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 — Complete Injection Guide
Mounjaro (tirzepatide) is a prescription-only, once-weekly injection used for weight management in UK adults. If your GP or prescriber has prescribed Mounjaro, it's important to understand exactly how to use it safely and effectively. This guide covers everything you need to know about administration, dosing, storage, and what to expect when you start treatment. Always follow your prescriber's instructions and read the patient information leaflet that comes with your 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 Mounjaro Doses and How to Inject
Mounjaro comes as a pre-filled KwikPen device that you inject once per week, always on the same day. Treatment starts with a 2.5 mg starter dose for the first four weeks, then increases gradually every four weeks — typically progressing to 5 mg, then 7.5 mg, 10 mg, 12.5 mg, and finally the maximum 15 mg dose.
Your prescriber will determine your dose schedule based on your individual response and tolerance. Each KwikPen contains four doses (four weeks' supply). To inject, choose a clean injection site on your stomach (abdomen), upper thigh, or upper arm, clean the skin with an alcohol wipe, pinch the skin, insert the needle at a 90-degree angle, and hold for three seconds before withdrawing. Rotate injection sites weekly to prevent irritation and tissue damage. Never inject into areas with bruising, hardness, lumps, or dimpling.
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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.
Storage, Side Effects, and When to Seek Help
Storage is critical: unopened Mounjaro pens must be refrigerated at 2–8°C (keep in original carton away from light). Once you've used a pen for the first time, you can store it at room temperature (below 30°C) for up to 30 days before discarding.
Common side effects during the first few weeks include nausea, diarrhoea, and stomach pain — these often improve as your body adjusts. If you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions, or gallbladder symptoms, seek urgent medical attention. If you miss a dose within four days, inject as soon as you remember and continue your normal schedule. If more than four days have passed, skip that dose and resume on your regular day.
Our GLP-1 weight loss calculator can help you understand potential weight loss outcomes based on clinical trial data, whilst our BMI calculator lets you track how your weight changes affect your health metrics over time. Use our free progress tracker to log your weight loss journey week by week and monitor your results alongside Mounjaro treatment.
- 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 do I know if I'm injecting Mounjaro correctly?
The injection should be quick and relatively painless if done correctly. Pinch the skin, insert at 90 degrees, hold for three seconds, then withdraw smoothly. If you feel sharp pain, you may have hit a nerve — choose a different site next time. Always watch the demonstration video provided with your KwikPen and ask your prescriber or pharmacist to show you the correct technique before your first injection.
What if I accidentally inject Mounjaro twice in one week?
Contact your prescriber or pharmacist immediately for guidance. Do not inject again until at least 72 hours have passed since your last dose. Taking an extra dose may increase side effects. Your healthcare team will advise whether to continue your normal schedule or adjust timing.
Can I switch between injection sites without a pattern?
You should rotate sites systematically — for example, stomach this week, left thigh next week, right thigh the week after. This prevents lipohypertrophy (thickened tissue) and ensures consistent absorption. Never inject into the same spot twice in a row, and avoid areas with existing bruises, lumps, or skin changes.
How long does Mounjaro stay in my system after I stop taking it?
Mounjaro has a half-life of about three to four days, meaning your body clears roughly half the dose every three to four days. However, full clinical effects may take several weeks to wear off. If you stop treatment, speak to your prescriber about managing the transition — some people experience appetite increases as the medication leaves their system.
Will Mounjaro affect my other medications?
Mounjaro may reduce how quickly your body absorbs oral medications, including oral contraceptives. If you take the contraceptive pill, use barrier protection or switch to a non-oral method for four weeks after starting Mounjaro and for four weeks after each dose increase. Always inform your prescriber of all medications and supplements you take before starting treatment.
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.