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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How Does Mounjaro Work for Weight Loss?
Mounjaro (tirzepatide) is a prescription weight loss injection that works differently from other weekly weight loss injections available in the UK. Unlike single-action medications, Mounjaro activates two natural hormone pathways simultaneously—GIP and GLP-1—to help regulate appetite, reduce food cravings, and support sustainable weight loss alongside lifestyle changes. Understanding how Mounjaro works can help you discuss whether this medication might be suitable for you with your GP or prescriber.
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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.
Mounjaro's Dual Mechanism: How GIP and GLP-1 Work Together
Mounjaro is a dual GIP and GLP-1 receptor agonist—this means it activates two separate hormone pathways in your body, setting it apart from single-action weight loss medications like Wegovy (semaglutide), which only activates GLP-1.
- GLP-1 pathway: Signals to your brain that you're full after eating, reduces appetite and food cravings, and slows gastric emptying (how quickly food leaves your stomach)
- GIP pathway: Helps regulate blood sugar response and supports how your body uses fat for energy
In the SURMOUNT-1 clinical trial, participants taking the maximum 15mg dose of Mounjaro lost an average of 20.9% of their body weight over 72 weeks—though individual results vary significantly. This dual action is why Mounjaro may produce different weight loss outcomes compared to single GLP-1 injections, though both require a reduced-calorie diet and increased physical activity to work effectively.
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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.
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Mounjaro vs Wegovy: Key Differences in How They Work
While both Mounjaro and Wegovy are prescription-only weekly injections licensed in the UK, they work via different mechanisms. Wegovy (semaglutide) is a GLP-1-only agonist, activating just the GLP-1 hormone pathway. In clinical trials (STEP-1), Wegovy resulted in an average weight loss of approximately 14.9% body weight at the maximum 2.4mg dose over 68 weeks.
The key difference: Mounjaro's dual GIP+GLP-1 action may produce different satiety and metabolic effects compared to GLP-1 alone. However, suitability depends on individual health factors, medical history, and prescriber assessment—not on trial averages. Both medications require a prescription and clinical assessment before dispensing.
Both Mounjaro and Wegovy work by mimicking natural hormones your gut produces after eating, but Mounjaro's dual mechanism may offer a different approach. Neither medication assures specific weight loss outcomes—results depend on your individual metabolism, adherence to dose titration schedules (Mounjaro starts at 2.5mg and increases weekly; Wegovy starts at 0.25mg), lifestyle changes, and how your body responds. Your prescriber will assess which medication, if either, is clinically appropriate based on your BMI, health conditions, and medical history.
- 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.
How quickly does Mounjaro work for weight loss?
Mounjaro works gradually. Most people begin noticing appetite changes within the first 2–4 weeks, but significant weight loss typically takes 8–12 weeks. The medication requires dose titration over approximately 20 weeks to reach maximum effectiveness. Weight loss accelerates as doses increase, with most losses occurring between weeks 8–72 in clinical trials.
Does Mounjaro work better than Wegovy?
Clinical trials show both medications produce clinically meaningful weight loss, with Mounjaro averaging 20.9% and Wegovy averaging 14.9% in their respective trials. However, "better" depends on individual factors—genetics, metabolism, medical history, and how your body responds. Your prescriber will recommend the most suitable option for your specific health profile.
Why does Mounjaro have a dual mechanism?
Mounjaro's dual GIP+GLP-1 action targets multiple appetite and metabolic pathways simultaneously. This may help regulate hunger signals more comprehensively than single GLP-1 activation alone. The dual mechanism also supports blood sugar stability and fat utilisation, though individual responses vary significantly.
How long does it take to see results with Mounjaro?
Initial appetite suppression typically appears within weeks 1–4. Measurable weight loss usually becomes apparent by weeks 8–12. Maximum weight loss benefits typically occur around week 72 of consistent use at therapeutic doses. Results depend on adherence to the medication schedule, calorie deficit, and physical activity.
Do I need to diet while taking Mounjaro?
Yes. Mounjaro is not a standalone weight loss solution. Clinical trials showed results only when combined with a reduced-calorie diet and increased physical activity. Mounjaro helps support adherence to these lifestyle changes by reducing appetite—it doesn't replace them. Your prescriber will discuss realistic dietary expectations.
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.