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
How Do Fat Loss Injections Work in the UK?
Fat loss injections work by mimicking natural hormones that regulate appetite, fullness, and blood sugar control. In the UK, three prescription slimming injections are licensed for weight management: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All three use different receptor mechanisms to help reduce food cravings and support sustainable weight loss alongside diet and exercise. Understanding how these medications work can help you make an informed decision about whether GLP-1 weight loss treatment might be suitable for you. Always consult your GP or a qualified healthcare professional before starting any prescription medication.
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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 the Three UK-Licensed Weight Loss Injections Work
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — the newest option available in the UK. It activates two natural hormone pathways simultaneously, which may explain why clinical trials showed it achieved average weight loss of 15–21% of body weight. Mounjaro is a once-weekly injection with doses ranging from 2.5 mg to 15 mg.
Wegovy (semaglutide) is a GLP-1 receptor agonist that activates a single hormone pathway. In clinical trials, it helped participants lose an average of approximately 15% of their body weight. Wegovy is also a once-weekly injection, with a maximum dose of 2.4 mg. Both Mounjaro and Wegovy slow how quickly food leaves your stomach, increase feelings of fullness, and reduce appetite signals to the brain.
Saxenda (liraglutide) is also a GLP-1 agonist but requires a daily injection rather than once weekly. Maximum dose is 3 mg daily. All three medications work best when combined with a reduced-calorie diet and increased physical activity — they are not standalone weight loss solutions.
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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.
NHS vs Private Access and Eligibility in the UK
Weight loss injections are prescription-only medicines in the UK, available through two routes. NHS access is through specialist tier 3 weight management services (usually hospital-based), typically requiring a GP referral. NHS availability and waiting times vary significantly by region. Private access is available through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber.
General eligibility criteria include a BMI of 30 kg/m² or above, or 25–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. For South Asian, Chinese, Black African, and Black Caribbean adults, lower BMI thresholds typically apply due to higher health risks at lower weights. Use our free BMI calculator above to check your current BMI and understand where you fall on the NHS weight categories. Your GP or prescriber will assess individual suitability during consultation.
- 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.
What is the difference between GLP-1 and GIP receptors?
GLP-1 (glucagon-like peptide-1) is a natural hormone that signals fullness and regulates blood sugar. GIP (glucose-dependent insulinotropic polypeptide) is another appetite-regulating hormone. Mounjaro activates both; Wegovy and Saxenda activate only GLP-1. The dual mechanism may contribute to Mounjaro's slightly higher average weight loss in clinical trials, though individual results vary significantly.
Do fat loss injections work on their own without diet changes?
No. All three UK-licensed injections are most effective when combined with a reduced-calorie diet and increased physical activity. Clinical trials showed weight loss only when participants also made lifestyle changes. Injections support these changes by reducing appetite and cravings, but they are not standalone weight loss solutions. Your prescriber will discuss diet and exercise expectations.
How quickly do fat loss injections start working?
Most people notice reduced appetite within 1–2 weeks, though weight loss typically becomes visible after 4–8 weeks. Doses are increased gradually over 4-week intervals to minimise side effects. Average weight loss progresses over months — sustainable loss is typically 0.5–1 kg per week. Try our free GLP-1 weight loss calculator to estimate potential results based on clinical trial data at different doses.
Can I switch between Mounjaro and Wegovy if one isn't working?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion chart, though approximate equivalences exist (e.g., Mounjaro 10 mg is roughly equivalent to Wegovy 1.7 mg). Any switch requires medical guidance to ensure safe dosing. Your GP or prescriber can discuss alternatives if one medication is not suitable for you.
Are weight loss injections available on the NHS in all UK regions?
No. NHS access to weight loss injections varies significantly by region depending on local integrated care board (ICB) funding and specialist service capacity. Some areas have long waiting lists; others may have limited availability. Private prescription through online pharmacies is available nationwide without NHS referral. Contact your local NHS weight management service or GP to check availability in your area.
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.