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
Are Slimming Injections Safe in the UK?
Slimming injections — commonly called weight loss injections or fat jabs — are prescription medications that have undergone rigorous safety testing before UK approval. The main options available in the UK are GLP-1 weight loss injections like Wegovy (semaglutide) and Mounjaro (tirzepatide), both licensed by the MHRA for weight management in adults. Safety depends on proper medical assessment, suitability for your individual health, and following prescriber guidance. This guide explains eligibility criteria, how to access them safely through both NHS and private routes, and what the assessment process involves.
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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.
Eligibility Criteria and Safety Assessments
Weight loss injections in the UK are only prescribed to eligible adults following a clinical assessment by a qualified healthcare professional. Before prescribing, your GP or private prescriber will assess your:
- BMI: Adults with a BMI of 30 kg/m² or above, or 27–29.9 kg/m² with weight-related health conditions (type 2 diabetes, high blood pressure, cardiovascular disease, sleep apnoea, or abnormal blood fats), may be eligible
- Medical history: Including any conditions affecting your stomach, pancreas, thyroid, or heart
- Current medications: To check for interactions with slimming injections
- Ethnicity: People of South Asian, Chinese, Black African, or Black Caribbean heritage may be eligible at lower BMI thresholds (27.5 or above) due to higher health risks at lower weights
- Pregnancy status: Prescription slimming injections are not suitable during pregnancy or breastfeeding
Use our free BMI calculator to check your starting point before consulting your GP or a prescriber.
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.
NHS vs Private Routes — How to Access
NHS access: Referral from your GP to a tier 3 specialist weight management service. NICE has approved tirzepatide (Mounjaro) and semaglutide (Wegovy) for NHS use, but availability varies significantly by region and waiting lists can be months or years long. Your GP will assess whether referral is appropriate for you.
Private access: Faster route through online consultation with a UK-licensed prescriber. You complete a health questionnaire, a prescriber reviews your suitability, and if approved, a prescription is issued to a GPhC-registered pharmacy. Cost ranges from approximately £129–£320 monthly depending on dose. Both routes require genuine clinical assessment — they are not simply available on request.
- 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.
What happens during a clinical assessment for slimming injections?
A prescriber will review your weight, BMI, medical history, current medications, and any weight-related health conditions. They'll ask about your diet and activity level, explain how the medication works, discuss potential side effects, and check you have no contraindications. You'll need to be honest about all health conditions and medications. This typically takes 15–30 minutes.
Are slimming injections safe for everyone?
No — they are not suitable for people under 18, those pregnant or breastfeeding, or people with a history of severe pancreatitis, gallbladder disease, or medullary thyroid cancer. They require careful medical assessment. Always inform your prescriber of all health conditions before starting. Individual safety depends on proper suitability assessment.
How long does it take to get slimming injections on the NHS?
Waiting times vary significantly by region (ICB area). Some areas have waiting lists of several months to over a year. You'll first need a GP appointment, then referral to tier 3 weight management services, then assessment and prescription. Private routes are typically faster — often 1–2 weeks from consultation to first injection.
Can I switch between Mounjaro and Wegovy?
Both are prescription weight loss medications licensed in the UK. Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist; Wegovy (semaglutide) is a GLP-1 agonist only. Any switch between medications must be managed by your prescriber — they'll assess whether a change is appropriate and manage the transition safely. Do not switch without medical guidance.
What side effects should I expect with slimming injections?
Common side effects include nausea, diarrhoea, constipation, and reduced appetite — usually mild and temporary. Less common are vomiting, dizziness, or stomach pain. Serious side effects are rare but include pancreatitis symptoms (severe abdominal pain) requiring urgent medical attention. Your prescriber will discuss all potential side effects before you start and monitor you during treatment.
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.