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 Weight Loss Injections Work in the UK
Weight loss injections, commonly called "slimming jabs" in the UK, are prescription-only medicines that may help support weight management alongside diet and exercise. These medications work by activating natural hormone pathways that regulate appetite and help you feel fuller for longer. The most widely discussed options are semaglutide (Wegovy) and tirzepatide (Mounjaro), both licensed in the UK for weight management. Individual results vary considerably, and these injections are only prescribed following a clinical assessment by a healthcare professional. Use our free BMI calculator above to check your starting point, and our GLP-1 weight loss calculator to explore potential outcomes with these medications.
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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 Injections Work
Weight loss injections contain medicines that activate GLP-1 or dual GIP/GLP-1 hormone receptors in your body. These natural pathways influence how your brain perceives hunger and fullness, potentially reducing appetite and cravings. The injections may also slow how quickly food leaves your stomach, helping you feel satisfied after smaller meals.
Semaglutide (Wegovy) is a GLP-1 receptor agonist licensed in the UK. Clinical trials (STEP studies) showed that on average, participants lost approximately 15% of their body weight over 68 weeks alongside a reduced-calorie diet and increased physical activity.
Tirzepatide (Mounjaro) is a newer dual GIP/GLP-1 agonist. The SURMOUNT trial showed average weight loss of 15–21% body weight depending on dose, again combined with lifestyle changes.
Both are prescription-only, administered as once-weekly injections, and require a clinical assessment before being prescribed.
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 Access and Eligibility
In the UK, weight loss injections may be prescribed through NHS specialist weight management services (tier 3) or private online prescribing following a clinical assessment. NHS access is typically limited and involves referral from your GP; waiting lists vary significantly by region and ICB.
You may be eligible if you have a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds may apply due to higher health risks at lower weights.
Your GP or a licensed prescriber will assess your individual suitability. Use our weight converter to check your current weight in kg, and our free BMI calculator to see where you stand in terms of NHS eligibility criteria and healthy weight guidance for your height.
- 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.
Do weight loss injections work without diet and exercise?
No. Clinical trials show that weight loss injections only work effectively when combined with a reduced-calorie diet and increased physical activity. These medications support your efforts but are not a standalone solution. Results vary significantly between individuals.
What are the main side effects of weight loss injections?
Common side effects include nausea, diarrhoea, vomiting, and constipation, particularly when starting or increasing doses. Most people find side effects settle after a few weeks. Rare but serious complications can include pancreatitis or gallbladder issues. Always discuss concerns with your GP or prescriber.
How quickly do weight loss injections start to work?
Most people notice reduced appetite within the first 1–2 weeks, though significant weight loss typically takes 4–8 weeks to become visible. Clinical trials measured results over 68+ weeks. Weight loss is gradual and sustainable rather than rapid.
Can I get weight loss injections on the NHS?
Yes, but access is limited. You need referral to a specialist tier 3 weight management service from your GP. Availability and waiting times vary significantly by region and ICB. Private online prescribing is an alternative route if NHS access is not available in your area.
What's the difference between Wegovy and Mounjaro?
Wegovy (semaglutide) is a GLP-1 agonist with trials showing ~15% average weight loss. Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist with trials showing 15–21% average weight loss. Both require prescription, though results vary between individuals and both must be combined with lifestyle changes.
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.