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 Are Mounjaro Slimming Jabs?
Mounjaro is a prescription-only injection medication licensed in the UK for weight management in adults. The medicine contains tirzepatide, which works as a dual GIP and GLP-1 receptor agonist—meaning it activates two natural hormone pathways to help reduce appetite and support feeling fuller after meals. Mounjaro is administered as a once-weekly injection using a KwikPen device and is available through NHS specialist weight management services or private prescription following a clinical assessment.
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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 Works and What to Expect
Mounjaro works differently from other weight loss medications because it targets two hormone systems simultaneously. Unlike Wegovy, which activates only the GLP-1 pathway, Mounjaro activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) receptors. This dual action may help regulate appetite more effectively, reduce food cravings, and support your body's ability to feel satisfied with smaller portions. The medication also slows how quickly food leaves your stomach, though this effect typically reduces over time with continued use.
In clinical trials (the SURMOUNT-1 study), participants taking Mounjaro alongside a reduced-calorie diet and increased physical activity experienced average weight loss of approximately 15–21% of their body weight, depending on the dose used. Individual results vary significantly, and weight loss is not assured. Mounjaro is injected once weekly into the stomach, thigh, or upper arm, with doses ranging from 2.5 mg (starting dose) up to 15 mg (maximum dose).
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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.
UK Eligibility, Access, and Mounjaro vs Wegovy
In the UK, Mounjaro may be prescribed to adults with a BMI of 30 kg/m² or above, or BMI of 27–29.9 kg/m² with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. People of South Asian, Chinese, Black African, or Black Caribbean heritage may be eligible at lower BMI thresholds due to higher health risks at lower weights. Access is available through NHS tier 3 specialist weight management services (with potential waiting lists) or privately through GPhC-registered pharmacies following an online clinical assessment.
Mounjaro vs Wegovy: Both are once-weekly injections, but Mounjaro uses dual GIP+GLP-1 mechanism versus Wegovy's single GLP-1 mechanism. Mounjaro doses range to 15 mg, while Wegovy's maximum is 2.4 mg. Try our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and use our BMI calculator to check your current eligibility.
Mounjaro is not suitable for people under 18, those who are pregnant or breastfeeding, or anyone with a known allergy to tirzepatide. Common side effects include nausea, diarrhoea, constipation, and stomach pain—these typically improve as your body adjusts. Serious side effects are rare but can include severe stomach pain (possible pancreatitis) or gallbladder problems; seek immediate medical attention if these occur. A UK GP or licensed prescriber will assess whether Mounjaro is appropriate for your individual health during a clinical assessment. Always inform your healthcare provider of any other medications you are taking, as Mounjaro may affect how some oral medications are absorbed.
- 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 Mounjaro the same as Ozempic?
No. Both contain semaglutide, but Ozempic is approved for type 2 diabetes (not weight loss in the UK), and Wegovy is the licensed weight management version. Mounjaro contains tirzepatide, a different medication with a dual GIP+GLP-1 mechanism, making it distinct from both Wegovy and Ozempic.
How quickly will I lose weight on Mounjaro?
Weight loss varies between individuals. Clinical trials showed average losses of 15–21% of body weight over several months when combined with diet and exercise. Doses start at 2.5 mg and increase gradually (typically every 4 weeks) to allow your body to adjust. Your GP or prescriber can discuss realistic timelines for your situation.
Can I get Mounjaro through the NHS in the UK?
Yes, Mounjaro is approved by NICE for NHS use through specialist tier 3 weight management services. However, availability and waiting times vary significantly by region. Your GP can refer you to your local NHS weight management service, or you can access it privately through GPhC-registered online pharmacies.
What are the main side effects of Mounjaro?
Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain. These typically improve within a few weeks as your body adjusts. Less common side effects include headache, fatigue, and injection site reactions. Serious side effects are rare but can include severe stomach pain or gallbladder issues—seek immediate medical attention if these occur.
How does Mounjaro compare to Wegovy for weight loss?
Both are once-weekly injections for weight management. Mounjaro uses a dual GIP+GLP-1 mechanism, while Wegovy uses only GLP-1. Mounjaro's maximum dose is 15 mg versus Wegovy's 2.4 mg. Clinical trials suggest Mounjaro may achieve slightly higher average weight loss, but results vary individually. Your prescriber can help determine which is more suitable for you.
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.