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
Slim Injection — What You Need to Know
Slim injections, commonly referred to as weight loss injections or "fat jabs" in the UK, are prescription-only medications designed to support weight management in adults. These are injectable medicines licensed for weight loss and work by mimicking natural hormones that regulate appetite and food intake. This UK health guide explains how slim injections work, who may be eligible, and how to access them through the NHS or private prescribing routes.
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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 Slim Injections Work and UK Eligibility
Slim injections are GLP-1 (and GIP/GLP-1) receptor agonist medications — they activate natural hormone pathways in your body that help regulate appetite, reduce cravings, and support feeling fuller after smaller meals. The most commonly prescribed slim injections in the UK are Wegovy (semaglutide) and Mounjaro (tirzepatide).
In the UK, slim injections may be prescribed to adults who:
- Have a BMI of 30 or above, or
- Have a BMI of 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, heart disease, sleep apnoea)
- For South Asian, Chinese, Black African, or Black Caribbean adults, lower BMI thresholds may apply
All slim injections are prescription-only medicines requiring a clinical assessment before prescribing. You cannot buy them without a prescription from a UK-licensed healthcare professional.
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.
Accessing Slim Injections in the UK
In the UK, you can access slim injections through two routes: NHS specialist weight management services (tier 3/4) or private online prescribing through GPhC-registered pharmacies. NHS access requires a GP referral to your local specialist weight management service, though waiting lists vary significantly by region and can be lengthy. Private online prescribing is available without NHS referral — a licensed prescriber conducts an online clinical assessment, and if appropriate, issues a prescription dispensed by a registered pharmacy. Check your BMI using our free BMI calculator to assess your eligibility, and discuss weight management options with your GP.
Results from slim injections vary significantly between individuals. Clinical trials show average weight losses of 14–21% of body weight over several months, but this occurs alongside reduced-calorie diet and increased physical activity — injections are not a standalone solution. Common side effects include nausea, diarrhoea, and constipation, which often improve over time. Your prescriber will monitor your progress and adjust your dose accordingly. Use our free GLP-1 weight loss calculator to estimate potential weight loss projections at different doses, and track your progress with our progress tracker tool.
- 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 slim injections available on the NHS in the UK?
Yes, slim injections are available on the NHS through specialist tier 3/4 weight management services following a GP referral. However, availability varies by region, and waiting lists can be several months. Private online prescribing offers faster access if you meet eligibility criteria and choose to pay privately.
What is the difference between slim injections and other weight loss methods?
Slim injections use prescription medications that mimic natural hormones to reduce appetite and support weight management. They differ from diet and exercise alone because they provide pharmacological support, though they work best when combined with lifestyle changes. Results vary individually — clinical trials showed average losses of 14–21% body weight.
How long do you stay on slim injections?
Duration depends on your individual response and health goals. Some people use them short-term (6–12 months) to kickstart weight loss, while others continue longer-term for weight maintenance. Your prescriber will discuss the right timeline for you based on your progress and health status.
Can anyone get a slim injection prescription?
No — eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions. You must have a clinical assessment by a licensed prescriber who will review your medical history, current medications, and contraindications before issuing a prescription.
How do I know if a slim injection is right for me?
This is a decision for you and your prescriber. Use our free BMI calculator to check your BMI category, then discuss weight management options with your GP. Your GP can refer you to specialist services or discuss private prescribing options based on your individual health needs and eligibility.
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.