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 5mg Pen — What You Need to Know
The Mounjaro 5mg pen is the first maintenance dose of tirzepatide, a prescription-only weight management injection licensed in the UK. This single-dose KwikPen contains enough tirzepatide for one weekly injection and is typically started after a 4-week initiation phase at 2.5mg. Understanding how this dose fits into your treatment plan, what to expect, and whether it's right for you requires knowledge of the dosing schedule and how Mounjaro works. This guide covers everything about the 5mg pen in UK health context.
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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 the 5mg Mounjaro Pen Fits Into Treatment
Mounjaro comes as a pre-filled KwikPen auto-injection device, and the 5mg dose is the standard first maintenance step after starting treatment. Here's how the dosing schedule works: Week 1–4 you take 2.5mg, then from Week 5 onwards you increase to 5mg. This 5mg dose is typically maintained for at least 4 weeks before considering an increase to 7.5mg, depending on your response and tolerability.
Each KwikPen contains a single 5mg dose—enough for one week's injection. You'll receive a prescription for multiple pens to cover your treatment period. The pen is stored in the refrigerator between 2°C and 8°C when unopened, or at room temperature (below 30°C) for up to 30 days after first use. Once you've used a pen, it can be stored outside the fridge for up to 30 days before disposal.
You can inject the 5mg dose into your stomach (abdomen), upper thigh, or upper arm. Rotating injection sites each week reduces irritation and helps your skin tolerate the injections better over time. The needle is very fine and most people report minimal discomfort.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
5mg Mounjaro: Expected Effects and What to Expect
At 5mg, you're typically reaching the first full maintenance dose where weight loss benefits become more noticeable. Clinical trials show that most weight loss with Mounjaro happens gradually over weeks and months—not overnight. Individual results vary significantly based on your starting weight, diet, physical activity, and how your body responds to the medication.
Common side effects at 5mg include nausea, diarrhoea, constipation, and reduced appetite. Nausea is very common initially but often improves over time as your body adapts. If side effects are severe or persistent, your prescriber may recommend staying at 5mg longer before increasing, or they may adjust your approach. Use our free GLP-1 weight loss calculator to understand potential outcomes, and our BMI calculator to track changes as your weight progresses. Many people find it helpful to log results in a progress tracker alongside other lifestyle changes.
- 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.
Is the Mounjaro 5mg pen prescription-only in the UK?
Yes, Mounjaro is prescription-only medicine in the UK. You cannot buy it over the counter. You require a prescription from a UK-licensed prescriber following a clinical assessment. Access is available through NHS specialist weight management services (though waiting lists vary by region) or through private online prescribing services following an online consultation.
How long do you stay on the 5mg Mounjaro pen before increasing?
You should stay on 5mg for at least 4 weeks before your prescriber considers increasing to the next dose (7.5mg). Some people stay on 5mg longer if they're experiencing good weight loss and can tolerate it well. Your prescriber will assess your response and side effects before recommending a dose increase.
What's the difference between the Mounjaro 5mg pen and Wegovy for weight loss?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whereas Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials show Mounjaro may produce slightly greater average weight loss (~20.9% body weight) compared to Wegovy (~14.9%), but individual results vary. Both are prescription-only, and both require clinical assessment before prescription.
Can you buy the Mounjaro 5mg pen without a prescription in the UK?
No. Mounjaro is classified as prescription-only medicine in the UK. You must have a valid prescription from a UK-licensed prescriber. This can be obtained through NHS specialist services or private online prescribing following an appropriate clinical assessment. Never purchase without a prescription.
How long does a Mounjaro 5mg pen last?
One Mounjaro 5mg KwikPen contains a single weekly dose, so it lasts exactly one week. You'll need a new pen each week. A standard prescription typically covers 4 weeks (4 pens), so you'll need to reorder monthly or arrange repeat prescriptions through your provider.
Ready to take control of your weight loss journey?
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