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-only weekly weight loss injection now approved for use within NHS specialist weight management services in the UK. Unlike other weight loss injections, Mounjaro works through a unique dual mechanism that activates two natural hormone pathways in your body, helping to regulate appetite and support sustainable weight management when combined with lifestyle changes. This guide explains the science, how it's used on the NHS, and what the evidence shows about its effectiveness for UK adults.
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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 Mechanism Works
Mounjaro contains tirzepatide, which is a dual GIP and GLP-1 receptor agonist — meaning it activates two separate hormone pathways in your body. This dual action is what distinguishes Mounjaro from other weekly weight loss injections available in the UK.
- GLP-1 pathway: This natural hormone signals to your brain that you're full, reducing appetite and food cravings. It also slows how quickly food leaves your stomach, helping you feel satisfied after smaller meals.
- GIP pathway: Glucose-dependent insulinotropic polypeptide helps regulate how your body uses fat and manages blood sugar levels after eating.
Together, these mechanisms work to reduce hunger signals, support feeling fuller on less food, and help regulate energy use. Clinical trials, including the SURMOUNT-1 study, showed that this dual approach resulted in an average weight loss of approximately 15–21% of body weight over 72 weeks — though individual results vary significantly. You can use our free GLP-1 weight loss calculator to estimate potential results at different doses, though your actual outcome will depend on your personal circumstances, diet, and activity level.
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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.
NHS Access and Eligibility for Mounjaro in the UK
In the UK, Mounjaro is available through NHS specialist tier 3 weight management services — not as a standard GP prescription. NICE (National Institute for Health and Care Excellence) has approved tirzepatide for use within these services for adults meeting specific criteria.
You may be eligible if you have: a BMI of 30 or above, or a BMI of 27–29.9 with at least one weight-related health condition (such as type 2 diabetes, high blood pressure, or obstructive sleep apnoea). Lower thresholds apply if you're of South Asian, Chinese, Black African, or Black Caribbean heritage. However, NHS availability varies significantly by region due to waiting lists and funding. Many areas have waiting times of several months to over a year. Private prescription via a UK-licensed prescriber remains an alternative route with shorter timescales. Check your local integrated care board (ICB) website for NHS tier 3 service availability in your area. Our free BMI calculator can help you check your current BMI status.
- 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?
No. Both are prescription weight loss injections used weekly in the UK, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro typically shows slightly higher average weight loss in clinical trials, but individual results vary. Both require prescription and clinical assessment.
How long does Mounjaro take to work?
Most people notice appetite reduction within the first 1–2 weeks, though this varies. Measurable weight loss usually appears within 4 weeks. The first 4 weeks at starting dose (2.5 mg) are considered adjustment period. Maximum benefit typically appears after 12–16 weeks as your dose is increased. Consistency with diet and activity matters significantly.
Will I lose weight on the NHS Mounjaro programme?
Clinical evidence shows Mounjaro can help support weight loss, but results vary individually. NHS specialist services provide ongoing support with diet and activity advice alongside the medication. Weight loss depends on your adherence to a reduced-calorie diet, physical activity, and how your body responds to the medication. Your weight management team will monitor progress.
What's the difference between Mounjaro and other weight loss injections like Saxenda?
Mounjaro and Saxenda (liraglutide) are both GLP-1 medications but Saxenda is a daily injection while Mounjaro is once-weekly. Mounjaro's dual mechanism (GIP + GLP-1) may offer different results than Saxenda's single mechanism. Average weight loss in trials differs between the two. Your prescriber will discuss which is most suitable for you.
Can I switch from Wegovy to Mounjaro on the NHS?
This depends on your NHS tier 3 service and individual circumstances. There is no official established dose conversion between the two medications — any switch must be managed by your prescriber. Some NHS services may offer switching, while others may not. Discuss this directly with your weight management team if you're considering a change.
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.