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 Patients Experience
Mounjaro (tirzepatide) is a prescription-only weight loss injection taken once weekly. People often ask what weight loss happens each week — but the reality is more complex. Weight loss with Mounjaro happens gradually over months, not dramatically week to week. Clinical trial data shows meaningful loss typically emerges around weeks 8–12, with continued progress over 16+ weeks. This guide explains the realistic week-by-week timeline, dose escalation schedule, and what clinical evidence actually shows about Mounjaro weight loss patterns in UK patients.
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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.
How Mounjaro Weight Loss Works Week By Week
Mounjaro works by activating GIP and GLP-1 hormone pathways, which regulate appetite and fullness signals. Weight loss is not linear — most people experience minimal loss in the first 2–4 weeks, then gradual increases as their body adjusts and they continue a reduced-calorie diet.
Typical timeline across the dose titration schedule:
- Weeks 1–4 (2.5 mg dose): Often no change or 0–1 kg loss as the body adapts. Some experience nausea, affecting early eating patterns.
- Weeks 5–8 (5 mg dose): Many people start to notice appetite reduction. Average loss: 1–2 kg over this period.
- Weeks 9–16 (7.5–10 mg doses): Weight loss typically accelerates. Average loss: 2–4 kg per 4-week block as medication effects strengthen.
- Weeks 17+ (12.5–15 mg maintenance): Loss continues but often slows. Additional 1–3 kg per 4 weeks is typical.
The SURMOUNT-1 clinical trial showed an average loss of approximately 20.9% body weight over 72 weeks at the maximum 15 mg dose — but individual results vary significantly. Some people lose more, some less. Weight loss depends on adherence to diet and activity changes, not just the injection.
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.
What Clinical Reviews Actually Show
The clinical trial data (SURMOUNT-1) published in the New England Journal of Medicine followed participants for 72 weeks. At the 15 mg dose, average weight loss was approximately 20.9% of starting body weight — typically 15–25 kg for people starting at 100+ kg. However, this was alongside reduced-calorie diet and increased physical activity. Without lifestyle changes, results would be less dramatic.
Real-world UK patient reviews on prescription forums and weight loss communities often report: nausea in weeks 2–4, reduced appetite from week 5 onwards, plateau phases lasting 2–3 weeks (normal, not failure), and the need to actively maintain diet structure to see continued loss. Using our free GLP-1 weight loss calculator can help you estimate realistic projections based on your starting weight and chosen dose.
Many UK patients find that tracking their progress weekly with our free progress tracker helps identify patterns — some weeks show loss, others show no change or slight gain (often water retention). This is normal and expected. A prescriber-led weekly weight loss injection schedule provides structured accountability. If you're considering Mounjaro, your GP or a licensed prescriber will assess your eligibility and discuss realistic expectations during your clinical assessment.
- 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 lose weight every single week on Mounjaro?
No. Weight loss with Mounjaro is gradual and often plateaus. Most people see minimal change in weeks 1–4, then gradual loss from weeks 5–8 onwards. You may have weeks with no loss or slight gains (water retention is normal). The focus is on overall trend over months, not weekly consistency. Clinical trials showed average loss of ~21% body weight over 72 weeks.
How much weight can I lose in the first month of Mounjaro?
In month one (weeks 1–4 at 2.5 mg starting dose), most people lose 0–2 kg. This is expected — the medication is still adjusting in your system, and nausea may reduce appetite naturally. Significant weight loss typically starts from week 5 onwards as the dose increases to 5 mg and your body adapts to the medication.
Is Mounjaro or Wegovy better for weekly weight loss?
Clinical trials show similar results: Mounjaro averaged ~clinically studied weight loss, Wegovy ~15% at maximum doses. Mounjaro is a dual GIP+GLP-1 agonist, Wegovy is GLP-1 only. Both are prescription-only and require clinical assessment. Individual response varies — your prescriber will recommend the best option for you. Try our Mounjaro vs Wegovy comparison for detailed information.
Can I access Mounjaro through the NHS with weekly injections?
Mounjaro is available through NHS specialist weight management services (tier 3 services), but access varies by region and waiting lists can be long. Private prescribing is also available through GPhC-registered online pharmacies following clinical assessment. Both routes offer the same weekly KwikPen injection schedule — your prescriber will manage the dose titration.
What should I track if I'm using Mounjaro weekly injections?
Track your weight weekly (use our free progress tracker), but focus on trends over 4-week blocks, not daily fluctuations. Also note: appetite changes, energy levels, fit of clothes, and how you feel — these often improve before the scales move significantly. Report any concerning side effects to your prescriber immediately.
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.