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
How Do Weight Loss Injections Work?
Weight loss injections are prescription medications that help reduce appetite and support weight management. In the UK, three licensed options are available: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). These medications work by mimicking natural hormones that signal fullness to the brain, slow digestion, and help regulate blood sugar. They're prescribed as part of a weight management programme alongside diet and physical activity changes. Understanding how they work can help you decide whether this weight management approach is suitable for your health goals.
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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 Weight Loss Injections and How They Work
Weight loss injections available in the UK fall into two categories: GLP-1 receptor agonists and dual GIP/GLP-1 agonists. These medications mimic hormones naturally produced in your gut after eating.
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 agonist that activates two hormone pathways, amplifying appetite reduction and supporting metabolic function. Maximum dose 15 mg once weekly.
- Wegovy (semaglutide) — a GLP-1 agonist available in doses up to 2.4 mg once weekly. Clinical trials showed average weight loss of around 15% of body weight.
- Saxenda (liraglutide) — a daily GLP-1 injection, typically used when weekly injections aren't suitable.
All three work by reducing hunger signals, slowing how quickly food leaves your stomach, and helping you feel fuller after smaller meals. Individual results vary significantly, and they're most effective when combined with a reduced-calorie diet and increased physical activity. Calculate your potential weight loss outcomes using our free GLP-1 weight loss calculator.
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 Access and Eligibility
In the UK, weight loss injections are available through both NHS specialist services and private prescribing. NHS access requires referral to a tier 3 weight management service from your GP, though availability varies significantly by region and waiting lists can be lengthy. Private access is available through GPhC-registered online pharmacies following an online clinical assessment, offering quicker access without NHS referral.
You may be eligible if you have a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. South Asian, Chinese, Black African, and Black Caribbean adults may qualify at slightly lower BMI thresholds due to higher health risks at lower weights. Speak to your GP about eligibility and which option suits your situation best.
- 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's the difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP/GLP-1 agonist (maximum 15 mg weekly) while Wegovy is a GLP-1 agonist (maximum 2.4 mg weekly). Mounjaro activates two hormone pathways, while Wegovy activates one. Clinical trial data shows Mounjaro achieved slightly higher average weight loss (around 20–21%) compared to Wegovy (around 15%), though individual results vary. Your prescriber will recommend the most suitable option based on your health profile.
How quickly do weight loss injections work?
Weight loss typically begins within the first few weeks, though the full effect develops over several months as doses are gradually increased. Most people reach their target dose by week 17–20. In clinical trials, significant weight loss was observed by 12–16 weeks, with continued loss over the full trial period (68–72 weeks). Your GP or prescriber can discuss realistic timelines for your specific situation.
Do weight loss injections work without diet and exercise changes?
No. Weight loss injections are most effective when combined with a reduced-calorie diet and increased physical activity. The medications support appetite reduction and metabolic function, but behavioural changes are essential for sustainable weight loss. Think of the injection as a tool that makes diet and lifestyle changes easier to stick to, not a replacement for them.
Who cannot take weight loss injections?
Weight loss injections are not suitable for people who are under 18, pregnant, breastfeeding, or have a personal or family history of thyroid cancer. They're also not recommended for those with severe gastroparesis (stomach paralysis) or certain other medical conditions. Your GP or prescriber will assess contraindications during your clinical assessment.
How can I find out if I'm eligible for weight loss injections in the UK?
Start by speaking to your GP about weight management options. They can refer you to NHS tier 3 specialist weight management services if eligible and available in your area. Alternatively, you can access private prescribing through GPhC-registered online pharmacies. Check your BMI using our free BMI calculator, and discuss your health history with a healthcare professional to determine suitability.
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.