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
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What Does Mounjaro Do — How It Works in the UK
Mounjaro (tirzepatide) is a prescription-only weight management injection that works by activating two natural hormone pathways in your body to help reduce appetite and support weight loss. Licensed in the UK and approved by NICE for eligible adults, Mounjaro differs from other weight loss injections like Wegovy because it uses a dual mechanism targeting both GIP and GLP-1 receptors. Understanding how Mounjaro works and whether it's right for you requires knowledge of NHS eligibility criteria and what to expect from treatment.
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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: The Dual Mechanism Explained
Mounjaro is a dual GIP and GLP-1 receptor agonist, which means it activates two natural hormone pathways instead of one. 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 helps regulate appetite by signalling to your brain that you're fuller after eating less food.
- Reduces cravings and desire to eat between meals
- Slows gastric emptying (how quickly food leaves your stomach), promoting satiety
- Helps your body utilise fat more efficiently
- May improve blood sugar control if you have type 2 diabetes or prediabetes
In clinical trials (SURMOUNT-1), adults on the maximum 15mg dose lost an average of approximately 21% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. Results vary significantly between individuals, and Mounjaro is administered once weekly via a pre-filled KwikPen device injected into your stomach, thigh, or upper arm.
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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.
Mounjaro on the NHS: Eligibility and Access Pathways
The NHS has approved tirzepatide (Mounjaro) through NICE guidance for use within specialist tier 3 weight management services across England, Scotland, Wales, and Northern Ireland. You cannot obtain Mounjaro on the NHS through your GP directly — you must be referred to a specialist weight management service, typically after other interventions. Eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, obstructive sleep apnoea). South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI thresholds due to higher health risks at these weights.
NHS waiting lists for tier 3 services vary significantly by region. Many areas experience waiting times of several months to over a year. If NHS access isn't available quickly, private prescribing through online clinics offers faster access following a clinical assessment with a UK-licensed prescriber. Our GLP-1 weight loss calculator can help you estimate potential outcomes at different Mounjaro doses.
- 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 Wegovy?
Both are weight loss injections but work differently. Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, whilst Wegovy (semaglutide) activates GLP-1 only. Mounjaro may produce slightly higher average weight loss in trials, but individual results vary. Both require a prescription and clinical assessment.
How long does it take Mounjaro to work?
Most people begin noticing reduced appetite within 1–2 weeks of starting. Measurable weight loss typically becomes apparent after 4–8 weeks at therapeutic doses. Full effects develop gradually as your dose increases over 5–6 months. Consistent use alongside dietary changes is essential for results.
Will I need to take Mounjaro forever?
Mounjaro is typically used as part of a longer-term weight management plan. Some people continue indefinitely; others stop after reaching goals and maintain through lifestyle changes. Your prescriber will advise based on your individual health. Stopping suddenly may result in gradual weight regain.
Can I get Mounjaro through my GP?
Not directly. Mounjaro on the NHS requires referral to a specialist tier 3 weight management service. Your GP can refer you, but NHS availability varies by region and waiting lists can be long. Private prescribing offers faster access through online clinics with UK-licensed prescribers.
What's the difference between Mounjaro and Saxenda?
Mounjaro (tirzepatide) is a newer dual GIP/GLP-1 agonist given once weekly, whilst Saxenda (liraglutide) is a GLP-1-only injection given daily. Mounjaro typically achieves greater average weight loss and is more convenient due to weekly dosing. Your prescriber can advise which suits you best.
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.