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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GLP-1 Weight Loss: Before and After Explained
GLP-1 medications like Wegovy (semaglutide) and Mounjaro (tirzepatide) are prescription-only weight management injections licensed in the UK. Clinical trials show these medications can help people lose weight alongside diet and exercise, but actual outcomes vary widely between individuals. Understanding realistic expectations is essential before starting any weight loss medication. This guide explains what clinical evidence shows about GLP-1 weight loss and factors that influence individual results.
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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.
What Clinical Trials Show About GLP-1 Weight Loss
Wegovy (semaglutide) achieved an average weight loss of approximately 14.9% of body weight in the STEP-1 clinical trial over 68 weeks. Mounjaro (tirzepatide) showed average weight loss ranging from approximately 15% to 21% of body weight depending on the dose used, with the highest dose (15 mg) producing the greatest average reduction.
These clinical trial results are significant — they represent weight loss medication nhs specialists and prescribers refer to when assessing whether treatment is suitable. However, "average" is crucial: some participants lost considerably more, while others lost less. Weight loss injections work by reducing appetite and helping regulate food intake, but they are not assured to produce the same outcome in every person.
- Clinical trials included participants following a reduced-calorie diet and increased physical activity
- Results were measured over specific trial periods (typically 68–72 weeks)
- Individual outcomes depend on dose, adherence, diet, and lifestyle changes
Clinically BackedWeight Loss
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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.
Why Results Vary Between Individuals
Weight loss with GLP-1 medication depends on multiple factors beyond the injection itself. Your starting weight, metabolism, how your body responds to the specific medication, dietary adherence, physical activity level, and even genetics all influence outcomes. Some people may experience significant appetite suppression and steady weight loss; others may need dose adjustments or may respond differently.
Prescription weight loss medication is not a replacement for lifestyle changes — it works alongside them. The most successful outcomes occur when people combine their prescribed treatment with consistent dietary modifications and movement. This is why prescribers always discuss lifestyle alongside medication during your clinical assessment. Track your progress using our free progress tracker to identify patterns and adjust your approach over time.
Before starting any glp-1 weight loss treatment, use our free BMI calculator to understand your current health context and what a healthy weight range looks like for your height. Our GLP-1 weight loss calculator can also help you explore potential outcomes at different doses — though remember these are estimates, not assures. Individual results vary significantly, and your own prescriber will provide personalised guidance based on your medical history and health goals.
- 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.
Will I definitely lose weight on GLP-1 medication?
Clinical trials show average weight loss, but individual results vary considerably. Some people lose more than average; others lose less. Success depends on the medication, your body's response, dose, diet, exercise, and consistency. Your prescriber will assess whether treatment is suitable for you specifically.
How quickly will I see weight loss results?
Weight loss typically begins within the first few weeks as your dose increases. Most significant changes occur over 12–16 weeks as your body adjusts and your dose reaches the therapeutic level. Sustainable, healthy weight loss is gradual — approximately 0.5–1 kg per week is considered safe and realistic.
What happens after I stop taking GLP-1 medication?
Weight may gradually return if lifestyle changes are not maintained. GLP-1 medications work while you take them; they are not permanent solutions. Long-term success requires sustained dietary changes and physical activity. Your prescriber will discuss maintenance strategies with you.
Are before and after photos realistic for weight loss prediction?
Before and after images can be misleading because they often don't show the full picture — they may be from very selected cases, use different lighting or angles, or represent the very best outcomes. Individual results vary widely. Focus on your own health metrics: weight, BMI, how you feel, and energy levels rather than comparing yourself to others' images.
Can I use GLP-1 medication without changing my diet?
Prescription weight loss medication works best alongside reduced-calorie diet and increased activity. Medication alone without lifestyle changes produces much smaller results. Your prescriber will discuss the importance of combining medication with behaviour change for the best outcomes.
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.