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 Weight Loss Injection Pen?
A weight loss injection pen is a prescription-only device that delivers medication to help support weight management. In the UK, the most commonly prescribed weight loss injection pens contain either semaglutide (Wegovy) or tirzepatide (Mounjaro) — medications that work by activating natural appetite-regulating hormones. These are not cosmetic injections; they are clinical medicines used alongside diet and physical activity for weight management. Access to weight loss injections in the UK is through either NHS specialist weight management services or private online prescribing, following a clinical assessment by a licensed healthcare professional.
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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 Injection Pens Work
Weight loss injection pens contain GLP-1 receptor agonists (or dual GIP/GLP-1 agonists in the case of Mounjaro). GLP-1 is a natural hormone your body produces after eating — it signals fullness to your brain and helps regulate blood sugar.
When you use a weight loss injection pen, the medication mimics and amplifies these natural signals, which may help you:
- Feel fuller after eating smaller meals
- Reduce cravings and snacking
- Eat less without feeling deprived
- Support sustainable weight loss alongside a reduced-calorie diet and increased physical activity
Clinical trials have shown that weight loss injection pens can help participants lose an average of 14–21% of their body weight — though individual results vary significantly. The most common weight loss injections available in the UK are Wegovy (semaglutide) and Mounjaro (tirzepatide), both administered once weekly.
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.
UK Eligibility and How to Access Weight Loss Injections
In the UK, you may be eligible for prescription weight loss medication 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 may apply if you are of South Asian, Chinese, Black African, or Black Caribbean heritage due to higher health risks at lower weights.
Access routes in the UK include NHS specialist weight management services (tier 3/4) — where waiting lists vary by region — or private online prescribing through a GPhC-registered pharmacy following an online clinical assessment. Your GP can refer you to NHS services, or you can seek a private assessment. Use our free BMI calculator above to check your eligibility range, and our GLP-1 weight loss calculator to explore potential outcomes at different doses.
- 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 a weight loss injection pen the same as insulin?
No. Weight loss injection pens contain GLP-1 receptor agonist medications (such as semaglutide in Wegovy), not insulin. They work differently — they signal fullness and regulate appetite, rather than controlling blood sugar directly. Insulin is used in diabetes treatment, whereas GLP-1 injections are specifically for weight management support.
How often do you use a weight loss injection pen?
Both Wegovy and Mounjaro are once-weekly injections. You inject yourself or receive an injection once per week, on the same day each week. Each pre-filled pen contains four weekly doses. The starting dose is low and gradually increases over weeks — your prescriber will guide the schedule.
What are the side effects of weight loss injection pens?
Common side effects include nausea, diarrhoea, constipation, and stomach pain — typically most noticeable when starting or increasing the dose, and often improve over time. Serious side effects are rare but can include severe stomach pain (possible pancreatitis) or allergic reactions. Always discuss potential side effects with your prescriber before treatment.
Can I get a weight loss injection pen on the NHS?
Yes — the NHS offers weight loss injections through specialist tier 3 weight management services. However, availability and waiting times vary significantly by region. Your GP can refer you, or you can ask about services in your area. Private online prescribing is also available without an NHS referral.
How much weight will I lose with a weight loss injection pen?
Clinical trials show an average weight loss of 14–21% of body weight, but individual results vary widely. Some people lose more, some less — factors include your starting weight, diet, activity level, genetics, and which medication you use. Your prescriber can give you personalised expectations 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.