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
Can You Take A Partial Dose Of Mounjaro?
No, you cannot take a partial dose of Mounjaro using the standard KwikPen. Each pre-filled pen is designed to deliver a complete dose (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg) in a single weekly injection. In the UK, Mounjaro (tirzepatide) is prescribed as a full dose based on your weight management plan, with dose adjustments made every 4 weeks under GP or specialist guidance. If you're experiencing side effects or have concerns about your current dose, your prescriber can adjust to a different dose level — but splitting a single pen is not recommended and may affect medication effectiveness.
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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.
Understanding Mounjaro Dose Titration In The UK
Mounjaro is prescribed using a structured dose titration schedule — a gradual escalation approach rather than dose splitting. In the UK, this typically follows a 4-week progression pattern:
- Weeks 1–4: 2.5 mg (starting dose)
- Weeks 5–8: 5 mg (first escalation)
- Weeks 9–12: 7.5 mg
- Weeks 13–16: 10 mg
- Weeks 17–20: 12.5 mg
- Week 21+: 15 mg (maximum dose)
Each KwikPen contains exactly 4 doses — meaning you have one pen per month at each dose level. Your GP or specialist prescriber in a tier 3 NHS weight management service will review your progress every 4 weeks and decide whether to move to the next dose or remain at your current level. If side effects such as nausea or stomach discomfort occur, your prescriber may recommend staying at your current dose longer, rather than splitting doses. This approach, supported by NICE guidance and MHRA approval, ensures you receive the full therapeutic benefit and maintains consistency with clinical trial protocols.
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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.
Why You Cannot Split A Mounjaro Pen
The Mounjaro KwikPen is a pre-filled, auto-injection device engineered to deliver a precise dose in one complete injection. Attempting to split a dose would be unsafe and ineffective:
- The pen cannot be adjusted to deliver partial amounts — it is programmed for a complete dose only
- Dividing the insulin cartridge manually could contaminate the medication or alter the dose unpredictably
- You may not receive an accurate amount, affecting your treatment outcome and increasing side effect risk
- Modifying a prescription medicine in this way falls outside NHS and MHRA approval, invalidating your prescription
If cost is a concern, discuss this with your GP. If side effects are limiting, your prescriber can pause progression or switch to a lower maintenance dose. Use our free GLP-1 weight loss calculator to understand projected outcomes at different dose levels, and discuss realistic expectations with your healthcare team.
- 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 should I do if I'm having side effects from my current Mounjaro dose?
Contact your prescriber immediately — your GP or NHS tier 3 specialist service. Do not skip doses or split pens. Your prescriber may recommend staying at your current dose for longer than 4 weeks, returning to a lower dose, or switching to an alternative medication like Wegovy. Side effects often improve over time as your body adjusts.
Can I request a specific dose when starting Mounjaro on the NHS?
No — all patients start at 2.5 mg regardless of weight or BMI. The 4-week titration schedule is standardised to ensure safety and allow your body to adapt. Your prescriber may adjust the escalation pace based on your individual tolerance and response, but everyone begins at the lowest starting dose.
Is the dose titration schedule different in private vs NHS Mounjaro prescribing?
The starting dose (2.5 mg) and titration protocol are the same in both NHS and private prescribing in the UK — this is based on MHRA approval and clinical evidence. However, private prescribers may offer slightly more flexibility in timing adjustments if you're managing side effects, whereas NHS services may have longer review cycles due to capacity.
What if I've been on Mounjaro and want to switch to Wegovy — do I need to restart at a low dose?
A switch from Mounjaro to Wegovy does not require restarting at the absolute lowest dose, though your prescriber will manage the transition carefully. Approximate equivalence exists (e.g., Mounjaro 10 mg ≈ Wegovy 1.7 mg), but there is no official conversion chart — your prescriber will assess the best starting point based on your tolerance and response to tirzepatide.
Can I take a Mounjaro pen and miss the injection if I want to reduce my dose?
No — you must either use the full pen dose or not use it that week. Missing an injection reduces your dose that week, but you cannot partially inject a pen and reuse it later. If you need a dose reduction, contact your prescriber and request a lower dose for your next prescription cycle.
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.