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
Free tracker included with every download this week
How to Use Mounjaro — UK Instructions for the KwikPen
Mounjaro (tirzepatide) is a prescription-only weight loss injection that comes as a pre-filled KwikPen device. This guide covers everything you need to know about using your Mounjaro prescription in the UK, including storage, injection technique, dose schedules, and what to do if you miss a dose. Always follow the specific instructions provided by your prescriber and pharmacy, and contact your GP or prescriber if you have questions about your treatment.
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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 Doses and the Titration Schedule
Mounjaro starts at a low dose and increases gradually over 20 weeks. This dose titration approach helps your body adjust and reduces side effects. Each dose is injected once weekly, and you must wait at least 4 weeks on each dose before increasing. The schedule is:
- Weeks 1–4: 2.5 mg (starting dose)
- Weeks 5–8: 5 mg (first increase)
- Weeks 9–12: 7.5 mg
- Weeks 13–16: 10 mg
- Weeks 17–20: 12.5 mg
- Week 21+: 15 mg (maximum maintenance dose)
Some people stay on lower doses if they achieve good results or experience side effects. Your prescriber will guide your individual schedule. Each KwikPen contains 4 weekly doses (4 weeks' supply).
Clinically BackedWeight Loss
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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, Injection Sites, and Missed Doses
Storage: Keep unopened pens refrigerated at 2°C to 8°C in the original carton. Once you start using a pen, it can be kept at room temperature (below 30°C) for up to 30 days, then discarded. Never freeze Mounjaro.
Injection sites: Inject into your stomach (most common), upper thigh, or upper arm. Rotate the site each week to avoid skin irritation. Missed dose: If you remember within 4 days, inject as soon as you remember, then continue your normal schedule. If more than 4 days have passed, skip that dose and inject on your regular day. Never double-dose. Try our progress tracker to record your injection dates and side effects.
- 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 inject Mounjaro using the KwikPen?
Wash your hands, remove the KwikPen from the fridge 15 minutes before use, clean the injection site with an alcohol wipe, pinch the skin, insert the needle at a 90° angle, and press the injection button until you hear two clicks. Hold for 3 seconds, then remove. Your pharmacy or prescriber will demonstrate the technique.
Can I switch between injection sites with Mounjaro?
Yes, rotate your injection site each week to prevent irritation, lumps, or thickening of the skin. You can use your stomach, thigh, or upper arm. Always ensure the site is at least 2cm away from your previous injection.
What should I do if I experience severe side effects after injecting Mounjaro?
Contact your GP or call NHS 111 if you have severe stomach pain (possible pancreatitis), serious allergic reactions, vision changes, or chest pain. For mild nausea or diarrhoea, these often improve as your body adjusts. Keep a record using our progress tracker to share with your prescriber.
Is Mounjaro the same as Wegovy or Ozempic?
No. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. Wegovy (semaglutide) is a GLP-1 agonist only. Ozempic also contains semaglutide but is licensed for diabetes, not weight loss in the UK. All are prescription-only but have different mechanisms and doses.
Can I adjust my Mounjaro dose myself?
No. Dose adjustments must be managed by your prescriber. Never increase or decrease your dose without clinical guidance. Contact your GP or prescriber if you want to discuss your dose or if you're experiencing concerning side effects.
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.