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 Monjaro Injection and How Does It Work?
Monjaro injection is a prescription-only weekly weight loss medication licensed in the UK for adults with obesity or overweight with related health conditions. It contains tirzepatide, a dual GIP and GLP-1 receptor agonist that works differently from other weight loss injections. Monjaro may help reduce appetite, support feeling fuller after meals, and assist with weight management when combined with a reduced-calorie diet and increased physical activity. Individual results vary, and a clinical assessment is required before prescription. Our GLP-1 weight loss calculator can help you explore potential outcomes at different doses.
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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 Monjaro Works and What Makes It Different
Monjaro's dual mechanism is what sets it apart. It activates two natural hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) — whereas other weight loss injections like Wegovy activate only GLP-1. This dual GIP receptor and GLP-1 receptor agonist action may help regulate appetite more effectively and support better satiety signals to the brain.
When injected once weekly, Monjaro slows how quickly food leaves your stomach, reduces hunger signals, and helps your body use fat more efficiently. It is not insulin and does not work in the same way as insulin medications. Clinical trials showed that participants using Monjaro alongside lifestyle changes lost an average of 15–21% of their body weight over 72 weeks — though results vary significantly between individuals. The injectable comes as a pre-filled KwikPen device delivered once per week, making it convenient for long-term use.
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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, Doses, and What to Expect
In the UK, you may be eligible for Monjaro if you have 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 heart disease. South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI thresholds due to higher health risk at lower weights. Check your eligibility using our free BMI calculator.
Monjaro starts at 2.5 mg weekly and increases gradually to a maximum of 15 mg. Each dose is held for at least four weeks before increasing. Access in the UK is through NHS specialist weight management services (with potentially long waiting lists) or private prescription following an online clinical assessment. Your prescriber will determine the right starting dose and escalation plan based on your individual health profile.
- 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 Monjaro available on the NHS in the UK?
Yes, NICE has approved tirzepatide (Monjaro) for NHS use within specialist tier 3 weight management services. However, access varies significantly by region and waiting lists can be very long. You can also access Monjaro privately through online prescribing following a clinical assessment without NHS referral.
What are the main side effects of Monjaro injection?
Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain — especially when starting or increasing doses. These often improve over time. Less common side effects include headache, fatigue, and hair loss. Serious side effects are rare but seek immediate medical attention for severe stomach pain or signs of pancreatitis.
How much weight can I expect to lose with Monjaro?
Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks, but individual results vary significantly. Weight loss depends on your starting weight, dose, adherence to the medication, diet, and physical activity. Our weight loss calculator can help estimate potential outcomes, though actual results are unique to each person.
Can I switch from Wegovy to Monjaro injection?
Some prescribers do switch patients between these medications, though there is no official established conversion chart — any switch must be managed by your prescriber. Monjaro's dual mechanism (GIP and GLP-1) differs from Wegovy's single GLP-1 action, so your prescriber may adjust your dose carefully when switching.
How do I inject Monjaro and where do I inject it?
Monjaro comes as a pre-filled KwikPen device injected under the skin once weekly. Common injection sites are the stomach, upper thigh, or upper arm (upper arm requires assistance). Rotate your injection site each week. Each pen contains four doses and can be stored at room temperature for up to 30 days after first use, then disposed of safely.
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.