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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What is 2.5mg Mounjaro in ML?
The 2.5mg dose of Mounjaro (tirzepatide) is supplied as 0.25 millilitres (0.25ml) in each pre-filled KwikPen. This is the standard starting dose for adults beginning weight management treatment in the UK. Each KwikPen contains four weekly injections, so one pen provides a month's supply at the 2.5mg starting dose. The dose is measured by the pen itself — you don't need to measure millilitres manually. After four weeks on 2.5mg, your prescriber will typically increase your dose following a structured titration schedule.
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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 Mounjaro Doses Are Measured and Delivered
Mounjaro is a prescription-only weight management injection that comes in a pre-filled auto-injection pen called a KwikPen. The 2.5mg starting dose contains 0.25ml of liquid, but you won't measure this yourself — the pen is pre-calibrated to deliver the exact dose when you inject. Each KwikPen holds four doses (enough for four weeks), and you take one injection per week on the same day each week.
- Starting dose: 2.5mg (0.25ml) — weeks 1–4
- Each KwikPen contains four pre-filled doses
- Injections are taken once weekly, on the same day
- After four weeks, your prescriber reviews your response and may increase to 5mg if needed
- Maximum time between doses: 7 days; minimum: 72 hours (3 days)
The dose titration follows a specific schedule to allow your body to adjust and minimise side effects. Your prescriber will guide you through each step based on your tolerance and progress. Use our free GLP-1 weight loss calculator to explore projected outcomes at different doses.
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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.
Storage, Injection, and What to Expect
Before use, store your Mounjaro KwikPen in the refrigerator at 2–8°C in its original carton. Once you've started using a pen, it can be kept at room temperature (below 30°C) for up to 30 days. The 2.5mg starting dose commonly causes nausea, diarrhoea, or constipation — these side effects often improve as your body adjusts. Inject into your stomach, thigh, or upper arm, rotating injection sites each week to prevent irritation.
If you miss a dose, inject within four days of the missed day and continue your normal weekly schedule. More than four days late? Skip that dose and take your next one on the usual day. Never double-dose. Always consult your GP or prescriber if you experience severe stomach pain, persistent vomiting, or signs of allergic reaction.
- 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 2.5mg Mounjaro the same as 0.25ml?
Yes. The 2.5mg dose of Mounjaro (tirzepatide) is delivered as 0.25ml in the pre-filled KwikPen. You don't measure the millilitres yourself — the pen automatically dispenses the correct dose. Think of it as a pre-set dose rather than something you measure manually.
How long does one Mounjaro pen last at 2.5mg?
One KwikPen at 2.5mg contains four weekly doses, so it lasts exactly four weeks. After four weeks on the starting dose, your prescriber will typically increase you to 5mg (unless your clinical situation requires staying at 2.5mg longer).
What's the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — a different mechanism from Wegovy (semaglutide), which is a GLP-1 agonist only. Mounjaro's starting dose is 2.5mg, whilst Wegovy starts at 0.25mg. Clinical trials suggest Mounjaro may support slightly greater average weight loss, though results vary between individuals. Both are prescription-only in the UK.
Can I get Mounjaro through the NHS in the UK?
Mounjaro is approved by NICE for use within NHS specialist weight management services (tier 3). Access varies significantly by region and waiting lists can be lengthy. Private online prescribing through a UK-licensed prescriber is also available without NHS referral. Your GP can advise you about local NHS access.
What happens if I miss my 2.5mg Mounjaro injection?
If you're fewer than four days late, inject as soon as you remember and continue your normal weekly schedule. If more than four days have passed, skip that dose and take your next one on the normal day. Never take a double dose. Allow at least three days between injections (72 hours minimum).
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.