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
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Slimming Jabs Side Effects and How to Access Them in the UK
Slimming jabs—colloquial terms for prescription weight loss injections like Mounjaro and Wegovy—have become increasingly sought-after in the UK for managing excess weight. These GLP-1 and GIP/GLP-1 receptor agonist medications can help reduce appetite and support weight loss alongside lifestyle changes, but they do come with potential side effects worth understanding. This guide explains what side effects to expect and how to access these prescription injections through either NHS or private routes in the UK.
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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.
Eligibility Criteria for Slimming Injections in the UK
To access prescription slimming injections in the UK, you must meet specific eligibility criteria set by NHS guidelines and clinical assessment standards.
Standard BMI thresholds:
- BMI of 30 kg/m² or above (obesity), OR
- BMI of 27–29.9 kg/m² with at least one weight-related health condition
Weight-related qualifying conditions include: type 2 diabetes or prediabetes, high blood pressure, abnormal blood fats (dyslipidaemia), obstructive sleep apnoea, or history of cardiovascular disease.
Lower thresholds for certain ethnic groups: Adults of South Asian, Chinese, Black African, or Black Caribbean heritage may be eligible at BMI 27.5 or above without additional conditions, as these groups face higher health risks at lower BMI levels. Age restrictions: you must be 18 or over. Pregnancy and breastfeeding are contraindications. Use our free BMI calculator above to check where you currently sit on the BMI scale.
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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.
Two Routes to Access Weight Loss Injections
NHS route: Request a referral from your GP to a tier 3 specialist weight management service. Assessment involves discussion of your weight history, health conditions, lifestyle, and medication review. Waiting times vary significantly by region—some areas have waiting lists of many months. Once approved, prescriptions are dispensed through NHS pharmacies at no charge beyond standard prescription costs.
Private route: Consultation with a UK-licensed prescriber through a GPhC-registered online pharmacy. The consultation is typically completed online via questionnaire or video call. If approved, your prescription is dispensed within days. This route is faster but involves private costs (typically £129–£320 per month depending on dose). Both routes require honest health disclosure and ongoing monitoring.
- 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 are the most common side effects of slimming jabs?
The most common side effects of GLP-1 and GIP/GLP-1 injections like Mounjaro tirzepatide and Wegovy semaglutide are nausea, diarrhoea, vomiting, constipation, and stomach pain. These typically occur during the first few weeks and often improve as your body adjusts. Other common effects include indigestion, injection site reactions, and fatigue. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and gallbladder problems—seek immediate medical attention if these occur.
Do I need to see my GP first to get slimming injections?
For NHS access, yes—you'll need a GP referral to specialist services. For private routes, you don't need a GP referral, but you will need to complete a clinical assessment with a UK-licensed prescriber online. Either way, a qualified healthcare professional must assess your suitability before prescribing. Your GP can also provide support even if you're accessing medication privately.
How long does it take to get slimming injections on the NHS?
NHS waiting times vary significantly by region and local demand. Some areas have waits of 3–6 months, whilst others may exceed 12 months. Specialist tier 3 weight management services are busy. Private routes are typically faster—you can often receive your first injection within 1–2 weeks of consultation. Check with your local NHS integrated care board (ICB) for your area's current waiting times.
What happens during a private online consultation for weight loss injections?
You'll complete a detailed health questionnaire covering your medical history, current medications, weight history, and lifestyle. Some providers also offer a video consultation with a prescriber. The clinician assesses your eligibility based on BMI, health conditions, and contraindications. If approved, a prescription is issued and dispensed by a GPhC-registered pharmacy. The entire process typically takes 24–48 hours.
Will slimming injections work for me?
Clinical trials show that GLP-1 and GIP/GLP-1 injections help many people lose weight—Mounjaro users lost an average of 15–21% of body weight, whilst Wegovy users lost around 15%. However, individual results vary significantly. These injections work best alongside a reduced-calorie diet and increased physical activity. Try our free GLP-1 weight loss calculator to estimate potential outcomes at different doses based on your starting weight.
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.