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 Jabs in the UK — Your Options Explained
A slimming jab is a prescription-only weight loss injection that may help reduce appetite and support weight management alongside diet and exercise. Three medications are currently licensed for this purpose in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Each works through similar mechanisms but with different doses, frequencies, and clinical evidence. Access is available through both NHS specialist services and private prescribing. This guide covers all your options, how they compare, and what eligibility typically looks like.
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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.
Comparing the Three UK-Licensed Weight Loss Injections
Mounjaro (tirzepatide) is a once-weekly injection that works as a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways. It's given as a pre-filled pen starting at 2.5mg, escalating up to a maximum of 15mg per week. Clinical trial data (SURMOUNT-1) showed average weight loss of approximately 15–21% of body weight at maximum dose over 72 weeks, alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is also a once-weekly injection but uses a single GLP-1 mechanism. It starts at 0.25mg, building to a maximum of 2.4mg per week. STEP-1 trial data showed average weight loss of around 14.9% of body weight at 68 weeks. Both are GLP-1 weight loss options, though Mounjaro's dual mechanism may produce different results in some people.
Saxenda (liraglutide) is a daily injection (not once-weekly) containing the same active ingredient as Victoza but at a higher dose. Maximum dose is 3mg daily. Clinical evidence shows modest weight loss results — typically 5–10% of body weight. It's less commonly prescribed now than Mounjaro and Wegovy but remains a licensed UK option. Individual results vary significantly across all three medications.
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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.
NHS vs Private Access — Eligibility and How to Get Started
NHS access: Weight loss injections are available through tier 3 specialist weight management services, typically following GP referral. Availability varies significantly by region and waiting lists can be lengthy. You'll need a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, sleep apnoea, or cardiovascular disease). Lower thresholds apply for South Asian, Chinese, Black African, and Black Caribbean adults (BMI 27.5+).
Private access: Available through online consultations with UK-licensed prescribers, dispensed by GPhC-registered pharmacies. No NHS referral needed. You'll be assessed on the same clinical criteria. Costs range from approximately £129–£320 monthly depending on dose and medication. Calculate your potential weight loss using our GLP-1 weight loss calculator, then discuss options with a prescriber. Track progress with our free progress tracker once you've started treatment.
- 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 slimming jab is most effective?
Clinical trials show Mounjaro (15–clinically studied weight loss) produces slightly higher average results than Wegovy (14.9%), but individual results vary significantly. Effectiveness depends on your personal response, consistency with injections, diet, and activity level. Your prescriber will help determine which suits your situation best.
Can I get slimming injections on the NHS?
Yes, through specialist tier 3 weight management services. You'll need a BMI of 30+ (or 27+ with weight-related health conditions) and typically require a GP referral. Waiting lists vary by region — some areas have delays of several months to over a year. Private access is available without NHS referral.
How do prescription slimming injections work?
They're GLP-1 (or GIP/GLP-1) receptor agonists that mimic natural gut hormones. They reduce appetite, increase feelings of fullness, slow stomach emptying, and may improve blood sugar control. They work best alongside a reduced-calorie diet and increased physical activity — they're not standalone treatments.
What are the main side effects of weight loss injections?
Most common side effects are nausea, constipation, diarrhoea, and stomach discomfort, particularly when starting or increasing doses. These often improve over time. Serious side effects are rare but can include pancreatitis or gallbladder issues. Always discuss side effect concerns with your prescriber.
How long do you stay on slimming injections?
This varies individually. Some people use them short-term to establish weight loss, others continue longer-term. NHS guidance typically involves ongoing review and reassessment. Stopping suddenly may lead to weight regain. Your prescriber will discuss the best duration for your situation and how to stop safely if needed.
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.