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? A Clear UK Explanation
Diet injections—also called slimming injections or weight loss jabs—are prescription-only medications that may help with weight management in UK adults. They are not cosmetic injections, but licensed pharmaceutical treatments that work by affecting how your body regulates hunger and fullness. Three options are currently available on prescription in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Each requires a clinical assessment and prescription from a qualified healthcare professional before you can access them, whether through NHS specialist services or private prescribing.
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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 Injection Options
The three prescription-only diet injections licensed for weight management in the UK are:
- Mounjaro (tirzepatide)—a once-weekly injection that works as a dual GIP and GLP-1 receptor agonist. Doses range from 2.5 mg to 15 mg weekly. In clinical trials, it helped participants lose an average of 15–21% of their body weight, though individual results vary significantly.
- Wegovy (semaglutide)—a once-weekly GLP-1 receptor agonist injection. Maximum dose is 2.4 mg weekly. Clinical trial data showed an average weight loss of around 15% of body weight alongside dietary changes and increased activity.
- Saxenda (liraglutide)—a once-daily injection (the only daily option). Doses range from 0.6 mg to 3 mg daily. It is also a GLP-1 receptor agonist and may help reduce appetite and support weight loss.
All three are prescription-only and require a clinical assessment before being prescribed. Access varies between NHS specialist weight management services and private prescribing routes.
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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.
How Diet Injections Work: The GLP-1 Mechanism
Diet injections work by mimicking GLP-1 (glucagon-like peptide-1), a natural hormone your gut produces when you eat. GLP-1 receptor agonists trigger several effects: they reduce appetite and food cravings, help you feel fuller after smaller meals, slow how quickly food leaves your stomach, and support better blood sugar regulation. Mounjaro adds a second mechanism—it also activates GIP receptors—which may enhance these effects. These medications are not clinically supported solutions; they work best when combined with a reduced-calorie diet and increased physical activity. Individual responses vary, and sustained weight loss requires ongoing lifestyle changes. Use our free GLP-1 weight loss calculator to estimate potential weight loss projections at different doses, and try our BMI calculator to see where you currently stand.
- 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. Diet injections like Mounjaro and Wegovy are prescription pharmaceutical medications that work on appetite regulation and hormones. Cosmetic fat-dissolving injections (such as aqualyx) are different products used for localised fat reduction and are not weight loss treatments. Diet injections are systemic medications requiring clinical assessment.
Can I get a diet injection on the NHS?
Yes, but access is limited. NHS provision is through specialist tier 3 weight management services. You typically need a referral from your GP, and eligibility varies by region. Waiting lists can be long in many areas. Private prescribing is also available through GPhC-registered online pharmacies following an online assessment.
What is weight loss injection eligibility in the UK?
You may be eligible if you have a BMI of 30 or above, or a BMI of 27–29.9 with a weight-related health condition (such as type 2 diabetes, high blood pressure, sleep apnoea, or heart disease). South Asian, Chinese, Black African, and Black Caribbean adults may have lower BMI thresholds. A prescriber will assess your individual circumstances.
How much weight will I lose with a diet injection?
Results vary considerably between individuals. Clinical trial averages show 15–clinically studied weight loss, but some people lose less, and some lose more. Weight loss depends on the specific medication, dose, your adherence to dietary changes, physical activity level, and individual metabolism. Your prescriber can discuss realistic expectations during your assessment.
Are there side effects with diet injections?
Yes. Common side effects include nausea, diarrhoea, constipation, vomiting, and stomach pain—usually most noticeable when starting or increasing doses. These often settle over time. Serious side effects are rare but can include pancreatitis symptoms or gallbladder problems. Always discuss potential side effects with your prescriber before starting any 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.