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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Green Poop on Mounjaro — What NHS Guidance Says
Green or discoloured stools can occur when taking Mounjaro (tirzepatide), though this is not listed as a common side effect in UK prescribing information. Most gastrointestinal side effects of Mounjaro — including nausea, diarrhoea, and constipation — typically settle within the first few weeks as your body adjusts. If you're experiencing green poop while on Mounjaro prescribed through NHS specialist services or private prescription, it's important to understand what's normal and when to contact your GP. This guide covers NHS eligibility for Mounjaro, what to expect, and how to access support.
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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.
NHS Eligibility and Mounjaro Access in the UK
The NHS has approved tirzepatide (Mounjaro) for weight management through NICE guidance, making it available on the NHS for eligible adults. To qualify for Mounjaro through NHS specialist weight management services, you must have a BMI of 30 kg/m² or above, or BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease.
Access to Mounjaro on the NHS is provided through tier 3 specialist multidisciplinary weight management services, not through your standard GP appointment. You'll typically need a referral from your GP to these services, though waiting times vary significantly by region. Once assessed by the specialist team, your dose titration will be managed carefully — starting at 2.5 mg and increasing gradually over weeks, with each KwikPen device containing four weekly doses.
If you're on an NHS-prescribed Mounjaro dose and experience green poop or other digestive changes, report this to your specialist service rather than stopping the injection independently.
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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.
NHS vs Private Mounjaro Access and Support
Access to Mounjaro in the UK is split between NHS specialist services and private prescribing. NHS access requires referral to tier 3 weight management services and can involve waiting lists of several months in many regions. Private access is available through GPhC-registered online pharmacies following a remote clinical assessment with a UK-licensed prescriber — no NHS referral needed. Both routes require a valid prescription; Mounjaro cannot be obtained without one.
Whether accessing Mounjaro through NHS or private channels, side effects like green poop, diarrhoea, or other GI changes should be discussed with your prescriber. NHS specialist services provide ongoing support and dose adjustments, while private prescribers are also obligated to monitor your response and manage any adverse effects. Our free GLP-1 weight loss calculator can help you understand whether you might be eligible for treatment and what to expect from the dose titration process.
- 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 green poop a known side effect of Mounjaro?
Green or discoloured stools are not listed as a common side effect in UK prescribing information for Mounjaro (tirzepatide). However, GI side effects including diarrhoea, constipation, and nausea are very common, especially in the first few weeks. Changes in stool colour can sometimes occur alongside digestive changes. If green poop persists or is accompanied by severe symptoms, contact your GP or prescriber.
How long do Mounjaro side effects like diarrhoea usually last?
Most gastrointestinal side effects of Mounjaro — including diarrhoea, nausea, and constipation — typically improve within 4–8 weeks as your body adjusts to the medication. However, individual experiences vary. Your NHS specialist team or private prescriber can advise on symptom management and whether dose adjustment might help.
How do I access Mounjaro through the NHS in the UK?
NHS access to Mounjaro is through tier 3 specialist weight management services. Your GP can refer you if you meet eligibility criteria (BMI ≥30, or ≥27 with weight-related health conditions). Waiting times vary by region. Private access is faster but available only through GPhC-registered online prescribers following clinical assessment.
What should I do if I experience green poop while on Mounjaro?
If you're experiencing green poop or other unusual digestive symptoms on Mounjaro, contact your prescriber — either your NHS specialist service or your private prescriber. Do not stop your injection without medical advice. Your prescriber may adjust your dose, offer symptom management strategies, or investigate whether another cause is present. Keep using your KwikPen as prescribed unless advised otherwise.
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.