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
Does Mounjaro Break Down Fat?
Mounjaro (tirzepatide) doesn't directly break down or "dissolve" existing fat stores, but it works through a different mechanism to support weight loss. As a dual GIP and GLP-1 receptor agonist, it helps regulate appetite, reduces food cravings, and supports your body's natural fat utilisation when combined with a reduced-calorie diet and increased physical activity. Understanding how Mounjaro actually works can help you set realistic expectations about this prescription-only weight management 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.
How Mounjaro Works in the Body
Mounjaro activates two natural hormone pathways in your body: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action is what makes tirzepatide different from other weight loss injections like Wegovy, which only activates the GLP-1 pathway. The medication works by: signalling to your brain that you feel fuller, faster; reducing appetite and food cravings; slowing how quickly food leaves your stomach; and supporting your body's natural ability to use fat for energy when you're in a calorie deficit. It also helps regulate blood sugar levels. Importantly, Mounjaro doesn't chemically break down fat or increase metabolism on its own — it works alongside lifestyle changes (reduced calories and exercise) to help your body naturally utilise stored fat more effectively.
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.
Fat Loss and Weight Loss — What's the Difference?
When people ask if Mounjaro "breaks down fat," they're often asking whether it reduces body fat specifically — rather than just losing weight on the scales. Clinical trials with Mounjaro showed average weight loss of approximately 15–21% of body weight at 72 weeks, though individual results vary significantly. This weight loss typically includes both fat loss and water loss. For the most accurate picture of how your body composition is changing, your GP can arrange body composition analysis or waist circumference measurement alongside your weight. Using our free BMI calculator and progress tracker can help you monitor your weight over time, while a healthcare professional can assess whether your weight loss includes meaningful fat loss.
- 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.
Does Mounjaro burn fat faster than dieting alone?
Mounjaro isn't a fat-burning injection — it helps reduce appetite and supports calorie deficit creation. Clinical trials showed people on Mounjaro alongside diet and exercise lost more weight than those on diet and exercise alone. However, weight loss still requires a calorie deficit. Results vary between individuals, and your GP can discuss whether it's suitable for you.
Will Mounjaro help me lose belly fat specifically?
Mounjaro can support overall body weight loss, but you cannot target fat loss from specific areas — your body loses fat from where it naturally chooses as you lose weight overall. Some people do notice reduced abdominal fat as overall weight decreases. Waist circumference measurement (available through your GP) is a useful health marker alongside weight.
How long before I see fat loss on Mounjaro?
Most people begin to see weight changes within 4–8 weeks of starting Mounjaro, though this varies. Weight loss typically continues gradually with each dose increase (every 4 weeks). The SURMOUNT clinical trials measured results at 72 weeks. Your GP or prescriber can discuss what to expect based on your individual circumstances.
Is Mounjaro better than Wegovy for fat loss?
Mounjaro and Wegovy (semaglutide) are both prescription-only weight loss injections licensed in the UK. Mounjaro is a dual GIP/GLP-1 agonist; Wegovy is GLP-1 only. SURMOUNT trials showed Mounjaro achieved ~clinically studied weight loss at maximum dose versus ~15% for Wegovy in STEP trials. However, individual results vary, and your prescriber will assess which is more suitable for you.
Does Mounjaro speed up metabolism?
Mounjaro doesn't significantly increase your metabolic rate on its own. It works primarily by reducing appetite, helping you eat fewer calories, and supporting your body's natural fat utilisation in a calorie deficit. Weight loss still depends on creating a calorie deficit through diet and activity changes — Mounjaro makes this easier by reducing hunger signals.
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.