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
Understanding Mounjaro Pens and How to Use Them
Mounjaro comes as a pre-filled injection pen called a KwikPen, making it simple to self-administer at home once weekly. Each pen contains one dose and is designed for easy use without needing to draw up medication. Understanding how the pen works, proper storage, and injection technique helps you get the most from this prescription weight management medication. Our GLP-1 weight loss calculator can help you explore potential outcomes at different doses.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
What Is a Mounjaro KwikPen and How Does It Work?
Mounjaro is supplied as a pre-filled, single-dose pen called a KwikPen. Each pen contains enough medication for one weekly injection and comes ready to use—you don't need to mix, measure, or draw up the dose yourself. The pen has a hidden needle that extends only during injection, making the process straightforward and less intimidating than traditional syringes.
The KwikPen features an easy-to-use dial mechanism where you select your dose (typically 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg) and press the button to inject. Each box of Mounjaro contains 4 pens—a four-week supply. The dual GIP and GLP-1 receptor agonist mechanism means Mounjaro works differently from single-action GLP-1 medications like Wegovy, potentially offering enhanced appetite regulation and weight loss support.
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, Shelf Life, and Using Your Mounjaro Pens
Unopened pens must be stored in a refrigerator between 2°C and 8°C in the original carton. Never freeze—frozen pens cannot be used. Unopened pens remain stable until the expiry date if refrigerated properly.
After first use, your pen can be kept at room temperature (below 30°C) for up to 30 days, making it convenient for daily life. Once 30 days have passed since first use, dispose of the pen safely in a sharps bin. Always protect your pens from direct sunlight and heat. Proper storage ensures the medication remains effective throughout your 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 often do I use a Mounjaro pen?
You inject one Mounjaro pen once per week, every seven days. Each pen is a single dose. You stay on the same dose for at least four weeks before your prescriber may increase it. Most people reach their maintenance dose within 16-20 weeks.
Can I inject my Mounjaro pen myself at home?
Yes. Mounjaro pens are designed for self-injection at home. You can inject into your stomach, upper thigh, or upper arm. Rotate injection sites each week. If you're unsure about technique, ask your pharmacist or healthcare provider to demonstrate before your first injection.
What happens if I miss a dose or inject late?
If you're within 4 days of your scheduled injection, take it as soon as you remember and continue your normal schedule. If more than 4 days have passed, skip that dose and inject on your usual day. Never double-dose. Ensure at least 72 hours between injections.
How is Mounjaro different from Wegovy pens?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1-only agonist. Mounjaro's dual mechanism may provide different appetite regulation. Both are once-weekly injections, but Mounjaro has a higher maximum dose (15 mg vs 2.4 mg) and faster dose escalation schedule.
Do I need a prescription to get Mounjaro pens in the UK?
Yes. Mounjaro is prescription-only and requires a clinical assessment from a UK-licensed prescriber. This assessment confirms you're eligible based on your BMI and health profile. Use our free BMI calculator to check where you stand against NHS eligibility thresholds, then discuss options with your GP or a private prescriber.
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.