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 a Diet Pen? A UK Health Guide
A "diet pen" is the colloquial UK term for prescription weight management injections—specifically GLP-1 and GIP/GLP-1 receptor agonist medications like Wegovy and Mounjaro. These are once-weekly injectable medications licensed in the UK to support weight management in adults who meet specific health criteria. They work by affecting appetite regulation and may help reduce food cravings. Diet pens are prescription-only medicines requiring a clinical assessment and are typically used alongside a reduced-calorie diet and increased physical activity. NHS guidance supports their use as part of comprehensive weight management programmes.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Diet Pens Work and UK Eligibility
Diet pens contain medications that activate GLP-1 or combined GIP/GLP-1 hormone pathways. These natural hormones help regulate appetite, increase feelings of fullness, and slow gastric emptying. In the UK, two main options are licensed: Wegovy (semaglutide) and Mounjaro (tirzepatide). Both are once-weekly injections delivered via a pre-filled pen device.
UK eligibility criteria: Adults with a BMI of 30 or above, or 27 or above with weight-related health conditions (type 2 diabetes, high blood pressure, sleep apnoea), may be eligible. South Asian, Chinese, Black African, and Black Caribbean adults have lower thresholds due to higher health risks at lower BMI levels.
- Prescription-only—requires clinical assessment by a UK-licensed prescriber
- Available through NHS specialist weight management services or private online prescribing
- Not suitable for pregnant women, those under 18, or those with certain medical conditions
- Results vary—average weight loss in clinical trials ranged from 14–21% of body weight
Your GP or a prescriber will assess whether this treatment is appropriate for your individual health needs.
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.
Accessing Diet Pens in the UK and Getting Support
In the UK, you can access diet pens through two main routes. NHS route: Referral to a tier 3 specialist weight management service through your GP—availability varies by region and waiting lists can be lengthy. Private route: Online consultation with a UK-licensed prescriber followed by prescription from a GPhC-registered pharmacy.
Before starting, use our free BMI calculator to check your current BMI category, and our weight converter to understand your weight in both metric and imperial measurements. Your GP can provide personalised advice about whether prescription weight loss treatment suits your situation. Once prescribed, a progress tracker helps you monitor your weight loss journey and observe patterns over time.
- 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 diet pens the same as slimming injections?
Yes—"diet pen," "slimming injection," "weight loss jab," and "fat jab" are all UK colloquial terms for the same prescription medications. They contain GLP-1 or GIP/GLP-1 receptor agonist medications licensed for weight management. They are not cosmetic injections; they are prescription-only pharmaceuticals requiring clinical assessment.
How much weight can I expect to lose with a diet pen?
Clinical trial data shows average weight loss of 14–21% of body weight over 68–72 weeks, alongside a reduced-calorie diet and increased physical activity. Individual results vary significantly—some people lose more, some less. A prescriber will discuss realistic expectations during your assessment. Our GLP-1 weight loss calculator can help estimate potential outcomes at different doses.
Can I get a diet pen on the NHS?
Yes, through specialist tier 3 weight management services. You'll need a referral from your GP. Availability varies by region—some areas have shorter waiting lists than others. Private prescription is also available through online prescribers if you prefer faster access. Your GP can advise on local NHS options.
What are the main side effects of diet pens?
Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain—especially when starting or increasing doses. These often improve over time. Serious side effects are rare but can include pancreatitis symptoms (severe stomach pain) and gallbladder issues. Always report concerns to your prescriber immediately.
Do I need to diet and exercise while using a diet pen?
Yes—diet pens work best alongside a reduced-calorie diet and increased physical activity. They are not replacements for lifestyle changes; they support them. A prescriber or dietitian will discuss diet and exercise goals. Track your progress with our free progress tracker to stay motivated and monitor results.
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.