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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Mounjaro 15 Mg Pen — The Maximum Dose Explained
The Mounjaro 15 mg pen is the maximum prescribed dose of tirzepatide, a dual GIP and GLP-1 receptor agonist licensed for weight management in UK adults. At this highest dose, clinical evidence shows participants lost an average of around 21% of their body weight in trials, though individual results vary significantly. This injectable medication is prescription-only and requires clinical assessment before being prescribed. The 15 mg dose is typically reached only after gradual escalation from lower doses over several months of treatment.
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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 the 15 Mg Dose Works and When It's Used
The 15 mg Mounjaro KwikPen is the maximum maintenance dose, reached after a careful titration schedule starting at 2.5 mg and increasing every 4 weeks. Not all patients require or reach this dose — many respond well at 5 mg or 10 mg. The tirzepatide in Mounjaro works by activating two natural hormone pathways (GIP and GLP-1), which helps regulate appetite, reduce cravings, and support feeling fuller after smaller meals. At 15 mg, the dual mechanism is fully activated. Your prescriber will assess whether dose escalation is appropriate for you based on your individual response and tolerability. Each KwikPen contains 4 doses (one month's supply) of the same strength.
- Prescribed only after clinical assessment and lower doses
- Takes approximately 5 months to reach from starting dose
- Works alongside a reduced-calorie diet and physical activity
- Individual response varies — not everyone needs or tolerates 15 mg
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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.
Side Effects and Eligibility at Maximum Dose
At the 15 mg dose, side effects follow the same pattern as lower doses but may be more pronounced in some individuals. Common side effects include nausea, diarrhoea, constipation, and stomach discomfort — these often improve over time as your body adjusts. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and gallbladder problems. You must inform your prescriber immediately of any severe symptoms. To be eligible for Mounjaro 15 mg, you need a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. Lower BMI thresholds apply if you're of South Asian, Chinese, Black African, or Black Caribbean heritage. Use our free BMI calculator to check your personal BMI instantly.
- 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 the 15 mg Mounjaro dose?
It typically takes approximately 5 months to reach 15 mg from the starting dose of 2.5 mg. You increase every 4 weeks: 2.5 mg (weeks 1–4), 5 mg (weeks 5–8), 7.5 mg (weeks 9–12), 10 mg (weeks 13–16), 12.5 mg (weeks 17–20), and finally 15 mg (week 21 onwards). Your prescriber may adjust this schedule based on your individual tolerance.
Is 15 mg Mounjaro better than lower doses?
Clinical trials show average weight loss of around 21% at 15 mg versus approximately 15% at 5 mg. However, many individuals achieve their weight loss goals at lower doses. Your prescriber will assess the right dose for you — higher dose doesn't automatically mean better results for everyone. Some people experience more side effects at higher doses.
Can I get Mounjaro 15 mg on the NHS in the UK?
Mounjaro is available on the NHS through specialist tier 3 weight management services, but access varies significantly by region and availability. You would need GP referral to your local specialist service. Waiting lists can be lengthy in many areas. Private access is available through UK-licensed prescribers following an online clinical assessment.
What happens if I miss a Mounjaro 15 mg injection?
If you miss a dose, inject as soon as you remember if it's within 4 days of the missed dose, then continue your normal schedule. If more than 4 days have passed, skip that dose and take your next injection on the normally scheduled day. Never take a double dose. Always maintain at least 72 hours between injections.
How does Mounjaro 15 mg compare to Wegovy?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both GLP-1-based weight management injections, but Mounjaro uses a dual GIP and GLP-1 mechanism versus Wegovy's single GLP-1 mechanism. Mounjaro's maximum dose is 15 mg, Wegovy's is 2.4 mg. Clinical data suggests similar average weight loss outcomes, though individual results vary. Your prescriber will advise which suits you best. Try our GLP-1 weight loss calculator to compare projected outcomes.
Ready to take control of your weight loss journey?
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