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
What Is an Injection Pen and How Does It Work in the UK?
An injection pen is a prescription medical device used to deliver medications through a pre-filled, single-dose or multi-dose pen system. In the UK, injection pens are commonly used for weight management medications including GLP-1 receptor agonists and dual GIP/GLP-1 agonists. These pens offer a convenient, portable way to self-administer weekly or daily injections at home, following clinical assessment and guidance from your GP or prescriber. They're an important tool in modern UK health information and weight management support.
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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 UK Injection Pens for Weight Management Work
Prescription injection pens used for weight management in the UK contain medications that activate natural hormone pathways to help regulate appetite and support weight loss alongside a reduced-calorie diet and increased physical activity. Common UK-licensed injection pens include Mounjaro (tirzepatide), a dual GIP and GLP-1 agonist, and Wegovy (semaglutide), a GLP-1 agonist. Each pen is pre-filled with a specific dose and designed for once-weekly or daily self-injection, depending on the medication. The pen mechanism delivers the medication subcutaneously (under the skin) into the stomach, thigh, or upper arm. Users receive clear training on injection technique, site rotation, and storage before beginning treatment. Results from clinical trials show these medications may help with weight loss when used as part of a comprehensive approach, though individual results vary significantly. Access in the UK is through NHS specialist tier 3 weight management services or private online prescribing following a clinical assessment.
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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.
UK Storage, Safety, and Support for Injection Pens
Storage requirements vary by medication but typically include refrigeration (2–8°C) for unopened pens and room temperature storage (below 30°C) for up to 30 days after first use. Never freeze injection pens, as this renders them unusable. Dispose of used pens safely using a sharps bin, available from your GP surgery or pharmacy. Missed dose guidance depends on the specific medication — check your patient information leaflet or contact your prescriber. Side effects are common initially but often improve over time; nausea, constipation, and diarrhoea are frequently reported. Use our free BMI calculator above to understand your baseline health metrics, and speak with your GP or prescriber about whether an injection pen medication is appropriate for your individual circumstances.
- 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.
Is an injection pen the same as an insulin pen?
No. While both are pre-filled pens that deliver medication via injection, weight management injection pens (like those containing GLP-1 or GIP/GLP-1 medications) are not insulin and work differently. They regulate appetite and support weight loss through hormone pathways. Always clarify the specific medication with your prescriber.
Can I buy an injection pen without a prescription in the UK?
No. All prescription weight management injection pens are prescription-only medicines in the UK. They require a clinical assessment from a UK-licensed prescriber before being dispensed. This assessment ensures the medication is safe and appropriate for your individual health.
How long does an injection pen last?
This depends on the medication and dose. Each pre-filled pen typically lasts 4 weeks (four once-weekly injections) or 28 days (for daily injections). Your prescriber will specify how long your pens should last and when to reorder.
What should I do if I accidentally freeze my injection pen?
If your injection pen has been frozen, do not use it — freezing damages the medication. Dispose of it safely and contact your prescriber or pharmacy for a replacement. Always store pens in a refrigerator (2–8°C), not a freezer.
Where can I get support and training on using an injection pen?
Your prescriber, GP surgery, or pharmacy should provide training before you start using an injection pen. NHS weight management services also offer support. Online resources and patient information leaflets are available with every pen. Ask your healthcare provider if you have any concerns about technique or side effects.
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.