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 One Mounjaro Pen Last?
Each Mounjaro KwikPen contains exactly 4 weekly doses, meaning one pen lasts 4 weeks (28 days). Since Mounjaro is a once-weekly injection, you'll need one new pen every month. The duration depends on your dose level — whether you're on 2.5 mg, 5 mg, 10 mg, 15 mg, or any dose in between — the pen always contains 4 weeks of supply. Understanding how long your supply lasts helps with planning refills and managing costs through the UK private prescription route or NHS specialist services.
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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 Much Does Mounjaro Cost in the UK?
Mounjaro is not routinely available on the NHS except through specialist tier 3 or tier 4 weight management services in some regions. Private prescription costs in the UK typically range from £129 to £320 per month, depending on your dose and the prescribing pharmacy. A single KwikPen (4 weeks) generally costs between £129–£160 at lower doses and up to £250–£320 at maximum doses like 15 mg. Costs can vary because private providers set their own pricing. Since you'll need one pen per month, annual costs could range from approximately £1,500 to £3,840 depending on your dose and whether you remain on the same dose or progress through titration. It's worth comparing prices across GPhC-registered online pharmacies to find the most cost-effective option for your situation.
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 vs Private Access to Mounjaro in the UK
NHS route: Mounjaro is available through NHS specialist weight management services following NICE approval, but access depends on your local Integrated Care Board (ICB). You'll need a referral from your GP and assessment by a specialist team. Waiting lists can be several months to over a year in many areas. Private route: You can access Mounjaro through a GPhC-registered online pharmacy following an online clinical assessment with a UK-licensed prescriber. This typically takes 24–48 hours. No NHS referral is needed, but you pay the full private cost. Use our free BMI calculator and GLP-1 weight loss calculator to assess your eligibility and estimate potential outcomes before you consult.
- 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.
Does every Mounjaro pen contain the same amount?
Yes. Every Mounjaro KwikPen contains exactly 4 weekly doses, regardless of dose strength. A 2.5 mg pen, 10 mg pen, or 15 mg pen all last 4 weeks. The difference is the concentration of tirzepatide in each weekly injection, not the volume of the pen.
What happens when my Mounjaro pen runs out?
Once your 4-week supply is exhausted, you'll need to request a repeat prescription from your prescriber. Via NHS specialist services or private prescribing, your provider can issue a new prescription. If you're on titration, your dose may increase at this point. Keep track of your pen expiry dates to avoid running out unexpectedly.
Can I use a Mounjaro pen after 30 days?
No. After first use, you must dispose of your Mounjaro KwikPen after 30 days, even if it still contains medication. This is a safety requirement. Unopened pens can be stored in the fridge until the expiry date printed on the box, but once you start using a pen, you have 30 days before it must be discarded.
Is Mounjaro cheaper on the NHS than private?
Yes — if you can access it through NHS specialist services, Mounjaro is free. However, NHS waiting lists are very long in many regions. Private prescription costs £129–£320 per month. Some patients choose private access for faster treatment, accepting the cost, rather than waiting months for NHS assessment.
How do I know if I'm eligible for Mounjaro?
You're generally eligible if you have a BMI of 30 or above, or 27–29 with weight-related health conditions like type 2 diabetes or high blood pressure. For South Asian, Black African, or Black Caribbean backgrounds, lower thresholds may apply. A prescriber will assess your full health history before offering Mounjaro or any tirzepatide treatment.
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.