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 Is the Mounjaro Fat Jab?
The term "fat jab" refers to Mounjaro (tirzepatide), a prescription-only weight management injection licensed in the UK. It's not actually a cosmetic injection — it's a once-weekly medicine that works by activating two natural hormone pathways to help regulate appetite and support weight loss. In clinical trials, Mounjaro helped people lose significant amounts of weight when combined with diet and exercise. Here's what you need to know about how it works, who can get it, and what to expect.
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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 the Mounjaro Fat Jab Works
Mounjaro is a dual GIP and GLP-1 receptor agonist — this means it activates two different hormone pathways in your body that influence hunger and fullness signals. Unlike Wegovy (semaglutide), which works through GLP-1 alone, the dual mechanism of Mounjaro may help it achieve slightly stronger results in some people.
The injection works by:
- Reducing appetite and food cravings
- Helping you feel fuller after smaller meals
- Slowing how quickly food leaves your stomach
- Helping your body regulate blood sugar and fat use
Each injection is given once per week, using a pre-filled KwikPen device. You inject under the skin (subcutaneously) into your stomach, thigh, or upper arm. Most people start at 2.5 mg and gradually increase over weeks — this is called dose titration. The maximum dose is 15 mg once weekly.
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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 Eligibility and How to Access It
To be eligible for Mounjaro in the UK, you typically need a BMI of 30 or above, or a BMI of 27–29.9 if you have a weight-related health condition (such as type 2 diabetes, high blood pressure, or sleep apnoea). Adults of South Asian, Chinese, Black African, or Black Caribbean heritage may qualify at slightly lower BMI thresholds due to higher health risk at these weights.
Access routes in the UK include:
- NHS: Through specialist tier 3 weight management services — requires GP referral, but availability varies widely by region
- Private: Through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber
You'll need a consultation to confirm Mounjaro is suitable for you before being prescribed.
- 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.
Is the "fat jab" the same as Wegovy?
No. Both are prescription weight loss injections, but Mounjaro (tirzepatide) and Wegovy (semaglutide) work differently. Mounjaro is a dual GIP+GLP-1 agonist; Wegovy is GLP-1 only. Clinical trials showed Mounjaro may achieve slightly higher average weight loss (~15–21% of body weight vs ~15% for Wegovy), but individual results vary significantly. Both are once-weekly injections licensed in the UK.
How much weight can I expect to lose with Mounjaro?
In the SURMOUNT-1 clinical trial, people taking the highest dose (15 mg) lost an average of approximately 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. However, results vary widely between individuals. Some people lose more, some less. Weight loss is not assured, and ongoing lifestyle changes are essential for results.
What are the most common side effects of Mounjaro?
The most common side effects include nausea, diarrhoea, constipation, vomiting, and stomach pain — especially when starting or increasing doses. These often improve over time. Less common side effects include dizziness, fatigue, headache, and injection site reactions. If you experience severe stomach pain, contact your healthcare provider immediately, as it could indicate pancreatitis.
Can I get Mounjaro on the NHS?
Yes, Mounjaro is approved for use on the NHS through specialist weight management services (tier 3). However, availability varies significantly by region, and waiting lists can be lengthy. You'll need a GP referral. Private access is also available through licensed online prescribers without an NHS referral, though this involves a cost.
What's the difference between Mounjaro and Ozempic?
Both contain similar active ingredients but are different medicines. Mounjaro (tirzepatide) is licensed specifically for weight management. Ozempic contains semaglutide and is licensed for type 2 diabetes — using it for weight loss is off-label in the UK. Wegovy (also semaglutide) is the licensed weight management version. Always use the medicine prescribed for you by your healthcare provider.
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.