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's Normal?
Green poop is an uncommon but reported side effect in people taking Mounjaro (tirzepatide) in the UK. This change in stool colour typically relates to how quickly food moves through your digestive system and changes to your gut bacteria. While it's usually harmless, understanding what's normal and when to contact your GP is important. Most people taking Mounjaro experience digestive changes like nausea or constipation rather than stool colour changes, but you're not alone if this has happened to you. Here's what NHS guidance suggests and when to seek advice.
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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.
Why Does Mounjaro Affect Stool Colour?
Mounjaro (tirzepatide) works as a dual GIP and GLP-1 receptor agonist, which affects how your body processes food and regulates appetite. One key effect is slowed gastric emptying — meaning food passes through your stomach and intestines more slowly than usual. This delay can alter the balance of your gut bacteria and change the colour, consistency, and frequency of your stools.
Green stool specifically may occur because:
- Faster transit through the colon before stool darkens (bile breaks down more slowly)
- Changes to your gut microbiome from the medication
- Dietary changes often made when taking weight loss medication — increased fibre, fewer calories
- Reduced food intake slows digestive processes
Most people experience more common digestive side effects like constipation or nausea instead. Green poop is not listed as a very common side effect in UK prescribing information, but it is recognised in clinical practice. If you're concerned, use our GLP-1 weight loss calculator to track other changes alongside your dose, and discuss any digestive concerns with your GP.
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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.
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When Should You Contact Your GP?
Green stool alone is usually not a reason for emergency care, but contact your GP if green poop occurs alongside: severe or persistent stomach pain, blood in stool, unexplained weight loss beyond expected loss from Mounjaro, or diarrhoea lasting more than a few days. These could indicate a different digestive issue unrelated to your medication.
Also speak to your GP if green poop is distressing you or if it's accompanied by other unusual symptoms. Your prescriber needs to know about any side effects affecting your quality of life, even if they seem minor. They can assess whether the issue relates to your Mounjaro dose, your diet, or another factor — and adjust your treatment plan if needed. Never stop taking Mounjaro without medical guidance.
- 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 common side effect of Mounjaro in the UK?
Green stool is not listed as a very common side effect in Mounjaro's official UK prescribing information, but it is occasionally reported by people taking tirzepatide. Much more common side effects include nausea, constipation, diarrhoea, and stomach pain. If you experience green poop, it's worth mentioning to your GP, but it's unlikely to be a cause for alarm on its own.
Can dietary changes from Mounjaro cause green poop?
Yes. When taking Mounjaro, many people reduce their calorie intake, eat differently, or increase fibre — all of which can affect stool colour. Reduced overall food intake may also slow digestion, allowing bile to break down differently. Track your dietary changes using our progress tracker alongside any stool colour changes to help identify patterns.
Does green poop mean my Mounjaro dose is too high?
Not necessarily. Green stool is not a recognised sign of dose toxicity. However, if it occurs alongside severe stomach pain, persistent diarrhoea, or vomiting, contact your GP immediately. They can assess whether your current dose is right for you and whether dose adjustment might help.
Should I stop taking Mounjaro if my poop is green?
No. Do not stop Mounjaro without medical guidance. Green stool alone is not a reason to discontinue treatment. Contact your GP to discuss it — they'll advise whether it's related to your medication or another cause, and whether any adjustments are needed.
How long does green poop last on Mounjaro?
This varies between individuals. For some people, digestive side effects settle within a few weeks as the body adjusts to tirzepatide. For others, they may persist. If green poop continues beyond a few weeks or worsens, mention it at your next GP appointment so they can investigate further.
Ready to take control of your weight loss journey?
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