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
Can You Get an Extra Dose From a Mounjaro Pen?
No, you cannot get an extra dose from a standard Mounjaro KwikPen. Each pre-filled pen contains exactly 4 doses (one month's supply) of a single strength, and each dose is already measured and sealed within the device. Once you've completed your 4 weekly injections, the pen is empty and should be disposed of safely. If you need additional doses or a dose increase, you'll need a new prescription from your GP or prescriber.
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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 KwikPens Are Designed and Dosed
Mounjaro (tirzepatide) is supplied in pre-filled KwikPen devices, each containing exactly 4 fixed doses. The pen is designed for once-weekly injection only, with each dose automatically measured and delivered. There is no way to extract additional medication from the device—the mechanism doesn't allow partial doses to be stored or reused. Each KwikPen comes in a single strength (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg), and you cannot combine doses from different pens or extract leftover solution.
After 4 weeks (4 injections), the pen is empty. Do not attempt to disassemble or refill the pen—this is unsafe and makes the device unusable. If you're on a dose titration schedule, your prescriber will issue a new prescription for the next strength when you're ready to increase.
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.
Getting Your Next Dose or Dose Increase Through the NHS
If you're receiving Mounjaro through an NHS specialist weight management service (tier 3), your prescriber will manage your dose schedule. You'll typically increase your dose every 4 weeks, following the standard titration protocol starting at 2.5 mg and progressing to your target dose. When you're due to increase, your tier 3 service will issue a new prescription. If you need an urgent dose or have missed a dose, contact your tier 3 clinic directly—they can advise on timing and issue a replacement prescription if needed. If you're on a private prescription, contact the prescriber or online pharmacy that issued your current supply.
- 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.
What should I do with an empty Mounjaro pen?
Place the empty pen in a sharps bin or return it to your local pharmacy for safe disposal. Never put it in household recycling or general waste. Your pharmacy can advise on local needle disposal services if you don't have a sharps bin at home.
Can I store leftover Mounjaro solution for later use?
No. Once a KwikPen has been opened (first injected), the medication inside must be used within 30 days even if unopened. You cannot extract or store solution separately—the pen is designed as a sealed, single-use device.
What if I accidentally miss a weekly dose?
If you miss a dose by 4 days or less, take it as soon as you remember and continue your normal schedule. If more than 4 days have passed, skip that dose and take your next dose on the scheduled day. Never take a double dose. Contact your prescriber if you're unsure.
How do I request a dose increase if I'm on NHS Mounjaro?
Contact your tier 3 specialist weight management service. They'll assess whether you're ready to increase and issue a new prescription for the next strength. Standard protocol is minimum 4 weeks at each dose before increasing.
Can I combine doses from two different Mounjaro pens?
No. Each pen contains a single, sealed dose for each week. You cannot mix or combine doses from different pens or strengths. Always use one new pen per month as prescribed.
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.