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
Free tracker included with every download this week
What Is Mounjaro 15mg?
Mounjaro 15mg is the maximum dose of tirzepatide available in the UK for weight management. This once-weekly injection is the highest strength in the dose escalation programme, typically reached after several months of treatment. At 15mg, Mounjaro works as a dual GIP and GLP-1 receptor agonist, helping to regulate appetite and support sustainable weight loss alongside diet and exercise. It is prescription-only and requires clinical assessment before starting.
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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 15mg Dosing Works
Mounjaro 15mg is reached gradually, not started immediately. The standard escalation pathway takes approximately 20–24 weeks:
- Weeks 1–4: 2.5mg (starting dose)
- Weeks 5–8: 5mg (first maintenance)
- Weeks 9–12: 7.5mg
- Weeks 13–16: 10mg
- Weeks 17–20: 12.5mg
- Week 21+: 15mg (maximum)
Each dose is a once-weekly injection delivered via the Mounjaro KwikPen device. Your prescriber will assess your response at each stage before increasing. You must stay on each dose for at least 4 weeks before escalating. Some patients respond well at lower doses and may not need to reach 15mg. The 15mg dose is only continued if your prescriber considers it appropriate for your individual health and response.
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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.
Weight Loss Results at 15mg
Based on clinical trial data, patients on Mounjaro 15mg achieved an average weight loss of approximately 20.9% of body weight over 72 weeks. However, individual results vary significantly — some people lose more, some less. Results depend on adherence, diet, physical activity, metabolism, and other personal factors.
For perspective, if you weigh 100kg, a 20.9% loss would be approximately 20.9kg (3 stone 5lbs). Always use our free weight converter and BMI calculator to understand your personal baseline and track realistic progress. Combined with a reduced-calorie diet and increased physical activity, Mounjaro 15mg may help achieve meaningful weight loss — though it is not a assures, and results are individual.
- 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 long does it take to reach Mounjaro 15mg?
Typically 20–24 weeks. You start at 2.5mg and increase by 2.5mg every 4 weeks, provided your response is good and side effects are manageable. Some patients reach 15mg faster or slower depending on tolerance. Your prescriber will guide your individual escalation.
Is 15mg the right dose for everyone?
No. Not everyone needs or should take 15mg. Your prescriber will assess your response at each dose level. Some patients achieve good results at 10mg or 12.5mg and remain on those doses. The 15mg dose is only used if it is appropriate for your individual circumstances.
What are the main side effects at 15mg?
Common side effects include nausea, diarrhoea, constipation, vomiting, and stomach pain. These are usually most severe when first starting or increasing doses, and typically improve over time. Serious side effects are rare but can include pancreatitis or gallbladder issues. Report persistent or severe symptoms to your prescriber immediately.
Can I get Mounjaro 15mg on the NHS in the UK?
Mounjaro is approved for NHS use through specialist tier 3 weight management services. However, availability varies significantly by region and waiting lists can be lengthy. Private prescription access is also available through UK-licensed prescribers following an online clinical assessment.
How does Mounjaro 15mg compare to Wegovy?
Both are once-weekly injections for weight management, but Mounjaro uses a dual GIP+GLP-1 mechanism (maximum 15mg), whilst Wegovy (semaglutide) uses GLP-1 alone (maximum 2.4mg). Clinical data suggests Mounjaro may produce slightly higher average weight loss, but results are individual. Your prescriber will recommend the most suitable option for 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.