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
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Mounjaro 5ml Pen — What You Need to Know in the UK
The Mounjaro 5ml pen (formally the KwikPen) is a pre-filled injection device containing tirzepatide, a prescription-only weight management medicine licensed in the UK. Each pen delivers a single weekly dose and contains 4 weeks of treatment. Understanding how the pen works, NHS eligibility criteria, and what to expect is essential before starting this medication. This guide explains UK prescribing context, NHS access routes, and practical information about the device itself.
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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 Prescribing in the UK — NHS Eligibility and Access
Mounjaro (tirzepatide) is available in the UK through NHS specialist weight management services and private prescribing routes. NICE has approved tirzepatide for use within NHS tier 3 and tier 4 specialist services for adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, hypertension, or cardiovascular disease.
For South Asian, Chinese, Black African, or Black Caribbean adults, lower BMI thresholds apply — eligibility may begin at BMI 27.5 or above due to higher health risks at lower weights.
NHS access typically involves:
- GP referral to your local tier 3 weight management service
- Clinical assessment and eligibility review
- Prescription through the NHS specialist service
- Significant regional variation in availability and waiting times
Private prescribing is also available through UK-licensed online healthcare providers following a clinical assessment, without NHS referral.
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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.
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The Mounjaro KwikPen — How It Works and Storage
The Mounjaro 5ml pen is a colour-coded, pre-filled auto-injection device designed for once-weekly self-administration. Each pen contains 4 doses (4 weeks of treatment). The device is simple to use: dial the dose, place against the skin, and press the button — the needle retracts automatically.
Storage requirements: Unopened pens must be refrigerated at 2–8°C and kept in the original carton. After first use, the pen can be stored at room temperature (below 30°C) for up to 30 days. Do not freeze — if a pen freezes, discard it. Rotate injection sites weekly, choosing the stomach, upper thigh, or upper arm. Check the expiry date before use, and dispose of the pen 30 days after first opening, even if it contains remaining doses.
- 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 dose does the Mounjaro 5ml pen contain?
The standard Mounjaro 5ml KwikPen contains 5 mg of tirzepatide per dose. This is typically a maintenance or escalation dose, not a starting dose. The starting dose is usually 2.5 mg, administered via a separate pen, then increased gradually over weeks. Always check your prescription to confirm your specific dose.
How do I know if I'm eligible for Mounjaro on the NHS?
NHS eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions (diabetes, hypertension, sleep apnoea, cardiovascular disease). South Asian and Black African/Caribbean adults may qualify at lower BMI thresholds. You'll need GP referral to a specialist tier 3 weight management service for assessment. Regional availability varies significantly.
What's the difference between the Mounjaro KwikPen and Wegovy?
Mounjaro is a dual GIP and GLP-1 agonist; Wegovy is GLP-1 only. Mounjaro's maximum dose is 15 mg weekly; Wegovy's is 2.4 mg. Mounjaro is delivered via KwikPen (once weekly); Wegovy uses an auto-injection pen. Clinical trial data shows both effective for weight loss, though results vary individually.
Can I switch from Wegovy to Mounjaro on the NHS?
Switching between medications requires prescriber guidance. There is no official NHS conversion protocol — your specialist team will assess whether switching is appropriate and manage the transition. Never switch medications without medical support, as dose timing and overlaps must be carefully managed.
What should I do if I miss a weekly Mounjaro injection?
If you miss a dose, inject within 4 days of the scheduled date and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and inject on your usual day. Never double-dose. If you're uncertain, contact your prescriber or pharmacist for guidance on your specific situation.
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