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 Is Injection Slimming?
Injection slimming refers to prescription weight loss injections — specifically GLP-1 and GIP/GLP-1 receptor agonist medications licensed in the UK for weight management. These once-weekly or daily injections may help reduce appetite and support weight loss alongside a reduced-calorie diet and increased physical activity. They are prescription-only medicines available through NHS specialist services or private clinical assessment. Individual results vary significantly, and these medications work best as part of a comprehensive healthy weight UK approach that includes lifestyle changes.
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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.
How Do Slimming Injections Work?
Slimming injections are GLP-1 and GIP/GLP-1 receptor agonists — medications that mimic natural hormones your body produces after eating. They work by:
- Signalling to your brain that you feel full, reducing appetite and food cravings
- Slowing how quickly food leaves your stomach, helping you feel satisfied longer
- Supporting better blood sugar control (particularly relevant if you have type 2 diabetes or prediabetes)
- Helping your body utilise fat more effectively
The most commonly prescribed slimming injections in the UK are Wegovy (semaglutide) — a GLP-1 agonist — and Mounjaro (tirzepatide) — a dual GIP/GLP-1 agonist. Clinical trials show these may help people lose an average of 15–21% of their body weight alongside a reduced-calorie diet. However, results vary considerably between individuals, and weight loss is not assured. A GP can discuss whether a slimming injection may be suitable for you based on your nhs health advice and personal circumstances.
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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 Eligibility and NHS Access
In the UK, you may be eligible for prescription slimming injections if you have a BMI of 30 or above, or a BMI of 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, eligibility may begin at a lower BMI due to higher health risks at these thresholds.
NHS access: Available through specialist tier 3 weight management services following GP referral — availability varies significantly by region and waiting lists can be lengthy. Private access: Available through GPhC-registered pharmacies following an online clinical assessment with a UK-licensed prescriber. Use our free BMI calculator to check if you might meet initial eligibility criteria, and our weight converter to understand your weight in different measurements. Your GP can provide personalised nhs guidance about whether injection slimming is appropriate for your specific health situation.
- 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 slimming injections safe?
Slimming injections are prescription-only medicines with a established safety profile from large clinical trials. Side effects are common (nausea, diarrhoea, constipation) but usually mild and tend to decrease over time. Serious side effects are rare. They are not suitable for pregnant women, those under 18, or people with certain medical histories. A prescriber will assess your individual safety before prescribing.
What's the difference between Wegovy and Ozempic?
Both contain semaglutide but are different medicines. Ozempic is licensed for type 2 diabetes (not weight loss in the UK). Wegovy is the licensed weight management version. Using Ozempic for weight loss would be off-label. Always ensure you receive the correct prescribed medication for your intended use.
How much weight can I expect to lose?
Clinical trials show average weight loss of 15–21% of body weight over 68 weeks alongside diet and exercise. However, individual results vary significantly — some people lose more, some less. Weight loss depends on starting weight, dose, adherence, diet, and exercise. A prescriber can discuss realistic expectations for your situation.
How do I access slimming injections on the NHS?
Ask your GP for referral to your local specialist (tier 3) weight management service. Eligibility and availability vary by region. Waiting lists are often lengthy. Your GP can also discuss whether you meet criteria and explore private options if NHS access is limited in your area.
Can I use slimming injections without changing my diet?
Slimming injections work best alongside a reduced-calorie diet and increased physical activity. Clinical trials show results are achieved with these lifestyle changes combined, not medication alone. A healthy weight UK approach includes sustainable dietary choices and regular movement. Try our free GLP-1 weight loss calculator to explore potential outcomes.
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.