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 Half Dose Mounjaro in the UK?
No — you should not half dose Mounjaro without medical guidance. Each Mounjaro KwikPen is pre-filled with a specific dose designed to be taken as a complete injection once weekly. Splitting doses or adjusting the prescribed amount without your prescriber's approval can affect how the medication works. If side effects are concerning or you're not responding well to your current dose, always speak to your GP or the prescriber managing your treatment.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Mounjaro Dosing Works in the UK
Mounjaro (tirzepatide) is a pre-filled KwikPen injection designed for once-weekly use. Each pen contains a fixed amount of medication in a specific concentration. The standard Mounjaro dosing schedule in the UK follows a careful dose titration process, starting at 2.5 mg and gradually increasing every 4 weeks to 5 mg, 7.5 mg, 10 mg, 12.5 mg, and up to a maximum of 15 mg weekly. This gradual escalation allows your body to adjust and helps minimise side effects like nausea and gastrointestinal discomfort. Your prescriber will monitor your response and tolerability at each stage. The pre-filled design means the full dose cannot be easily divided — attempting to do so risks incorrect dosing, medication waste, and unpredictable effects on your weight management and metabolic control.
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.
What to Do If You're Struggling with Your Current Dose
If you're experiencing troublesome side effects or feel your current Mounjaro dose isn't working for you, contact your GP or prescriber rather than adjusting the dose yourself. Options available include staying on your current dose for longer before increasing, switching to the next scheduled dose if you've plateaued, or in some cases, temporarily pausing treatment whilst side effects settle. Your prescriber can also discuss whether Mounjaro remains the right choice or whether an alternative like Wegovy might suit you better. The NHS and NICE guidance emphasises individualised dosing — your prescriber will tailor the approach to your specific health circumstances, not a one-size-fits-all schedule.
- 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.
Can you open a Mounjaro pen and use half?
No. Mounjaro pens are sealed, pre-filled devices designed to deliver the full dose as a single injection. Opening a pen risks contamination, medication degradation, and loss of sterility. Each KwikPen contains exactly one week's dose — attempting to use it differently may render the medication unsafe or ineffective.
What if I'm having severe side effects from my Mounjaro dose?
Contact your prescriber or GP immediately. Do not reduce the dose yourself. Your prescriber can extend the time at your current dose before escalating, switch you to a lower dose schedule, or explore alternative treatments. Managing side effects is part of the standard prescribing pathway.
Is there a lower starting dose of Mounjaro available in the UK?
The standard UK starting dose is 2.5 mg weekly. This is the lowest available strength in the KwikPen range. If even this causes problems, your GP or specialist can discuss whether to delay escalation or try a different medication entirely — but halving this dose is not recommended.
Can your GP adjust your Mounjaro dose outside the standard schedule?
Yes, if medically necessary. Your prescriber has flexibility to extend time at each dose or pause treatment if side effects are significant. However, this adjustment is made by prescription, not by you manually altering injections. Always involve your healthcare team.
What's the difference between Mounjaro dose titration and dose adjustment?
Titration is the planned weekly increase schedule (2.5 mg → 5 mg → 7.5 mg, etc.). Adjustment is when your prescriber changes this plan based on your individual response — but adjustments are made via new prescriptions, not by self-medicating with half doses.
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.