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 to Use Mounjaro Injection — Complete UK Guide
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection now available in the UK through NHS specialist services and private prescribing. If you've been prescribed Mounjaro, understanding how to use it correctly — from storage to injection technique — is essential for safety and effectiveness. This guide explains the practical steps, how it works compared to other weight loss injections like Wegovy, and what to expect during your treatment. Use our free GLP-1 weight loss calculator to explore projected outcomes at different doses alongside your clinical assessment.
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Calculate Your BMI Instantly
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.
Mounjaro vs Wegovy: Mechanism and Trial Data
Mounjaro and Wegovy are both prescription-only weekly weight loss injections, but they work differently. Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two natural hormone pathways to regulate appetite and blood sugar. Wegovy is a GLP-1 receptor agonist only, working through a single mechanism.
Clinical trial results (SURMOUNT vs STEP trials): Mounjaro participants lost an average of 20.9% body weight at the 15mg dose over 72 weeks. Wegovy participants lost an average of 14.9% body weight at the 2.4mg maximum dose over 68 weeks. Both trials included participants following a reduced-calorie diet and increased physical activity. Individual results vary significantly.
- Mounjaro: doses from 2.5mg to 15mg, escalated over weeks
- Wegovy: doses from 0.25mg to 2.4mg, escalated over weeks
- Both require a clinical assessment before prescription
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Using Mounjaro: Practical Steps and Suitability
Mounjaro comes as a pre-filled KwikPen device containing four weekly doses. Store unopened pens in a refrigerator (2–8°C). After first use, you can keep the pen at room temperature (below 30°C) for up to 30 days. Inject once weekly into your stomach, thigh, or upper arm, rotating injection sites each week.
Who can use Mounjaro: Adults with BMI 30 or above, or BMI 27–29.9 with weight-related health conditions (diabetes, high blood pressure, cardiovascular disease). Eligibility is lower for adults of South Asian, Chinese, Black African, or Black Caribbean heritage. Your GP or prescriber will assess whether Mounjaro or Wegovy is more suitable based on your individual health, medical history, and preferences. Use our free BMI calculator to check your starting point, then discuss prescription options with your healthcare provider.
- 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.
What is the difference between Mounjaro and Wegovy injections?
Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 receptor agonist only. Clinical trials showed Mounjaro resulted in an average of clinically studied weight loss versus Wegovy at 14.9%. Both require a prescription and clinical assessment. Individual results vary based on dose, lifestyle changes, and personal factors.
How do I store and use a Mounjaro KwikPen?
Store unopened Mounjaro pens in a refrigerator at 2–8°C. After first use, keep the pen below 30°C for up to 30 days. Inject once weekly into your stomach, upper thigh, or upper arm, rotating sites each week. Always follow your prescriber's instructions for your specific dose and timing.
Which weight loss injection should I choose — Mounjaro or Wegovy?
Your prescriber will recommend the most suitable option based on your health, BMI, medical history, and any weight-related conditions. Both are prescription-only and require clinical assessment. Wegovy may be recommended for some patients; Mounjaro for others. Discuss your personal circumstances with your GP or prescriber.
What side effects should I expect from Mounjaro?
Very common side effects include nausea, diarrhoea, constipation, and stomach pain — often reducing over time. Common side effects include injection site reactions, fatigue, and headache. Serious side effects are rare but include severe stomach pain (possible pancreatitis) or gallbladder problems. Report any concerns to your prescriber immediately.
Is Mounjaro available on the NHS in the UK?
Yes, Mounjaro (tirzepatide) is NICE-approved for NHS use through specialist tier 3/4 weight management services. Availability and waiting times vary by region. Private prescribing is also available through registered online services. Both routes require a clinical assessment before prescription.
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.