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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How to Half Dose Mounjaro in the UK
Half-dosing Mounjaro (tirzepatide) is not standard practice and should only be considered under direct guidance from your prescriber. While the standard tirzepatide UK prescription starts at 2.5 mg and increases weekly, individual circumstances sometimes require dose adjustments. This guide explains NHS and private prescribing contexts, why dose changes matter, and how to safely discuss modifications with your healthcare provider. Always consult your GP or prescriber before making any changes to your injection regimen.
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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.
NHS Mounjaro Dosing and Adjustment
Standard Mounjaro dosing in the UK follows a strict titration schedule: 2.5 mg weekly for weeks 1–4, then increasing by 2.5 mg every 4 weeks until reaching a maintenance dose (typically 10 mg or 15 mg). The KwikPen pre-filled device delivers a full dose only — individual pens contain exactly 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg per injection.
If you're struggling with side effects or your prescriber believes a dose adjustment is necessary, they may:
- Extend the interval at your current dose before increasing
- Delay escalation to allow your body to adjust
- Switch to a lower dose tier if side effects are severe
- Pause treatment temporarily under medical supervision
NHS specialist weight management services (tier 3) manage these adjustments through clinical assessment. You cannot physically half a KwikPen — each pen is a pre-filled device designed for a single full dose. Attempting to divide doses risks inaccurate administration and loss of medication.
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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.
When to Discuss Dose Changes with Your Prescriber
Contact your prescriber if you experience severe or persistent side effects such as unmanageable nausea, vomiting lasting beyond the first week of a new dose, or significant stomach pain. These symptoms often improve with time, but your prescriber may recommend staying at your current dose longer before escalating.
On the NHS, adjustments are managed through your tier 3 specialist service — contact them directly rather than your GP. Privately, contact the prescriber who issued your prescription. Never attempt to alter your dose without guidance. Use our free GLP-1 weight loss calculator to track your progress and share results with your prescriber during consultations, helping them assess whether your current dose is working effectively for you.
Contact your prescriber if you experience severe or persistent side effects such as unmanageable nausea, vomiting lasting beyond the first week of a new dose, or significant stomach pain. These symptoms often improve with time, but your prescriber may recommend staying at your current dose longer before escalating.
On the NHS, adjustments are managed through your tier 3 specialist service — contact them directly rather than your GP. Privately, contact the prescriber who issued your prescription. Never attempt to alter your dose without guidance. Use our free GLP-1 weight loss calculator to track your progress and share results with your prescriber during consultations, helping them assess whether your current dose is working effectively for you.
- 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 split a Mounjaro KwikPen in half?
No. Each KwikPen is a pre-filled device delivering a single fixed dose (2.5 mg through 15 mg). You cannot safely split or divide a pen. If you need a lower dose, your prescriber will issue a different pen strength or recommend extending your current dose interval.
What happens if Mounjaro side effects are too severe?
Contact your prescriber immediately. They may extend the duration at your current dose, delay your next escalation, or temporarily pause treatment. Most side effects improve within a few days; extending your dosing timeline often helps your body adjust without abandoning treatment.
Is half-dosing Mounjaro available on the NHS?
Half-dosing is not a standard NHS or private option because KwikPens deliver fixed doses only. If you need a lower dose, your tier 3 service will prescribe a lower-strength pen or adjust your titration schedule under clinical supervision.
How long should I stay at each Mounjaro dose?
The standard UK protocol requires at least 4 weeks at each dose before increasing. Your prescriber may recommend longer (6–8 weeks) if you're experiencing side effects or responding well. Use our BMI calculator to monitor your progress alongside your prescriber's guidance.
What should I tell my prescriber about dose concerns?
Document any side effects, their severity, and when they occur. Note your weight loss progress using our progress tracker. Share this information with your NHS tier 3 team or private prescriber to help them make informed decisions about whether to adjust, pause, or continue your current dose.
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.