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 Long Does The Mounjaro Pen Last In The UK?
Each Mounjaro KwikPen contains four weekly doses, meaning one pen lasts approximately four weeks. After that, you'll need a new prescription and a fresh pen. The cost and how you access Mounjaro — whether through the NHS or privately — affects how long you can sustain treatment. This guide explains pen duration, storage requirements, UK prescription routes, and what to expect financially so you can plan your weight management journey effectively.
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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 Long One Mounjaro Pen Lasts And Storage Requirements
A single Mounjaro KwikPen contains four pre-filled doses, each administered once weekly. This means one pen provides a four-week supply of medication. Every pen is pre-loaded and ready to use — you simply inject once per week and discard the pen after four weeks.
Storage is critical for pen longevity. Unopened pens must be refrigerated at 2–8°C and kept in their original carton away from light. An unopened pen remains usable until its printed expiry date if stored correctly. Once you start using a pen, you can keep it at room temperature (below 30°C) for up to 30 days. After 30 days from first use, even if doses remain, the pen must be discarded. Never freeze Mounjaro — if accidentally frozen, it cannot be used safely.
- One pen = four weekly injections = one month's supply
- Unopened: refrigerate at 2–8°C
- Opened: can use room temperature for 30 days maximum
- After 30 days from first use: discard regardless of remaining doses
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.
NHS Versus Private Access: Cost And Timeline In The UK
NHS access to Mounjaro for weight management is through specialist tier 3 weight management services, typically accessed via GP referral. Availability varies significantly by region, and waiting times can be several months or longer depending on your local integrated care board (ICB). Once prescribed through the NHS, you pay standard prescription charges (currently £11.65 per item in England, or free if eligible).
Private prescription route is often faster. A UK-licensed prescriber conducts an online consultation, assesses your eligibility based on BMI and health conditions, and issues a prescription if appropriate. The cost ranges from approximately £129–£320 per month depending on your dose and the provider. This route avoids NHS waiting lists but requires upfront private payment. Our GLP-1 weight loss calculator can help you understand potential outcomes and costs across different doses before committing to either route.
- 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 happens if I run out of Mounjaro pens?
You'll need a new prescription from your prescriber before you can obtain more pens. If you're on the NHS, contact your weight management service. If private, contact your prescribing provider. There's no automatic repeat supply — each prescription must be requested and issued separately, so plan ahead to avoid gaps in treatment.
Can I keep using the same Mounjaro pen after 30 days if doses remain?
No. Mounjaro must be discarded 30 days after first use, even if doses remain unused. This is a safety requirement to ensure the medication remains effective and sterile. Always plan your prescription timing to avoid wasting doses.
Is Mounjaro cheaper on the NHS or through a private prescription?
On the NHS, you pay the standard prescription charge (£11.65 per item in England). Private prescriptions typically cost £129–£320 per month depending on dose. NHS is significantly cheaper, but waiting times are often much longer. Private is faster but more expensive.
How does Mounjaro's cost compare to tirzepatide from other sources?
Mounjaro is the brand name for tirzepatide manufactured by Eli Lilly. Other generic or branded versions of tirzepatide may eventually become available, potentially affecting cost. Currently in the UK, Mounjaro is the primary licensed option. Always obtain tirzepatide through a valid prescription only.
What's the difference between Mounjaro dose titration and pen duration?
Dose titration is the schedule of increasing your injection strength over weeks (starting 2.5 mg, then escalating every 4 weeks to a maintenance dose). Pen duration is simply that each pen lasts four weeks regardless of which dose strength it contains. Higher dose pens don't last longer — one pen always equals four weekly injections.
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.