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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Weight Loss Pens: How GLP-1 Injections Work in the UK
Weight loss pens — commonly called "slimming injections" or "fat jabs" — are prescription-only injectable medications designed to help with weight management. Available in the UK through both NHS specialist services and private prescribing, these pens deliver GLP-1 receptor agonist medications once weekly. They work by mimicking natural hormones that regulate appetite and blood sugar, potentially helping you feel fuller for longer. This guide explains what weight loss pens are, how they work, and how to access prescription weight loss medication in the UK through your GP or a licensed prescriber.
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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 Weight Loss Pens Work
Weight loss pens contain GLP-1 receptor agonist medications — drugs that activate natural hormone pathways in your body. When you eat, your gut naturally produces GLP-1, which signals to your brain that you're full. Weight loss pen medications amplify this effect, which may help reduce appetite and food cravings.
The two main weight loss pens licensed in the UK are Wegovy (semaglutide) and Mounjaro (tirzepatide). Both are prescription-only and administered as once-weekly injections. In clinical trials, these medications helped participants lose an average of 14–21% of their body weight — though individual results vary significantly. Weight loss is most effective when combined with a reduced-calorie diet and increased physical activity. Use our GLP-1 weight loss calculator to estimate potential weight loss at different dose levels.
- May reduce appetite and cravings
- Help you feel fuller after smaller meals
- Support sustainable weight management
- Require prescription and clinical assessment
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.
Who Can Access Weight Loss Pens in the UK
In the UK, you may be eligible for prescription weight loss pens if you have a BMI of 30 or above, or a BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. Lower BMI thresholds apply if you're of South Asian, Chinese, Black African, or Black Caribbean heritage, due to higher health risks at lower weights.
NHS access: Referral through your GP to a tier 3 specialist weight management service. Waiting times vary significantly by region. Private access: Through online consultation with a UK-licensed prescriber, followed by dispensing from a GPhC-registered pharmacy. Check your BMI using our free BMI calculator to understand your eligibility category. All access requires a clinical assessment to confirm suitability.
- 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.
Are weight loss pens the same as insulin pens?
No. While both are injectable medications in pen devices, weight loss pens contain GLP-1 receptor agonists, not insulin. They work on appetite regulation pathways, not blood sugar control in the same way insulin does. Weight loss pens are prescription-only medications and require clinical assessment before use.
How long do you need to use weight loss pens?
There's no fixed time limit. Some people use them long-term for sustained weight management, while others may use them for a defined period. Your prescriber will discuss treatment duration based on your individual response and health goals. Stopping medication often results in weight regain, so ongoing support is important.
Can you get weight loss pens on the NHS in the UK?
Yes, NICE has approved tirzepatide (Mounjaro) and semaglutide (Wegovy) for NHS use through specialist tier 3 weight management services. However, availability varies by region and waiting lists can be lengthy. Your GP can refer you to check eligibility and availability in your area.
What's the difference between Wegovy and Ozempic?
Both contain semaglutide, but Wegovy is licensed for weight management (up to 2.4mg dose), while Ozempic is approved for type 2 diabetes (up to 2mg). Only Wegovy is the licensed weight loss option in the UK. Using Ozempic for weight loss would be off-label use.
How much do weight loss pens cost privately in the UK?
Private costs vary between providers, typically ranging from £129–£320 per month depending on the medication and dose. Prices change frequently, so it's worth checking current costs with licensed UK pharmacies. NHS access is free but involves referral and waiting times. ```
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.