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 a Slim Injection?
A slim injection is a prescription-only weight management injection that helps reduce appetite and support healthy weight loss. In the UK, slim injections typically refer to GLP-1 receptor agonist medications like Wegovy (semaglutide) or Mounjaro (tirzepatide). These are licensed medications that work alongside diet and exercise to help eligible adults achieve sustainable weight loss. They're not cosmetic injections — they're NHS-aligned prescription medicines requiring a clinical assessment before use.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Slim Injections Work
GLP-1 slim injections work by activating the GLP-1 hormone pathway, which is naturally produced in your gut after eating. This mechanism helps your brain recognise fullness more quickly, reduces food cravings, and slows how fast food leaves your stomach. Mounjaro (tirzepatide) is slightly different — it activates both GLP-1 and GIP pathways for a dual effect. Clinical trials show these injections may help adults lose an average of 14–21% of their body weight when combined with a reduced-calorie diet and increased physical activity. Individual results vary significantly, and weight loss is not assured. You can check your current BMI using our free BMI calculator to see if you fall within eligibility thresholds. The weight converter on this page can also help you track your progress across different units (kg, stone, lbs).
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, slim injections are available through two routes: NHS specialist services (tier 3 weight management) and private online prescribing. NHS access requires referral from your GP, though availability varies by region and waiting times can be lengthy. Private access is available through GPhC-registered pharmacies following an online clinical assessment. Eligibility typically requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Adults from South Asian, Chinese, Black African, or Black Caribbean backgrounds may have lower BMI thresholds (27.5+) due to higher health risks. Use our GLP-1 weight loss calculator to explore projected outcomes at different doses, and speak to your GP or a licensed prescriber for personalised advice.
- 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 the same as Ozempic?
Both Wegovy and Ozempic contain semaglutide, but they're different. Ozempic is licensed for type 2 diabetes (up to 2mg). Wegovy is the licensed weight management version (up to 2.4mg). Using Ozempic for weight loss is off-label use. In the UK, Wegovy is the approved semaglutide option for weight management.
How quickly do slim injections work?
Weight loss typically begins within 2–4 weeks, but most significant changes occur after 8–12 weeks of consistent use at a therapeutic dose. Results depend on dose, diet, exercise, and individual factors. Clinical trials showed average loss of 14–21% over 68 weeks at maximum dose.
What side effects should I expect from slim injections?
Common side effects include nausea, diarrhoea, constipation, and stomach pain — especially during dose escalation. These usually improve over time. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and gallbladder problems. Always inform your prescriber if side effects persist.
Can I get slim injections on the NHS?
Yes, through tier 3 specialist weight management services if referred by your GP and meeting eligibility criteria. However, NHS availability varies significantly by region, and waiting lists can be months or years. Private prescribing offers faster access through online pharmacies.
Do I need to change my diet and exercise while using slim injections?
Yes. Slim injections work best alongside a reduced-calorie diet and increased physical activity. They're a support tool, not a replacement for lifestyle changes. Clinical trials showed weight loss only occurred when combined with behaviour change. Your GP or prescriber can advise on sustainable approaches.
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.