pen is a pre-filled device used to deliver prescription weight management medication. In the UK, injection pens are used for GLP-1 and GIP/GLP-1 receptor agonist medications like Wegovy (semaglutide) and Mounjaro (tirzepatide). These pens deliver a precise dose once weekly by subcutaneous injection. Understanding how to use, store, and handle your injection pen correctly is essential for safe and effective treatment. This UK health guide explains everything you need to know about weight management injection pens.
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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 Injection Pens Work and What They Contain
Injection pens for weight management are pre-filled, single-use or multi-use devices that deliver a measured dose of medication under the skin. The most common pens used in the UK for weight management are:
- Wegovy (semaglutide): Pre-filled KwikPen containing 4 weekly doses. Available in doses from 0.25 mg to 2.4 mg. Once the pen is opened, it can be used for up to 30 days, then must be disposed of safely.
- Mounjaro (tirzepatide): Pre-filled KwikPen containing 4 weekly doses. Available in doses from 2.5 mg to 15 mg. Also usable for 30 days after first use.
Each injection is administered once weekly, typically on the same day each week. The needle is very fine (29-gauge) and the injection is given into the stomach, thigh, or upper arm. Many people find the injection less painful than expected. These medications work by activating GLP-1 (and in Mounjaro's case, also GIP) hormone pathways, which may help reduce appetite, support feeling fuller after smaller meals, and assist with weight management when used alongside a reduced-calorie diet and increased physical activity.
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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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Storage, Safety, and Using Your Injection Pen Correctly
Proper storage is essential for medication safety. Unopened pens must be kept refrigerated at 2°C to 8°C in their original carton — do not freeze. Once you open a pen for the first time, you can store it at room temperature (below 30°C) for up to 30 days. After 30 days, even if doses remain, the pen must be disposed of safely.
When injecting, rotate your injection site weekly to avoid skin irritation. Common sites include the stomach (most popular), upper thigh, or upper arm. Always wash your hands and the injection site with soap and water before injecting. If you miss a dose within 4 days, take it as soon as you remember and continue your normal schedule. If more than 4 days have passed, skip that dose and take your next injection on your usual day. Never take a double dose. Dispose of used pens in a sharps bin — ask your pharmacy for a free sharps bin if you don't have one.
- 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.
Can I use an injection pen if I'm not diabetic?
Yes. In the UK, Wegovy and Mounjaro injection pens are licensed for weight management in adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions, regardless of diabetes status. These medications are prescription-only and eligibility is assessed individually by a prescriber.
How do I know if I'm using my injection pen correctly?
Your prescriber or pharmacist will demonstrate correct injection technique when you first receive your pen. Key steps: clean the injection site, pinch the skin, insert the needle at a 90-degree angle, press the button slowly until you hear/feel a click, wait 3 seconds, then remove. If you're unsure, ask your pharmacist to watch your technique.
What should I do if my injection pen gets too warm or is accidentally frozen?
Do not use a pen that has been frozen — discard it safely. If a pen is exposed to temperatures above 30°C for extended periods, contact your pharmacist or prescriber for advice. Heat and freezing can affect medication stability. When in doubt, it's safer to ask your pharmacist.
Can I travel with my injection pen?
Yes, but plan ahead. Keep your pen in an insulated cooling bag with ice packs for flights over 2 hours. Inform airport security that you carry prescription medication — bring your prescription letter. Different countries have different rules, so check your destination's regulations before travel.
Is there support available if I find injecting difficult?
Yes. Your GP, practice nurse, or pharmacist can help. Some people find auto-injector pens (which hide the needle) less anxiety-inducing. Your prescriber may also discuss alternative formats if available. Speaking to your healthcare team about concerns is important — many people find injecting becomes routine after the first few doses.
Ready to take control of your weight loss journey?
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