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 Mounjaro Does: How This Weight Management Injection Works
Mounjaro is a prescription-only weight management injection that works differently from other weight loss medications available in the UK. It contains tirzepatide, which activates two natural hormone pathways in your body to help regulate appetite and support weight loss alongside diet and exercise. Understanding how Mounjaro works—and what it actually does—is essential before considering whether it might be right for you. This guide explains the mechanism, expected effects, and UK eligibility criteria in straightforward terms.
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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's Dual Hormone Mechanism Works
Mounjaro is a dual GIP and GLP-1 receptor agonist, which means it activates two separate hormone pathways your body uses naturally to manage appetite and blood sugar. This dual action is what sets it apart from other weight loss injections like Wegovy (semaglutide), which works on GLP-1 alone.
- GLP-1 pathway: Signals to your brain that you feel full, reduces food cravings, and slows how quickly food moves through your stomach
- GIP pathway: Works alongside GLP-1 to enhance satiety signals and may help your body use stored fat more effectively
Together, these hormones may help you eat smaller portions, experience fewer cravings, and maintain a reduced-calorie diet more easily. Clinical trials showed that adults using Mounjaro lost an average of 15–21% of their body weight over 72 weeks—though individual results vary significantly and depend on dose, adherence, diet, and physical activity.
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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
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What Mounjaro Actually Does (Expected Effects)
In practice, people using Mounjaro typically report feeling fuller after smaller meals, reduced hunger between meals, and fewer cravings for high-calorie foods. These effects usually develop gradually as your dose increases over the first 4–5 months of treatment.
Mounjaro is delivered as a once-weekly injection via a KwikPen device, with doses ranging from 2.5 mg to a maximum of 15 mg weekly. Most people reach an effective maintenance dose between 5 mg and 15 mg, depending on their individual response and tolerance. Side effects—mainly nausea, constipation, and diarrhoea—often improve after the first few weeks. Use our free GLP-1 weight loss calculator above to estimate potential weight loss at different doses, or try our BMI calculator to see where you currently sit on the NHS healthy weight scale.
- 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 the same active ingredient (semaglutide for Ozempic, tirzepatide for Mounjaro), but Mounjaro uses a dual GIP+GLP-1 mechanism while Ozempic is a GLP-1 only. More importantly, Mounjaro is licensed for weight management in the UK, whilst Ozempic is licensed for type 2 diabetes—using it for weight loss is off-label.
How quickly does Mounjaro start working?
You may notice reduced appetite and cravings within 1–2 weeks of your first injection, though effects gradually build as your dose increases over 16–20 weeks. Weight loss typically becomes noticeable after 4–8 weeks, but individual timelines vary based on diet, exercise, and starting weight.
Do I need a prescription to get Mounjaro in the UK?
Yes, Mounjaro is prescription-only. You can access it through NHS specialist weight management services (via GP referral) or privately through a UK-licensed prescriber following an online clinical assessment. A healthcare professional must assess your eligibility based on BMI and health conditions.
What's the difference between Mounjaro and Wegovy for weight loss?
Mounjaro uses dual GIP+GLP-1 activation (tirzepatide), whilst Wegovy uses GLP-1 alone (semaglutide). Clinical trials suggest Mounjaro may produce slightly greater average weight loss (15–21% vs ~15%), but both are effective—results vary by individual.
Will I keep the weight off after stopping Mounjaro?
Weight regain is common after stopping any weight loss medication if diet and exercise habits don't change. Mounjaro is most effective as part of a long-term lifestyle approach including healthy eating and physical activity. Your GP can discuss long-term management strategies with 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.