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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Diet Injections Side Effects — What You Need to Know
Diet injections, commonly called slimming injections or weight loss jabs, are prescription-only medicines licensed in the UK for weight management in adults. These are not cosmetic injections—they are injectable medications that work by mimicking natural hormones to regulate appetite and support weight loss alongside dietary changes. Understanding what diet injections are, how they work, and their potential side effects is essential before considering treatment. This guide explains the three UK-licensed options available and the mechanism behind how they function to help you make an informed decision.
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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.
Three UK-Licensed Weight Loss Injections
Three prescription-only weight loss injections are currently licensed in the UK for weight management in adults. Mounjaro (tirzepatide) is a once-weekly injection that activates both GIP and GLP-1 hormone pathways, with an average weight loss of 15–21% of body weight in clinical trials. Wegovy (semaglutide) is also a once-weekly GLP-1 injection, with trials showing average weight loss around 15% of body weight. Saxenda (liraglutide) is a daily GLP-1 injection for those preferring this frequency. All three are prescription-only and require a clinical assessment before dispensing.
- Mounjaro: Once weekly, doses 2.5–15 mg
- Wegovy: Once weekly, doses 0.25–2.4 mg
- Saxenda: Once daily, doses 0.6–3 mg
Access in the UK is available through NHS specialist weight management services or private online prescribing following a clinical consultation. Individual results vary, and these medications work best alongside a reduced-calorie diet and increased physical activity. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses.
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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 GLP-1 Weight Loss Injections Work
GLP-1 receptor agonist medications (and Mounjaro's dual GIP+GLP-1 mechanism) work by activating natural hormone pathways in the body. When you eat, your gut naturally produces GLP-1, which signals your brain that you are full, slows stomach emptying, and helps regulate blood sugar. These slimming injections amplify and prolong these effects, helping you feel fuller for longer and reducing food cravings. This supports a natural reduction in calorie intake without the need for extreme restriction.
The effect typically develops gradually over the first few weeks as your dose increases. Most people notice reduced appetite and improved fullness signals within 2–4 weeks of starting treatment. Check your BMI using our free BMI calculator to understand where you sit on NHS health thresholds, and discuss with your GP whether prescription weight loss treatment might be appropriate for your individual circumstances.
- 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 diet injections and slimming injections the same thing?
Yes. Diet injections, weight loss jabs, slimming injections, and fat jabs are all colloquial UK terms for the same prescription-only medicines—GLP-1 receptor agonists (or dual GIP+GLP-1 agonists in the case of Mounjaro). They are medical injectable medications, not cosmetic treatments.
What is the difference between Mounjaro and Wegovy?
Both are once-weekly injections for weight management, but Mounjaro activates two hormone pathways (GIP and GLP-1), while Wegovy activates one (GLP-1 only). Mounjaro starts at 2.5 mg and goes up to 15 mg; Wegovy starts at 0.25 mg and goes up to 2.4 mg. Clinical trials showed similar average weight loss results, though individual outcomes vary.
Can I get diet injections on the NHS?
Mounjaro and Wegovy are approved by NICE for NHS use through specialist tier 3 weight management services. Access varies significantly by region and involves referral from your GP. Waiting lists can be lengthy. Private access is available through online consultations with UK-licensed prescribers registered with the GPhC.
How quickly do weight loss injections work?
You typically notice reduced appetite within 2–4 weeks of starting. Most significant weight loss occurs between weeks 8–24 as your dose increases. The medication continues to support weight loss for as long as you take it, alongside a reduced-calorie diet and physical activity. Results vary between individuals.
Are slimming injections suitable for everyone?
No. They are not suitable during pregnancy or breastfeeding, for people under 18, or those with a history of certain thyroid or pancreatic conditions. A UK-licensed prescriber will assess your medical history and determine suitability during a clinical consultation. Always discuss with your GP or a qualified healthcare professional first.
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.