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 Are Skinny Pens? A UK Health Guide
"Skinny pens" is a colloquial UK term for prescription weight loss injection medications, particularly GLP-1 receptor agonists like Wegovy (semaglutide) and Mounjaro (tirzepatide). These are legitimate, prescription-only medicines licensed for weight management in UK adults with a BMI of 30 or above, or 27 or above with weight-related health conditions. They're not cosmetic or unregulated — they require clinical assessment and a prescription from a licensed healthcare professional. This health guide explains how they work, who may be eligible, and how to access them in the UK.
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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 Weight Loss Injections Work
Wegovy (semaglutide) is a GLP-1 receptor agonist — it mimics a natural hormone your body produces after eating. This hormone signals to your brain that you're full, reduces appetite and food cravings, slows how quickly food leaves your stomach, and helps regulate blood sugar. In the STEP-1 clinical trial, Wegovy helped participants lose an average of 14.9% of their body weight over 68 weeks, alongside a reduced-calorie diet and increased physical activity.
Mounjaro (tirzepatide) works differently — it's a dual GIP and GLP-1 receptor agonist, activating two natural hormone pathways instead of one. Clinical trials showed average weight loss of 15–21% of body weight, depending on dose. Both medicines are once-weekly injections, available only on prescription, and require a clinical assessment before being prescribed. Results vary significantly between individuals.
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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.
Eligibility and How to Access in the UK
To be eligible for prescription weight loss injections in the UK, you typically need a BMI of 30 or above, or 27 or above if you have a weight-related health condition such as type 2 diabetes, high blood pressure, or sleep apnoea. You can access these medicines through two main routes: NHS specialist tier 3 weight management services (via GP referral — availability varies by region and waiting lists can be lengthy), or private online prescribing services where a UK-licensed prescriber assesses your suitability. Both routes require a clinical consultation. Use our free BMI calculator to check your current BMI and understand whether you might meet eligibility criteria.
Side effects of weight loss injections are common and include nausea, constipation, diarrhoea, and stomach pain — usually mild and temporary as your body adjusts. Serious side effects are rare but can include severe stomach pain (possible pancreatitis) or gallbladder problems. These medicines are not suitable if you're pregnant, breastfeeding, or have a personal or family history of medullary thyroid cancer. Always discuss your full medical history and any concerns with your prescriber or GP before starting treatment.
- 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 "skinny pens" the same as Ozempic?
No. Both Wegovy and Ozempic contain semaglutide, but Ozempic is licensed for type 2 diabetes (up to 2 mg dose) and is not approved for weight loss in the UK. Wegovy is the licensed weight management version of semaglutide (up to 2.4 mg). Using Ozempic for weight loss is off-label and not recommended.
How quickly will I lose weight on these injections?
Results vary significantly. Clinical trials showed average weight loss of 15–21% over 68 weeks, but individual outcomes depend on your starting weight, diet, exercise, dose, and genetics. Weight loss typically begins within 4–8 weeks. Your prescriber can discuss realistic expectations for you.
Can I get these injections on the NHS?
Yes, but access is through tier 3 specialist weight management services, typically via GP referral. NHS availability varies significantly by region, and waiting lists can be long in many areas. Private online prescribing is an alternative route if you meet clinical eligibility criteria.
Do I need to change my diet and exercise when taking these injections?
Yes. Weight loss medications work best alongside a reduced-calorie diet and increased physical activity. These injections support weight management by reducing appetite — they're not a replacement for lifestyle changes. Your prescriber will advise you on diet and exercise alongside treatment.
What's the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist; Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro shows slightly higher average weight loss (20.9% at maximum dose vs 14.9% for Wegovy in trials), but both are effective. Your prescriber will discuss which is more suitable for you based on your health profile.
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.