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
What Are Diet Injections in the UK?
Diet injections, commonly called "slimming injections" or "fat jabs" in the UK, are prescription-only weight loss medications that work by regulating appetite and helping you feel fuller after eating. Three medications are currently licensed for weight management in UK adults: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). These are not cosmetic injections—they are clinically approved medicines that work by mimicking natural hormones your body produces after eating. To find out whether you might be eligible, you can use our free GLP-1 weight loss calculator to explore how these medications work.
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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.
Three UK-Licensed Weight Loss Injections
Mounjaro (tirzepatide) is a once-weekly injection that activates two natural hormone pathways (GIP and GLP-1), helping regulate appetite and blood sugar. Doses range from 2.5 mg to 15 mg. In clinical trials, Mounjaro helped participants lose an average of 15–21% of their body weight over 72 weeks, though individual results vary significantly.
Wegovy (semaglutide) is also a once-weekly injection that activates the GLP-1 pathway alone. The maximum dose is 2.4 mg. Clinical trial data showed an average weight loss of around 15% of body weight, again with results varying between individuals.
Saxenda (liraglutide) is a daily GLP-1 injection (not weekly like the others) with a maximum dose of 3 mg per day. It has been available in the UK longer than Mounjaro or Wegovy. All three require a prescription following clinical assessment and are not available over-the-counter.
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 Do Diet Injections Work?
All weight loss injections work by mimicking GLP-1 (glucagon-like peptide-1), a natural hormone your gut produces after eating. This hormone signals your brain that you're full, reducing appetite and food cravings. The injections also slow how quickly food leaves your stomach, helping you feel satisfied for longer. Mounjaro goes further by also activating GIP, another natural appetite-regulating pathway. These medications are prescription-only because they require proper clinical assessment and monitoring. Your GP or a licensed prescriber will evaluate your health, BMI, and any weight-related conditions before deciding if a diet injection is suitable for you. Use our free BMI calculator to check where your current weight sits on the NHS scale.
- 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 diet injections the same as cosmetic fat-dissolving injections?
No. Weight loss injections like Mounjaro and Wegovy are prescription medications that work systemically (throughout your body) by regulating appetite hormones. Cosmetic injections are different treatments entirely. Diet injections must be prescribed by a UK-licensed healthcare professional following clinical assessment.
Can I get diet injections through the NHS?
Access to weight loss injections on the NHS varies by region. They are available through specialist tier 3/4 weight management services, but waiting lists can be long. Your GP can refer you to local services. Private prescribing through online consultations is also available if you meet eligibility criteria.
What's the difference between Mounjaro and Wegovy?
Both are once-weekly injections, but Mounjaro (tirzepatide) activates two hormone pathways (GIP and GLP-1), while Wegovy (semaglutide) activates GLP-1 only. Mounjaro goes up to 15 mg per week; Wegovy's maximum is 2.4 mg. Clinical trial data suggests slightly higher average weight loss with Mounjaro, but individual results vary considerably.
Who is eligible for diet injections in the UK?
Generally, adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, sleep apnoea), may be eligible. People of South Asian, Chinese, Black African, or Black Caribbean descent may qualify at lower BMI thresholds. A prescriber will assess your individual circumstances.
Do diet injections work without diet and exercise changes?
No. Weight loss injections are most effective alongside a reduced-calorie diet and increased physical activity. They support appetite regulation and help you stick to healthier choices, but they are not replacements for lifestyle changes. Track your progress with our free progress tracker alongside any treatment plan.
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.