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
What to Expect During Weight Loss Week 3
Week 3 of weight loss is often a critical point where sustainable patterns start forming. Whether you're managing weight through lifestyle changes, NHS weight management programmes, or prescription weight loss medication like GLP-1 injections, understanding what happens during this period helps you stay motivated and realistic about your progress. This guide covers realistic expectations, common experiences, and how to maintain momentum as you move through the first month of weight management.
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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.
Weight Loss Progress in Week 3
By week 3, most people following a reduced-calorie diet with increased physical activity typically see measurable progress. The NHS recommends losing 0.5–1 kg per week for sustainable results, meaning you may have lost 1.5–3 kg by the end of week 3. However, weight loss is not linear — some weeks show more progress than others due to water retention, hormonal fluctuations, and digestive changes.
If you're taking prescription weight loss medication such as GLP-1 injections (Wegovy or Mounjaro), week 3 is often when appetite-suppressing effects become more noticeable. Semaglutide (Wegovy) and tirzepatide (Mounjaro) work by activating natural hormone pathways that reduce hunger and support feeling fuller after smaller meals. Clinical trials showed these medications can help with weight loss when combined with dietary changes and increased physical activity — though individual results vary significantly.
Track your progress using our free progress tracker to monitor weight changes, not just the scales. Measurements, how clothes fit, and energy levels all matter alongside the number on the scales.
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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.
Common Week 3 Experiences and Challenges
Many people experience appetite reduction in week 3, which is positive and indicates your body is responding to lifestyle or medication changes. Some report increased energy levels as fitness improves and initial weight loss reduces strain on joints and cardiovascular systems. Others notice digestive changes — increased fibre intake, reduced calorie consumption, or GLP-1 medication effects can cause mild constipation, diarrhoea, or nausea.
Motivation can dip in week 3 as initial excitement fades. This is normal. Remember that sustainable weight management is a marathon, not a sprint. If you're using prescription weight loss medication through NHS weight management services or private prescribing, your GP or prescriber can offer practical support. Calculate your current BMI using our free BMI calculator to understand where you are in your weight management journey and what realistic targets look like for your height and health profile.
- 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.
How much weight should I lose by week 3?
Most people lose 1.5–3 kg in the first three weeks following a reduced-calorie diet and increased physical activity. The NHS recommends losing 0.5–1 kg per week. Weight loss is not linear — some weeks show more progress than others due to water retention and other factors. Individual results vary significantly, especially with prescription medication.
Is nausea normal in week 3 of weight loss medication?
If you're taking GLP-1 medication like Wegovy or Mounjaro, mild nausea can occur, especially in early weeks. This typically improves as your body adjusts. Nausea is listed as a very common side effect. Eat smaller meals, stay hydrated, and avoid fatty or spicy foods. Contact your GP or prescriber if nausea is severe or persistent.
Why has my weight loss plateaued by week 3?
Weight loss plateaus are completely normal and don't mean progress has stopped. Water retention from exercise, hormonal changes, and digestive adjustments can mask fat loss. Continue following your calorie deficit and increased physical activity. Real progress is often visible in measurements, energy levels, and how clothes fit before the scales show movement.
Should I increase my exercise in week 3?
Gradually increasing physical activity is sensible, but avoid sudden dramatic increases which raise injury risk and can cause temporary water retention. The NHS recommends 150 minutes of moderate activity weekly. If you're taking GLP-1 medication, ensure you're eating enough to fuel exercise — appetite suppression means you must consciously eat sufficient calories for activity levels.
Can I expect faster weight loss with prescription weight loss injections?
Clinical trials (STEP-1 for Wegovy, SURMOUNT-1 for Mounjaro) showed participants lost an average of 15–21% of body weight over several months when combined with lifestyle changes. Results vary between individuals. Week 3 is very early — most significant progress appears from weeks 4–12 as medications reach therapeutic doses and dietary changes become habits.
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.