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
What to Do With an Empty Mounjaro Pen in the UK
When you've completed all four weekly doses from your Mounjaro KwikPen, you'll need to obtain a new prescription for your next pen. Whether you're accessing Mounjaro through the NHS or privately in the UK, the process for getting a refill depends on how you're currently receiving treatment. This guide explains your options, costs, and what to expect when your pen runs out.
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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 Refill Costs and Access Routes in the UK
Private prescription costs: A single Mounjaro pen (containing four weekly doses) typically costs between £130–£220 through UK private online pharmacies, depending on your dose strength and the pharmacy. Costs are usually lower at higher doses per milligram. If you're on dose titration, you may need to purchase different strength pens as you progress through 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg doses.
NHS access: If prescribed through the NHS, Mounjaro is free on the NHS prescription charge (or £9.90 per item in England if eligible for prescription charges). However, NHS access is limited to specialist tier 3/4 weight management services, and availability varies significantly by region. Waiting lists can be lengthy in many areas.
Online prescription process: For private prescriptions, you'll complete a brief online consultation with a UK-licensed prescriber, who will assess your eligibility based on your BMI and health history. If approved, a prescription is issued and your pen is dispensed by a GPhC-registered pharmacy — typically within 1–3 working days.
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.
Switching Doses and Managing Your Mounjaro Supply
Most people taking Mounjaro progress through different dose strengths during their first 5–6 months — this is called dose titration. Each dose increase (2.5 mg → 5 mg → 7.5 mg, etc.) typically lasts at least four weeks. When planning refills, you'll need a new pen prescription each time you step up to the next strength. If you're accessing privately, inform your prescriber of your current dose so they issue the correct strength. If taking Mounjaro via the NHS, your specialist clinic will manage dose progression and prescriptions. Use our GLP-1 weight loss calculator to understand projected results at different doses, and track your progress with our free progress tracker to monitor your weight loss journey alongside your dose increases.
- 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 many doses are in one Mounjaro pen?
One KwikPen contains four pre-filled doses — enough for four weeks (one injection per week). When empty, you need a new prescription for a fresh pen. Each pen is single-use and cannot be refilled.
Can I get Mounjaro on the NHS when my pen is empty?
If you're already receiving Mounjaro through an NHS specialist weight management service, your next prescription should be issued automatically. However, NHS access varies by region and services may have waiting lists. Ask your clinic about their prescription renewal process before your current pen runs out.
What's the difference between getting a refill privately vs through the NHS?
NHS prescriptions are free (or £9.90 in England), but access is limited to specialist services with regional variation. Private refills cost £130–£220 per pen but are accessible immediately through online prescribers without NHS referral. Speed and cost vary depending on your route.
How do I know when to order my next Mounjaro pen?
Plan ahead — order your refill prescription at least one week before your current pen runs out. This gives time for online assessment, prescription issue, and pharmacy dispatch. Don't wait until you've used your final dose, as delays could interrupt your treatment.
Do I need a new clinical assessment every time I refill my Mounjaro pen?
For NHS refills through an established service, no — your clinic will manage ongoing prescriptions. For private prescriptions, some providers may ask brief check-in questions at refill, but full reassessment isn't always required if you're an established patient. Check with your pharmacy or 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.