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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Mounjaro Weight Loss by Week — What the Clinical Data Shows
If you're considering Mounjaro as a weekly weight loss injection in the UK, you're probably wondering how quickly you'll see results. The reality is that weight loss doesn't happen in a straight line — some weeks you'll lose more, some weeks less. Clinical trials show average weight loss over 72 weeks, but what happens week-by-week? This guide explains realistic timelines based on SURMOUNT trial data, how dose increases affect weight loss, and why individual results vary so significantly. Remember, Mounjaro is a prescription-only medicine that works alongside dietary changes and physical activity — not instead of them.
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Calculate Your BMI Instantly
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.
Clinical Trial Weight Loss Timeline (SURMOUNT Data)
The SURMOUNT-1 trial tracked participants on Mounjaro over 72 weeks (approximately 17 months). Weight loss wasn't immediate — most participants experienced gradual, progressive loss as they progressed through dose titration (starting at 2.5 mg and increasing weekly doses every 4 weeks). In the first 4 weeks on the starting dose, weight loss was modest — typically 0.5–1.5 kg. By week 12, participants averaged 2–3 kg loss. The most significant weight loss occurred between weeks 12–36 as doses increased to 10 mg and 15 mg. By week 72, participants on the maximum 15 mg dose had lost an average of approximately 20.9% of their body weight — roughly 15–20 kg for someone starting at 100 kg, though individual results varied considerably.
- Weeks 1–4 (2.5 mg): Minimal loss, adjustment phase
- Weeks 5–12 (5 mg): Steady loss, 1–2 kg per week typical
- Weeks 13–36 (7.5–15 mg): Accelerated loss as appetite suppression increases
- Weeks 37–72 (maintenance): Loss continues but rate plateaus
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.
Why Your Week-to-Week Results Won't Match a Straight Line
It's crucial to understand that weight naturally fluctuates week-to-week due to water retention, hormonal changes, digestion, and salt intake — not just fat loss. You might lose 2 kg one week and 0.2 kg the next. This is completely normal and not a sign that Mounjaro has stopped working. Many people find it helpful to track weight loss over monthly intervals or use our free progress tracker rather than obsessing over weekly numbers. Additionally, as you lose weight, your metabolic rate decreases slightly, which is why weight loss typically slows after the first 3–6 months — this is a normal physiological adaptation, not failure. Weight loss injections work best when combined with consistent healthy eating, regular activity, and patience with the natural fluctuations of the process.
- 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 do people lose in the first week of Mounjaro?
Very little — typically 0.5–1 kg in week one, if any at all. The first week is usually an adjustment period as your body adapts to the medication. Most weight loss begins by week 3–4 and accelerates after dose increases. Don't expect dramatic early results; this is normal.
Does Mounjaro work faster than Wegovy for weight loss?
Clinical trials suggest Mounjaro (tirzepatide, a dual GIP/GLP-1 agonist) may produce slightly greater average weight loss than Wegovy (semaglutide, a GLP-1 agonist) — approximately 20.9% versus 14.9% body weight over similar timeframes. However, individual results vary significantly, and both are prescription-only medications requiring clinical assessment.
Will I lose weight every single week on Mounjaro?
No — weight naturally fluctuates week-to-week due to water retention, hormones, digestion, and other factors unrelated to fat loss. Week-to-week variations are normal. Tracking monthly or using a progress tracker helps you see the real trend rather than worrying about normal weekly ups and downs.
When does weight loss plateau on Mounjaro?
Most people notice the rate of weight loss slowing after 4–6 months as their body adapts and their metabolic rate decreases — this is a normal physiological response, not a sign of failure. Some continue losing weight for several months; others plateau earlier. Your GP or prescriber can advise on next steps.
Can I use your BMI calculator to track my weekly progress on Mounjaro?
Yes — our free BMI calculator and progress tracker on this page help you log weekly or monthly weights and see how your BMI changes over time. This helps separate normal fluctuations from genuine progress and keeps you motivated during your weight loss journey.
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.