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 15mg Pen — UK Dose Information
The Mounjaro 15mg pen is the maximum dose of tirzepatide available in the UK for weight management. It represents the highest strength in the Mounjaro dose range and is prescribed only after gradual dose escalation over several weeks. This guide explains what the 15mg dose means, how it's used, and what to expect if you're prescribed this strength. Always consult your GP or a qualified healthcare professional before starting any prescription medication.
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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.
Understanding the Mounjaro 15mg Maximum Dose
Mounjaro 15mg is the maximum dose strength available in the UK. It is a once-weekly injection of tirzepatide, a dual GIP and GLP-1 receptor agonist that works by activating two natural hormone pathways to help regulate appetite and support weight management. Patients do not start at 15mg — the dose is increased gradually every 4 weeks, beginning at 2.5mg and escalating through 5mg, 7.5mg, 10mg, and 12.5mg before reaching 15mg. Each dose increase allows your body to adjust and helps your prescriber assess tolerability. Clinical trials showed that participants on the 15mg dose achieved an average weight loss of approximately 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. Individual results vary significantly based on personal factors including metabolism, adherence to dietary changes, and physical activity level. The 15mg pen contains 4 weekly doses (a 4-week supply) and must be stored in a refrigerator at 2–8°C before use.
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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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Who Can Access Mounjaro 15mg in the UK
Mounjaro 15mg is a prescription-only medicine in the UK and requires a clinical assessment before being prescribed. You may be eligible if you have a BMI of 30 kg/m² or above, or a BMI of 27–29.9 kg/m² with weight-related health conditions such as type 2 diabetes, prediabetes, high blood pressure, or cardiovascular disease. If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds may apply. You cannot take Mounjaro if you're under 18, pregnant, breastfeeding, or have a history of serious allergic reactions. Access in the UK is through NHS specialist tier 3/4 weight management services or through private online prescribing following an assessment with a UK-licensed prescriber. Waiting times vary significantly by region.
- 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 15mg Mounjaro dose?
It takes approximately 20–24 weeks (5–6 months) to reach the maximum 15mg dose. You start at 2.5mg and increase by 2.5–5mg every 4 weeks, depending on tolerability. Your prescriber may slow or pause escalation if you experience side effects.
What are the side effects at the 15mg Mounjaro dose?
At 15mg, side effects are typically the same as lower doses but may be more pronounced. Most common are nausea, diarrhoea, vomiting, and constipation. These usually improve over time. Serious side effects are rare but include severe stomach pain (pancreatitis), gallbladder problems, and vision changes.
How much weight can I expect to lose on Mounjaro 15mg?
Clinical trials showed an average weight loss of about 20.9% of body weight — but individual results vary widely. Some people lose more, some less. Weight loss depends on diet, physical activity, metabolism, and how your body responds to the medication. Your prescriber can discuss realistic expectations.
Can I switch from Wegovy to Mounjaro 15mg?
Switching between medications requires prescriber guidance. Wegovy (semaglutide) and Mounjaro (tirzepatide) are different medications — Mounjaro has a dual mechanism (GIP + GLP-1) versus Wegovy's single GLP-1 action. Any switch must be managed by your prescriber to ensure safe transition.
Is the Mounjaro 15mg pen available on the NHS?
Mounjaro is available through NHS specialist weight management services (tier 3/4), but availability and waiting times vary significantly by region. Private access is available through UK-licensed online prescribers. Your GP can refer you to NHS services, or you can access private prescription routes independently.
Ready to take control of your weight loss journey?
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