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 Long Does Mounjaro Take To Start Working?
Most people notice appetite reduction within 2–4 weeks of starting Mounjaro, though this varies individually. Some experience effects by their second dose, whilst others take longer. Weight loss typically becomes measurable after 4–8 weeks. Unlike immediate changes, Mounjaro works by gradually adjusting your body's hunger signals — it's not an instant effect. Understanding the timeline helps set realistic expectations when starting this prescription weight management medication.
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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.
Mounjaro vs Wegovy: When You'll Notice Effects
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) work through similar mechanisms — activating GLP-1 pathways — but Mounjaro adds a dual GIP receptor component, which may affect onset timing slightly. In the SURMOUNT-1 clinical trial, Mounjaro users reported reduced appetite within the first 2–3 weeks. Wegovy users in the STEP trials showed similar onset, though individual variation is significant.
- Weeks 1–2: Initial dose (2.5mg Mounjaro or 0.25mg Wegovy) — subtle appetite reduction may begin; nausea is common
- Weeks 3–4: Many people notice clearer appetite suppression and reduced cravings; dose increases to next level
- Weeks 4–8: Weight loss becomes measurable as appetite control stabilises; side effects often improve
The key difference: Mounjaro's dual mechanism may produce slightly faster appetite control in some people, but clinical evidence shows both medications produce comparable results over 12+ weeks. Your prescriber will discuss individual suitability based on your health profile.
Important factors affecting how quickly you'll feel effects: Starting dose, your individual metabolism, whether you're also reducing calorie intake, and your body's sensitivity to GLP-1 activation. Some people on a tirzepatide KwikPen feel effects by week 2; others by week 6. Both Mounjaro and Wegovy require a UK prescription and clinical assessment, available through NHS specialist weight management services or private online prescribing.
- Consistent diet and exercise changes amplify and accelerate results
- Skipping doses or not advancing your dose as prescribed may delay visible weight loss
- Medical conditions (gastroparesis, diabetes) may affect onset and tolerance
- Expected weight loss timeline: 5–10% body weight by week 12; 15–21% by week 68 (SURMOUNT data)
Use our free GLP-1 weight loss calculator above to project potential outcomes at different dose levels, and track progress with our progress tracker once you start treatment.
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 To Expect In Your First Month
Your first month on Mounjaro or Wegovy will focus on tolerance and dose adjustment, not gradual weight management. Week 1–2: You'll self-inject your starting dose weekly. Nausea, mild stomach discomfort, and reduced appetite are very common — these usually ease by week 3. You may lose 2–4 lbs initially from reduced calorie intake. Week 3–4: Your dose increases; appetite suppression strengthens. Constipation or diarrhoea may occur as your gut adjusts. Measurable weight loss typically starts now.
Both medications follow a gradual dose titration schedule — neither reaches full effect at the starting dose. This protects against side effects and allows your body to adapt. If you experience severe nausea or other concerns, contact your prescriber immediately; they may adjust your timeline. Patience and consistency with diet changes, alongside the medication, produce the best outcomes.
- 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 before Mounjaro suppresses appetite?
Most people notice reduced appetite within 2–4 weeks of starting Mounjaro. Some feel effects by their second injection (week 2), whilst others take 4–6 weeks. Individual variation is normal and depends on metabolism, starting dose, and body sensitivity to GLP-1 activation.
Is Mounjaro faster than Wegovy?
Mounjaro and Wegovy show similar onset timelines in clinical trials (2–4 weeks for appetite reduction). Mounjaro's dual GIP+GLP-1 mechanism may produce slightly faster hunger control in some people, but results are comparable over 12+ weeks. Your prescriber will recommend the most suitable option for you.
When will I see weight loss on Mounjaro?
Measurable weight loss typically appears after 4–8 weeks of consistent use alongside reduced calorie intake and physical activity. The SURMOUNT-1 trial showed an average 15–clinically studied weight loss over 68 weeks, but individual results vary significantly.
Why does Mounjaro take weeks to work?
Mounjaro works by gradually resetting your appetite hormones and hunger signals — it's not an instant effect. Additionally, prescription weight loss injections use a dose titration schedule, starting low and increasing every 4 weeks to minimise side effects and allow your body to adapt safely.
Can I check my BMI and weight loss progress?
Yes. Use our free BMI calculator above to track your starting point, and our progress tracker to log weekly or monthly weight changes. These tools help visualise progress and keep you motivated through the slower early weeks when appetite control is building.
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.