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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How Fast Does Mounjaro Work — Timeline & Comparison
Mounjaro (tirzepatide) starts working within days of your first injection, but meaningful weight loss typically becomes noticeable after 4–8 weeks of treatment. However, the timeline depends on your individual response, dose level, and how consistently you follow a reduced-calorie diet and physical activity plan. Compared to Wegovy (semaglutide), Mounjaro may work faster for some people due to its dual GIP and GLP-1 mechanism, though results vary significantly. This guide explains what clinical trials showed about speed of weight loss, how the two medications compare, and what to expect during your first weeks of treatment.
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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 Data — Mounjaro vs Wegovy Timeline
Mounjaro (SURMOUNT trial) showed measurable weight loss within the first 4 weeks for most participants, with average weight loss of approximately 3–5% of body weight by week 8. At 16 weeks, average loss reached around 10%, and by week 72 (final assessment), participants on 15 mg achieved an average 20.9% body weight reduction — though individual results varied considerably.
Wegovy (STEP-1 trial) showed similar early response, with noticeable weight loss typically appearing by weeks 4–8. However, the maximum dose (2.4 mg) takes longer to reach — the dose escalation phase spans approximately 16 weeks. At 68 weeks, Wegovy achieved an average weight loss of approximately 14.9% body weight.
The key difference: Mounjaro's dual GIP + GLP-1 mechanism may produce faster initial appetite suppression for some users, while Wegovy's single GLP-1 mechanism works effectively but slightly differently. Both require consistent diet and exercise to achieve results — the injection alone does not clinically studied weight loss.
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.
What Affects How Quickly Mounjaro Works
Your speed of weight loss depends on several factors: starting dose (2.5 mg takes longer than escalated doses), individual metabolism (genetics affect how your body responds), calorie deficit (you must eat fewer calories for weight loss to occur), physical activity level, adherence to the injection schedule, and existing health conditions like type 2 diabetes (which may slow initial response).
Some people see results by week 2–3; others notice changes after 8+ weeks. Your GP or prescriber can discuss realistic expectations based on your personal health profile. Use our free GLP-1 weight loss calculator to estimate potential timelines at different Mounjaro doses. Remember: slower, steady weight loss (0.5–1 kg per week) is more sustainable than rapid loss and better for preserving muscle mass.
- 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 long until Mounjaro starts working?
Mounjaro begins affecting appetite within 24–48 hours of your first injection. Measurable weight loss typically appears within 2–4 weeks for most people, though some don't notice changes until week 6–8. Clinical trials showed an average 3–clinically studied weight loss by week 8. Individual timelines vary based on starting dose, metabolism, and adherence to diet and exercise.
Does Mounjaro work faster than Wegovy?
In clinical trials, Mounjaro showed slightly faster weight loss for some participants due to its dual GIP and GLP-1 mechanism, but the difference is modest. SURMOUNT achieved 20.9% average loss at 15 mg; STEP achieved 14.9% at 2.4 mg Wegovy. Both work effectively — speed varies individually and depends on dose, diet consistency, and personal response.
Why doesn't Mounjaro work immediately?
Your body needs time to adjust to the injection and for appetite suppression to take full effect. The dose is also escalated gradually (starting at 2.5 mg, increasing every 4 weeks) to minimise side effects. Higher doses produce stronger appetite suppression. Additionally, weight loss requires a calorie deficit — the injection supports this but doesn't create weight loss without dietary changes.
Can I speed up Mounjaro weight loss?
You can optimise results by maintaining a consistent calorie deficit (approximately 500 kcal/day for 0.5 kg/week loss), staying physically active, following your dose escalation schedule, and taking injections on the same day each week. gradual weight management (more than 1 kg/week) increases muscle loss risk. Your GP can advise on safe, sustainable approaches tailored to your health.
What's the UK prescription process for Mounjaro?
Mounjaro (tirzepatide) is prescription-only in the UK. NHS access requires referral to a specialist tier 3 weight management service — availability and waiting times vary by region. Private UK prescribing is available through online consultations with licensed prescribers, followed by dispensing via a GPhC-registered pharmacy. Both routes require clinical assessment to confirm eligibility.
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.