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 Injection — Complete UK Guide
Mounjaro is a prescription-only weight loss injection licensed in the UK that requires careful, step-by-step administration. This guide explains exactly how to use your Mounjaro KwikPen, from storage and dose timing to injection technique and what to expect. Whether you're accessing Mounjaro through NHS specialist services or private prescribing, understanding the correct usage is essential for safety and effectiveness. Use our free GLP-1 weight loss calculator to estimate your potential results at different doses, and our BMI calculator to track your baseline and progress.
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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.
How Mounjaro Works and UK Dosing Schedule
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — a weekly weight loss injection that activates two natural hormone pathways to help regulate appetite and support feeling fuller after meals. It is available in the UK as a pre-filled KwikPen device containing a single dose ready to inject.
UK dosing schedule:
- Week 1–4: 2.5 mg (starting dose)
- Week 5–8: 5 mg (first escalation)
- Week 9–12: 7.5 mg (optional increase)
- Week 13–16: 10 mg (optional increase)
- Week 17–20: 12.5 mg (optional increase)
- Week 21+: 15 mg (maximum dose)
Each dose is injected once per week on the same day. Your prescriber will advise which starting dose is right for you and whether dose escalation is appropriate. Most patients start on 2.5 mg and progress at 4-week intervals if tolerated well. You must wait a minimum of 4 weeks on each dose before increasing.
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.
Storage, Injection Technique, and Practical Tips
Store unopened Mounjaro pens in a refrigerator at 2–8°C in the original carton — never freeze them. Once you start using a pen, it can be stored at room temperature below 30°C for up to 30 days, then must be disposed of. Rotate injection sites weekly between your stomach (abdomen), upper thigh, and upper arm. Inject slowly and steadily, leaving the needle in for 6 seconds after pressing the injection button. If you miss a dose within 4 days, inject as soon as you remember and resume your normal schedule; if more than 4 days have passed, skip that dose and take your next injection on the scheduled day. Common early side effects like nausea or constipation usually improve within the first few weeks as your body adjusts.
- 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 get Mounjaro on the NHS in the UK?
Yes, Mounjaro (tirzepatide) is approved by NICE for use within NHS specialist tier 3 weight management services. However, NHS access is limited and regional variation is significant. You'll need a GP referral to a specialist weight management service, and waiting lists can be several months to over a year in many areas. Private prescribing through a GPhC-registered pharmacy offers faster access without NHS referral.
What is a KwikPen and how do I use it?
The KwikPen is Mounjaro's pre-filled injection device. Each pen contains one week's dose. Remove it from the fridge 15 minutes before injecting to allow it to reach room temperature. Check the liquid is clear and colourless. Pinch the skin where you'll inject, place the pen at a 90-degree angle, and press the injection button firmly until you hear two clicks. Hold for 6 seconds, then remove and dispose of the needle safely in a sharps bin.
What happens if I accidentally miss a weekly injection?
If you remember within 4 days of your missed dose, inject immediately and continue your normal weekly schedule from that point. If more than 4 days have passed, skip that missed dose and take your next injection on your normal scheduled day. Never double up doses. Always ensure at least 72 hours (3 days) between injections.
Are there any medications that interact with Mounjaro?
Mounjaro may reduce the absorption of oral medications, including oral contraceptives — women using the pill should use a barrier method for 4 weeks after starting and after each dose increase. If you take diabetes medications like insulin or sulphonylureas, your prescriber may adjust your doses. Always inform your prescriber of all medications you're taking before starting Mounjaro.
How long does it take to see weight loss results with Mounjaro?
Weight loss typically begins within the first 2–4 weeks, but many people see more significant changes after 8–12 weeks of consistent weekly injections. Clinical trials showed average weight loss of 15–21% over 72 weeks. Results vary widely depending on dose, adherence, diet, and physical activity. Use our GLP-1 weight loss calculator to estimate your potential progression and our progress tracker to monitor your actual results week by week.
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.