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?
Fat loss injections, often called slimming jabs in the UK, are prescription-only medicines that help regulate appetite and promote sustainable weight loss. They work by mimicking natural hormones your body already produces, sending signals to your brain about hunger and fullness. Understanding how these injections function is essential before considering whether they might be suitable for you alongside diet and exercise changes.
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.
Three UK-Licensed Fat Loss Injection Options
Three prescription slimming injections are currently licensed for weight management in the UK, all working through similar appetite-regulation mechanisms:
- Mounjaro (tirzepatide) — A dual GIP and GLP-1 receptor agonist given once weekly via injection pen. Maximum dose is 15mg, with clinical trials showing average weight loss of 15–21% of body weight alongside dietary changes.
- Wegovy (semaglutide) — A GLP-1 receptor agonist administered once weekly. Maximum dose reaches 2.4mg, with clinical evidence supporting average weight loss around 15% of body weight in trials.
- Saxenda (liraglutide) — A daily GLP-1 injection (not weekly) with a maximum dose of 3mg. All three medications require a prescription and clinical assessment before being prescribed. Access via the NHS involves referral to specialist weight management services, while private options are available through GPhC-registered prescribers following online consultation.
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.
How the GLP-1 Mechanism Actually Works
GLP-1 is a natural hormone your gut releases after eating. These medications amplify this signal, helping your brain recognise fullness sooner and reducing overall hunger. The injections slow how quickly food moves through your stomach, increase feelings of satisfaction after smaller meals, and may support better blood sugar regulation. Results vary significantly between individuals—some people experience appetite suppression within days, whilst others take weeks to notice changes. Our free GLP-1 weight loss calculator can help estimate potential outcomes at different doses, though actual results depend on your lifestyle changes, diet adherence, and individual metabolism. These are not cosmetic injections; they are prescription medicines requiring ongoing medical monitoring.
- 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 fat loss injections the same as cosmetic injections?
No. Fat loss injections are prescription-only medicines containing GLP-1 or GIP/GLP-1 compounds. They are not cosmetic injections like those used in aesthetic treatments. They work internally to regulate appetite hormones and must be prescribed following clinical assessment by a licensed healthcare professional.
How quickly do fat loss injections start working?
Effects vary considerably between individuals. Some people notice reduced appetite within the first week, whilst others take 4–6 weeks to experience noticeable changes. Doses are gradually increased over weeks, and maximum benefit typically appears after several months of consistent use alongside dietary changes.
Can I get fat loss injections on the NHS?
Yes, but access is limited. You need referral to a specialist tier 3 weight management service through your GP. Eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions. NHS availability varies significantly by region. Private prescriptions are available faster through online assessments.
What happens if I stop using fat loss injections?
Weight typically returns if dietary habits don't change. These injections are tools to support lifestyle modifications, not permanent solutions. Most people regain weight within months of stopping unless they've established sustainable eating and exercise patterns during treatment. Your GP can advise on long-term management.
Are fat loss injections suitable for everyone?
No. They're not suitable during pregnancy, for people under 18, or those with certain medical conditions. You must have a clinical assessment to check eligibility. Medical history, current medications, and specific health conditions determine suitability. Always discuss with your GP or a licensed prescriber before starting any weight loss medication.
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.