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? How It Works
Mounjaro (tirzepatide) doesn't directly "break down" fat in the way you might imagine, but it does support weight loss through multiple biological pathways. It's a prescription-only dual GIP and GLP-1 receptor agonist that works by reducing appetite, slowing digestion, and helping your body use energy more efficiently. Understanding how Mounjaro actually works helps set realistic expectations about this weight management injection and whether it might be right for you.
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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 Actually Works — The Science Behind the Injection
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, which means it activates two natural hormone pathways in your body. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are hormones your gut naturally produces after eating. By mimicking and amplifying these signals, Mounjaro creates several effects that support weight loss:
- Reduces appetite: Signals to your brain that you feel fuller, reducing cravings and desire to eat
- Slows gastric emptying: Food moves through your stomach more slowly, helping you feel satisfied longer
- Improves fat utilisation: Helps your body use stored energy more efficiently
- Stabilises blood sugar: May reduce energy crashes that trigger overeating
Rather than chemically breaking down fat deposits, Mounjaro works by creating a calorie deficit through reduced food intake. Clinical trials showed that the KwikPen formulation helped participants lose an average of 15–21% of their body weight over 72 weeks, though individual results vary significantly. Weight loss happens because you consume fewer calories, not because the medication directly dissolves fat tissue.
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.
UK Cost and Access: NHS vs Private Prescriptions
Accessing Mounjaro in the UK depends on your route: NHS access is through specialist tier 3 weight management services (requiring GP referral), though availability varies significantly by region and waiting lists can be lengthy. Private prescription routes are available through UK-licensed online prescribers following a clinical assessment—costs range from approximately £129–£280 per month depending on your dose level. A 2.5 mg starting KwikPen costs less than a 15 mg maintenance dose. Your prescriber will assess your eligibility based on BMI (30+, or 27+ with health conditions) and medical history. Use our free BMI calculator to check where you stand, and our GLP-1 weight loss calculator to estimate potential results at different dose levels.
- 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 actually break down existing fat cells?
No—Mounjaro doesn't directly dissolve or chemically break down fat tissue. Instead, it reduces your appetite and helps you eat fewer calories, which creates the deficit needed for weight loss. Your body then uses stored fat for energy. It's appetite suppression, not fat dissolution.
How long does Mounjaro take to work?
Most people notice reduced appetite within the first week of starting Mounjaro. However, measurable weight loss typically becomes apparent after 2–4 weeks. Full effects are seen over several months as you progress through dose titration (increasing from 2.5 mg to your target dose every 4 weeks).
Is Mounjaro available on the NHS in the UK?
Yes, NICE has approved tirzepatide (Mounjaro) for NHS use through specialist weight management services. However, NHS access requires referral to tier 3 services and availability varies by region. Many people access it privately through online prescribers while waiting for NHS services.
What's the difference between Mounjaro and Wegovy?
Both are prescription-only injections, but Mounjaro is a dual GIP+GLP-1 agonist (maximum 15 mg weekly), whilst Wegovy is GLP-1 only (maximum 2.4 mg weekly). Clinical trials suggest Mounjaro may produce slightly higher average weight loss, though individual results vary. Both work through appetite suppression, not direct fat breakdown.
Can I get a private Mounjaro prescription after an online consultation?
Yes. UK-licensed prescribers can assess your eligibility online and issue a prescription if appropriate. You'll need to provide medical history and confirm you meet eligibility criteria (BMI 30+, or 27+ with weight-related health conditions). The prescription is then dispensed by a GPhC-registered pharmacy.
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.