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
How Quickly Do You Lose Weight With Mounjaro?
Weight loss with Mounjaro varies significantly between individuals, but clinical trials show average weight loss occurs gradually over weeks and months. In the SURMOUNT-1 trial, participants receiving the maximum 15 mg dose lost approximately 20.9% of their body weight over 72 weeks — though results depend on your starting weight, lifestyle changes, dose level, and individual metabolism. This page explains realistic timelines, what the evidence shows, and factors that influence how quickly you may lose weight with this prescription injection.
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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.
What Do Clinical Trials Show About Mounjaro Weight Loss?
The SURMOUNT-1 clinical trial is the main evidence for how quickly Mounjaro works. Participants taking the 5 mg dose lost approximately 15% of their body weight over 72 weeks. Those on the 10 mg dose lost around 19.5% of their body weight, and participants on the maximum 15 mg dose achieved average weight loss of 20.9%. However, these are average results — individual weight loss was highly variable.
Weight loss was not immediate. Most participants experienced gradual loss starting in weeks 4–8, with the majority of weight loss occurring between weeks 12–52. The trial combined Mounjaro with a reduced-calorie diet and increased physical activity — medication alone does not studied results.
Important context: these trial participants were carefully selected and monitored. Real-world results may differ, and individual factors like your starting weight, metabolism, adherence to lifestyle changes, and dose level significantly affect your personal timeline.
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.
Factors That Affect How Quickly You Lose Weight
Your individual weight loss speed depends on several key factors. Dose level matters — higher doses (10 mg and 15 mg) showed faster average weight loss than lower doses (5 mg) in trials. Starting weight also influences speed; people with higher starting weights often see faster initial weight loss. Your diet and physical activity directly impact results — Mounjaro works alongside lifestyle changes, not instead of them.
Metabolism, age, genetics, and other medications also play a role. Some people experience noticeable weight loss within 4–8 weeks; others take 12+ weeks to see significant change. This prescription weight loss injection requires patience and consistency with both the medication and lifestyle modifications.
- 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 most people lose with Mounjaro in the first month?
Weight loss is typically gradual. Most people do not see significant results in the first 4 weeks because the body is adjusting to the medication. The dose is also slowly increased each month (dose titration). Noticeable weight loss usually begins around weeks 4–8, but individual timelines vary considerably. Starting doses (2.5 mg) are lower than maintenance doses.
Is Mounjaro faster than Wegovy for weight loss?
Clinical trials suggest Mounjaro may produce slightly faster average weight loss than Wegovy (semaglutide), though both are effective. SURMOUNT-1 showed Mounjaro 15 mg achieved 20.9% average weight loss versus Wegovy's approximately 15% in comparable trials. However, individual results vary significantly, and the best medication depends on your personal health profile and prescriber assessment.
What if I am not losing weight after 3 months on Mounjaro?
After 12 weeks, discuss your progress with your prescriber. Weight loss may be slow but steady, or your dose may need adjustment. Your lifestyle (diet and exercise) directly affects results — Mounjaro supports weight loss but does not work without calorie deficit. Some people respond more slowly than others. Your prescriber can review your progress and adjust your treatment plan.
Can you lose weight quickly with Mounjaro without changing diet or exercise?
Clinical trials combined Mounjaro with reduced-calorie diet and increased physical activity. Using the medication without lifestyle changes significantly reduces results. Mounjaro helps reduce appetite and cravings, but it is most effective alongside dietary changes and movement. Realistic, sustainable weight loss requires both medication and lifestyle modifications.
When should I expect to see results with Mounjaro?
Most people notice initial changes (reduced appetite, fuller feeling) within the first 1–2 weeks. Measurable weight loss on the scales typically appears between weeks 4–12, though this varies. Significant weight loss usually builds gradually from weeks 12–24 onwards. Use our free progress tracker to monitor your journey over weeks and months — celebrate non-scale changes too, like increased energy and better fitting clothes.
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.