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?
Mounjaro (tirzepatide) is a prescription-only weight loss injection that works differently from other slimming injections available in the UK. Instead of targeting a single hormone pathway, Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone systems in your body simultaneously. This dual mechanism may help regulate appetite, reduce food cravings, and support feeling fuller after smaller meals. Understanding how Mounjaro works can help you decide whether this medication might be appropriate for your weight management goals alongside a reduced-calorie diet and increased physical activity.
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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.
The Dual Mechanism: GIP and GLP-1
Mounjaro's unique dual action sets it apart from other weight loss medications like Wegovy (semaglutide), which targets only the GLP-1 pathway. GLP-1 (glucagon-like peptide-1) is a natural hormone produced in your gut after eating. It signals to your brain that you are full, slows how quickly food leaves your stomach, and helps regulate blood sugar levels. GIP (glucose-dependent insulinotropic polypeptide) is a second hormone that also plays a role in appetite control and fat storage.
By activating both pathways, Mounjaro may provide a more comprehensive effect on appetite suppression and energy regulation. In clinical trials (the SURMOUNT-1 study), Mounjaro helped participants lose an average of 15–21% of their body weight over 72 weeks, though individual results vary significantly. These results were achieved alongside a reduced-calorie diet and increased physical activity — the medication alone is not effective without lifestyle changes.
- GLP-1 reduces hunger signals to the brain
- GIP may support better control of fat metabolism
- Together, they may enhance appetite suppression compared to single-pathway medications
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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.
How Mounjaro Is Used and What to Expect
Mounjaro is administered as a once-weekly injection using a pre-filled KwikPen device. You inject under the skin (subcutaneously) into your stomach, thigh, or upper arm. Treatment begins with a starting dose of 2.5 mg, increasing gradually every 4 weeks up to a maximum of 15 mg weekly. Your prescriber will adjust your dose based on your response and tolerance.
Common side effects during the first few weeks include nausea, diarrhoea, and reduced appetite — these often improve as your body adjusts. Mounjaro is prescription-only and requires a clinical assessment before being prescribed. In the UK, you can access Mounjaro through NHS specialist weight management services (tier 3) or via private online prescribing following a clinical consultation. Try our free GLP-1 weight loss calculator to see projected weight loss 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?
No. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Both are licensed for weight management in the UK, but Mounjaro's dual mechanism may produce different results. Clinical trials show similar average weight loss (15–21% with Mounjaro vs ~15% with Wegovy), but individual responses vary. Your prescriber will assess which is more appropriate for you.
How quickly does Mounjaro start working?
Many people notice reduced appetite within 1–2 weeks of starting Mounjaro, though weight loss typically becomes measurable after 4–8 weeks. Full effects develop gradually as your dose increases over several months. Remember, Mounjaro works alongside lifestyle changes — diet and exercise remain essential.
Will I need to stay on Mounjaro permanently?
This is a question for your prescriber. Some people continue long-term; others may stop after reaching their weight goal. Stopping Mounjaro may result in some weight regain, which is why ongoing lifestyle changes are important. Your GP or weight management specialist can advise on the best approach for you.
Can I get Mounjaro through the NHS?
Yes, but access is limited. NICE has approved tirzepatide for NHS specialist tier 3 weight management services. However, availability varies significantly by region and waiting lists can be long. Your GP can refer you to your local specialist service, or you can access Mounjaro privately through licensed UK prescribers.
What happens if I miss a Mounjaro injection?
If you miss a dose and remember within 4 days, take it as soon as you remember and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and inject on your usual day. Never take a double dose. Always speak to your prescriber if you're unsure about missed doses.
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.