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
What Happens When You Take Mounjaro?
When you start Mounjaro (tirzepatide), your body experiences changes that work to support weight loss. Mounjaro is a prescription-only, once-weekly injection that activates two natural hormone pathways — GIP and GLP-1 — which regulate appetite, food cravings, and how your body processes food. Understanding what to expect helps you prepare for the journey ahead and recognise whether the medication is working as intended.
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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 Works in Your Body
Mounjaro's dual mechanism sets it apart from other weight loss injections. When you inject the medication, tirzepatide attaches to GIP and GLP-1 receptors in your brain and gut, triggering several changes:
- Appetite suppression: Signals to the brain increase feelings of fullness, reducing hunger and food cravings
- Slower stomach emptying: Food moves more slowly from your stomach to your intestines, prolonging satiety
- Improved blood sugar regulation: The medication helps your body process glucose more effectively
- Fat utilisation: Your body may better utilise stored fat for energy
These changes typically begin within days to weeks, though individual responses vary significantly. Using our free GLP-1 weight loss calculator can help you estimate potential weight loss at your planned dose.
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 When Starting Mounjaro
Most people experience noticeable changes within the first two to four weeks. Many report reduced appetite, decreased cravings for certain foods (especially sugary or fatty items), and feeling fuller after smaller meals. Some experience nausea initially, which typically improves as your body adjusts. Weight loss usually becomes visible after four to six weeks on your current dose. The Mounjaro kwikpen is injected once weekly, and doses are gradually increased every four weeks — you won't jump straight to the maximum dose. Your prescriber will assess your response at each stage and adjust accordingly.
- 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 quickly does Mounjaro start working?
Most people notice reduced appetite within 3–7 days of their first injection. Noticeable weight loss typically appears after 4–6 weeks. However, individual responses vary — some experience changes faster, others more gradually. Consistency with your reduced-calorie diet and physical activity is essential for best results.
Will I feel different physically when taking Mounjaro?
Yes. Common physical changes include reduced hunger, earlier fullness during meals, less interest in certain foods, and sometimes mild nausea (especially in the first few weeks). Most people describe these as positive changes that support weight loss. Side effects usually decrease over time as your body adjusts.
Can I stop taking Mounjaro whenever I want?
Yes, Mounjaro is not addictive. However, stopping suddenly may cause appetite and cravings to return quickly, potentially leading to weight regain. Discuss any plans to stop with your prescriber, who can advise on the best approach for you.
Does Mounjaro work better than Wegovy for weight loss?
Clinical trials show Mounjaro (tirzepatide) achieved slightly higher average weight loss (~15–21% of body weight) compared to Wegovy (semaglutide, ~15% of body weight). Both are effective — individual results vary. Your prescriber will assess which is more suitable based on your health profile and eligibility.
What if Mounjaro doesn't seem to be working?
If you haven't noticed changes after 4–6 weeks, contact your prescriber. You may need a dose increase (doses are escalated every four weeks). Ensure you're also following a reduced-calorie diet and moving more — medication works best alongside lifestyle changes. Your GP or prescriber can help troubleshoot if progress stalls.
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.