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 in the UK
Mounjaro (tirzepatide) is a prescription-only, once-weekly injection for weight management in UK adults. If your GP or prescriber has issued you a prescription, you'll receive a pre-filled KwikPen device. This guide explains how to use Mounjaro safely, including injection technique, dose titration schedules, storage requirements, and what to expect. Always follow your prescriber's specific instructions alongside this information.
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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.
Mounjaro KwikPen — Injection Technique and Dose Schedule
Mounjaro comes as a pre-filled KwikPen auto-injection device containing four doses (one month's supply). Each dose is 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg — your prescriber determines your starting and escalation doses based on your individual health profile.
- Starting dose: typically 2.5 mg once weekly for weeks 1–4
- Escalation: increase by 2.5 mg every 4 weeks until reaching your maintenance dose or maximum 15 mg
- Injection day: choose the same day each week (e.g. every Monday)
- Timing: can be taken any time of day, with or without food
The dose titration process helps your body adjust and minimises nausea. Never skip doses or increase faster than prescribed. If you miss a dose, inject within 4 days of the missed date, then continue your normal weekly schedule.
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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.
Injection Technique, Storage, and NHS Access in the UK
How to inject: Wash hands, rotate injection sites weekly (stomach, thigh, or upper arm), and inject just beneath the skin at a 90-degree angle. Use a new needle each time. Store unopened KwikPens in a refrigerator at 2–8°C; do not freeze. After first use, pens can be kept at room temperature (below 30°C) for up to 30 days. In the UK, Mounjaro is available via NHS tier 3 specialist weight management services (typically requiring GP referral and significant waiting times) or through private online prescribing following a clinical assessment. Your weight loss injection UK prescription must come from a registered UK prescriber.
- 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.
Can I inject Mounjaro myself at home?
Yes. Mounjaro is designed for self-injection at home using the KwikPen device. Your prescriber or pharmacy should provide injection training. If you're unsure about technique, ask for a demonstration or watch the official instructional video provided by the manufacturer.
What's the difference between Mounjaro and Wegovy?
Both are prescription weight loss injections available in the UK, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro starts at 2.5 mg and escalates to 15 mg; Wegovy starts at 0.25 mg and goes to 2.4 mg. Clinical trials showed slightly higher average weight loss with Mounjaro, though individual results vary significantly.
How long does a Mounjaro prescription last on the NHS?
NHS access to Mounjaro is through specialist tier 3 weight management services, which vary by region. Waiting times can be several months to over a year depending on your local integrated care board (ICB). Private prescribing is faster but involves upfront costs. Your GP can advise on NHS availability in your area.
What should I do if I experience severe side effects?
Common side effects include nausea and diarrhoea, which often improve over time. Seek immediate medical attention if you experience severe, persistent stomach pain (possible pancreatitis), serious allergic reactions, or changes in vision. Contact your prescriber or NHS 111 for guidance on managing side effects.
Can I switch from Mounjaro to Wegovy if needed?
Only under prescriber guidance. Whilst both are GLP-1-related medications, they have different doses and mechanisms — your prescriber may recommend a specific switching protocol or may suggest continuing your current medication. Never switch without professional advice.
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.