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
Is There an Extra Dose in a Mounjaro Pen?
Each Mounjaro KwikPen contains exactly 4 pre-filled doses of tirzepatide — one for each week of your treatment cycle. There is no extra dose built into the pen. Each pen is calibrated to deliver your prescribed dose precisely, and once all 4 doses are used, the pen should be disposed of safely. If you're concerned about pen accuracy or think you've received an incorrect dose, contact your GP or pharmacist immediately.
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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 Pens Are Dosed and Designed
Mounjaro comes as a pre-filled KwikPen auto-injection device. Each pen is manufactured to contain exactly 4 weekly doses of your prescribed tirzepatide concentration. The dose titration schedule typically moves you through: 2.5 mg (weeks 1–4), 5 mg (weeks 5–8), 7.5 mg (weeks 9–12), 10 mg (weeks 13–16), 12.5 mg (weeks 17–20), and up to 15 mg (week 21+). Each KwikPen clearly displays which dose it contains on the label.
The pen mechanism is designed so that once you've injected your weekly dose by holding the button down for 3 seconds, the needle retracts automatically. There is no additional dose remaining — the pen empties completely with each of the 4 injections. After 4 weeks (4 injections), the pen is empty and must be disposed of in a sharps bin, never in regular household waste.
- Each pen = 4 weeks of treatment
- Each injection = 1 week's dose
- No residual medication remains after the 4th use
- Dispose safely in a sharps container
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.
What to Do If You're Unsure About Your Pen
If you suspect your Mounjaro pen may not be delivering the correct dose, or if you notice anything unusual — such as the pen not clicking, the needle not retracting, or liquid appearing after injection — do not use it again. Contact your GP, pharmacist, or the pharmacy that supplied your medication immediately.
You should also report any concerns to the MHRA (UK Medicines and Healthcare products Regulatory Agency) via their Yellow Card scheme if you believe you've experienced a dosing error or product defect. When you receive a new prescription, always check the pen label matches your prescribed dose before using it. Keep the original packaging and batch number in case you need to report an issue.
- Report pen issues to your GP or pharmacist
- Use the MHRA Yellow Card for product concerns
- Keep packaging and batch numbers
- Never attempt to refill or reuse a pen
- 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 an extra dose out of my Mounjaro pen if I inject slowly?
No. The Mounjaro KwikPen is engineered to deliver a fixed dose in 3 seconds. Injecting slowly or holding the button longer will not release additional medication — the pen mechanism controls the exact volume released. Attempting to manipulate the injection will not provide extra doses and may damage the device.
What's the difference between Mounjaro and Wegovy pens?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are different medications in different pen devices. Mounjaro is a dual GIP/GLP-1 agonist; Wegovy is a GLP-1 agonist only. Both are prescription-only and dosed weekly. They cannot be interchanged — your prescription specifies which medication you need.
How long does a Mounjaro pen last?
One Mounjaro KwikPen lasts exactly 4 weeks, containing 4 weekly injections. At your prescribed dose, you'll need a new pen every 4 weeks. Your pharmacy will issue a new prescription in time for your next dose — there's no overlap or extra doses included.
What happens if I accidentally inject twice in one week?
If you accidentally inject twice, contact your GP or pharmacist immediately. Do not skip the following week's dose. Your prescriber may advise monitoring for side effects. This is why keeping to your scheduled injection day (same day each week) is important — use a calendar or phone reminder to stay consistent.
Is Mounjaro available on the NHS in the UK?
Yes, Mounjaro (tirzepatide) is available on the NHS through specialist tier 3 weight management services in England, following NICE approval. Eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Access varies by region, and waiting times can be lengthy. Private prescribing is also available through registered UK pharmacies.
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.