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?
Slimming injections—often called weight loss injections or "fat jabs" in the UK—are prescription-only medicines that may help with weight management. They are not cosmetic injections, but licensed medications used alongside lifestyle changes. This guide explains what they are, how they work, and what safety evidence shows in the UK context.
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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.
UK-Licensed Slimming Injections Explained
Three prescription slimming injections are currently licensed for weight management in the UK:
- Mounjaro (tirzepatide)—a dual GIP and GLP-1 receptor agonist, injected once weekly at doses from 2.5 mg to 15 mg. Clinical trials showed average weight loss of 15–21% body weight over 72 weeks when combined with reduced-calorie diet and physical activity.
- Wegovy (semaglutide)—a GLP-1 receptor agonist, injected once weekly at doses up to 2.4 mg. STEP trials showed average weight loss around 15% body weight at maximum dose.
- Saxenda (liraglutide)—a GLP-1 receptor agonist, injected daily at doses up to 3 mg. Requires daily rather than weekly dosing.
All three are prescription-only and require a clinical assessment before being prescribed. Access in the UK is through NHS specialist weight management services or private online prescribing following consultation with a licensed prescriber.
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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.
How Do Slimming Injections Work?
Slimming injections work by mimicking or enhancing natural gut hormones that regulate appetite and fullness. GLP-1 receptor agonists activate the GLP-1 hormone pathway, which signals to the brain that you are satisfied after eating, reduces food cravings, and slows how quickly food leaves the stomach. Mounjaro adds a second mechanism by also activating GIP receptors, which may enhance fat regulation.
These medications are most effective when used alongside a reduced-calorie diet and increased physical activity. Individual results vary significantly—some people may lose more weight, others less. Your GP or prescriber can advise whether weight loss injections are suitable for your personal health situation using our free BMI calculator and GLP-1 weight loss calculator to assess potential benefit.
- 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 to use?
Prescription slimming injections are licensed medicines that have undergone clinical trials in the UK and internationally. Side effects are common (nausea, vomiting, diarrhoea) but usually temporary. Serious side effects are rare. Safety depends on individual health—your GP or prescriber assesses this through clinical consultation before prescription. Report any concerns to your healthcare provider.
What are the most common side effects of weight loss injections?
Very common side effects include nausea, diarrhoea, constipation, vomiting, and stomach pain. These typically occur early in treatment and often improve over time. Less common side effects include headache, fatigue, dizziness, and injection site reactions. Serious side effects like severe pancreatitis are rare. Always read the patient information leaflet.
Who should not use slimming injections?
Slimming injections are not suitable for people who are pregnant, breastfeeding, or planning pregnancy, those with a personal or family history of thyroid cancer, people with severe gastroparesis, or anyone allergic to the medication. People under 18 cannot use them. Your prescriber will assess your full medical history.
How long do you need to take slimming injections?
Treatment duration varies by individual. Some people continue long-term; others stop after reaching their goal weight. NHS guidance and research suggest that stopping may result in weight regain, so ongoing use is often recommended. Discuss your personal plan with your GP or prescriber, who can advise on duration based on your health goals.
Are slimming injections available on the NHS in the UK?
Yes, prescription weight loss injections including Mounjaro and Wegovy are available through NHS specialist tier 3 weight management services. However, NHS access is limited and varies significantly by region. Waiting lists can be very long. Private access is available through licensed prescribers and registered pharmacies without NHS referral.
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.