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
When Will You Start To Notice Weight Loss?
Weight loss from prescription medication typically becomes noticeable within 2–4 weeks, though this varies significantly between individuals. In the UK, adults eligible for weight management medication like Mounjaro or Wegovy may experience appetite reduction before seeing physical changes on the scales. The timeline depends on starting dose, individual metabolism, and lifestyle factors. This guide explains realistic expectations when using prescription weight loss options in the UK, and how to track your progress effectively alongside your GP.
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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 Prescription Weight Loss Options and Timeline
Mounjaro (tirzepatide) and Wegovy (semaglutide) are the main prescription weight management options available in the UK. Both are GLP-1 receptor agonists (Mounjaro is dual GIP/GLP-1). Clinical trials showed participants began noticing appetite changes within days to weeks, though visible weight loss typically takes 4–8 weeks at therapeutic doses.
- Weeks 1–2: Many people notice reduced hunger and food cravings, though scale weight may not change significantly
- Weeks 2–4: Small weight reductions may become visible (0.5–1 kg)
- Weeks 4–8: More noticeable weight loss emerges as medication reaches stable doses
- Weeks 8+: Progressive weight loss continues with continued lifestyle changes
Weight loss with weight management medication in the NHS follows the same timeline. Individual results vary based on adherence to reduced-calorie diet, physical activity, starting dose, and personal metabolism. Your GP or prescriber can discuss realistic expectations for your situation.
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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.
Factors That Affect How Quickly You Notice Weight Loss
Several factors influence when weight loss becomes noticeable. Calorie deficit remains essential—medication supports appetite control but doesn't replace dietary changes. Starting dose matters: lower doses (2.5 mg Mounjaro, 0.25 mg Wegovy) take longer to show results than therapeutic doses. Physical activity, baseline weight, age, metabolism, and whether you're taking other medications all play roles.
Using our free GLP-1 weight loss calculator above can help you estimate realistic timelines and projected outcomes at different doses. Tracking your progress with our free progress tracker helps you notice non-scale victories—improved energy, better-fitting clothes, or reduced joint pain—which may appear before significant scale weight loss, especially in the first 2–4 weeks.
- 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 notice weight loss in the first week of prescription weight loss medication?
Most people don't see scale weight loss in the first week, though many notice reduced appetite and fewer food cravings within days. These appetite changes are the medication working—sustainable weight loss typically appears after 2–4 weeks as calorie deficit accumulates. Everyone is different, so discuss your timeline with your prescriber.
What if I'm not losing weight after 4 weeks on Mounjaro or Wegovy?
Several factors could affect early results: calorie intake, physical activity level, medication dose (you may still be titrating), underlying health conditions, or medications affecting absorption. Contact your GP or prescriber—they may adjust your dose, review your diet, or investigate other causes. Weight loss timelines vary individually.
How much weight can I expect to lose per week on prescription weight loss medication?
The NHS recommends 0.5–1 kg per week as a safe, sustainable rate. With prescription medication and reduced-calorie diet combined, many people achieve this average. Clinical trials showed average weight loss of 15–21% over several months. However, results vary—some lose faster initially, others more slowly. Your GP can advise what's realistic for you.
Should I use the BMI calculator to track progress with weight loss medication?
Yes, our free BMI calculator helps track your BMI change alongside weight loss. However, remember BMI has limitations—it doesn't distinguish muscle from fat. Scale weight, how you feel, energy levels, and clothes fit are equally important progress markers, especially in the first few weeks.
Can I get weight loss medication through the NHS in the UK?
Yes, Mounjaro and Wegovy are available through NHS specialist weight management services (tier 3). Eligibility requires BMI ≥30 (or ≥27 with weight-related health conditions), and you'll need referral from your GP. NHS availability varies by region. Discuss with your GP to find out what's available in your area and whether you might qualify. ```
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.