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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Slimming Injections in the UK — Your Complete Guide
Slimming injections, often called "fat jabs" or "weight loss jabs", are prescription-only medicines licensed in the UK for weight management in adults. Three main options are currently available: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Each works through similar hormone-based mechanisms but differs in how they're administered, the doses available, and their clinical evidence. Understanding your options helps you have an informed conversation with your GP or prescriber about which might be suitable for your circumstances.
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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.
UK-Licensed Slimming Injections — How They Compare
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — the newest mechanism available in the UK. It's administered as a once-weekly injection with doses ranging from 2.5 mg to 15 mg. Clinical trial data (SURMOUNT-1) showed average weight loss of approximately 15–21% of body weight over 72 weeks alongside a reduced-calorie diet and physical activity.
Wegovy (semaglutide) is a GLP-1-only receptor agonist, also injected once weekly, with doses from 0.25 mg to 2.4 mg. The STEP-1 clinical trial demonstrated average weight loss of around 14.9% of body weight at 68 weeks. Both Mounjaro and Wegovy are administered as pre-filled pens, making them convenient for home use.
Saxenda (liraglutide) is also a GLP-1 receptor agonist but requires daily injections rather than weekly — doses range from 0.6 mg to 3 mg. It has longer clinical history in weight management in the UK. Individual results vary significantly across all three medications.
- Mounjaro: dual mechanism, weekly, stronger clinical average
- Wegovy: single GLP-1, weekly, well-established evidence
- Saxenda: daily injections, longer track record in UK practice
Your prescriber will assess which option suits your health profile, preferences, and lifestyle.
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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.
Accessing Slimming Injections — NHS vs Private Routes
NHS access: Weight loss injections are available through specialist tier 3/4 NHS weight management services in England, Scotland, Wales, and Northern Ireland. You'll typically need a GP referral. NICE has approved tirzepatide (Mounjaro) for NHS use, and other GLP-1 medications may be available depending on your region and local commissioning decisions. NHS waiting times vary significantly — some areas have long waiting lists.
Private access: All three medications are available through GPhC-registered online prescribing services. You complete an online consultation with a UK-licensed prescriber, and if clinically appropriate, they issue a prescription dispensed by a registered pharmacy. This route doesn't require NHS referral.
Eligibility criteria apply to both routes: You typically need a BMI of 30 kg/m² or above, or 27–29.9 kg/m² with weight-related health conditions (type 2 diabetes, high blood pressure, sleep apnoea, cardiovascular disease). Lower BMI thresholds apply for South Asian, Chinese, Black African, and Black Caribbean adults. All three medicines are prescription-only — none are available over-the-counter. Use our free BMI calculator to check your BMI and discuss your options with a healthcare professional.
- 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.
Which UK slimming injection causes the least nausea?
Nausea is common with all three medications, particularly when starting or increasing doses. Individual tolerance varies. Mounjaro and Wegovy both show nausea in around 25–40% of users. Saxenda has similar side effect profiles. Your prescriber will discuss strategies to manage nausea, such as gradually increasing doses or dietary adjustments.
Can I switch between Mounjaro, Wegovy and Saxenda?
Switching between medications is possible but must be managed by your prescriber. There's no official conversion chart — approximate dose equivalence exists (e.g. Mounjaro 10mg ≈ Wegovy 1.7mg), but any switch requires clinical assessment. Never switch without medical guidance.
How quickly do slimming injections work?
Weight loss typically begins within the first 2–4 weeks, though some people notice effects on appetite sooner. Most significant weight loss occurs between weeks 8–20 as doses are escalated. Results vary; some people lose weight steadily, others in plateaus. Combine injections with reduced-calorie diet and exercise for best results.
Are slimming injections safe long-term?
Long-term safety data continues to accumulate. Current clinical trials span 68–72 weeks. These medications carry Black Triangle (▼) monitoring status with the MHRA, meaning they're monitored closely for safety signals. Your prescriber will discuss individual risk-benefit balance, contraindications, and when to stop treatment.
Can I get slimming injections on the NHS if I'm overweight but not obese?
Yes, if you're overweight (BMI 27–29.9) with at least one weight-related health condition such as type 2 diabetes, high blood pressure, or sleep apnoea. Lower BMI thresholds apply for certain ethnic groups. Your GP can assess your eligibility and refer you to specialist NHS weight management services.
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.