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
Green Poop on Mounjaro vs Wegovy — What's Normal?
Green or unusual-coloured stools can occur with both Mounjaro and Wegovy, though it's not listed as a common side effect in the formal safety data. This happens because GLP-1 receptor agonists slow how quickly food moves through your digestive system. Both medications can cause diarrhoea, constipation, and changes to digestion, which may alter stool appearance. Most people find digestive symptoms improve as their body adjusts to the medication. If you experience persistent green poop or other concerning digestive changes whilst taking either injection, speak to your GP or the prescriber who issued your prescription.
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.
Digestive Side Effects: Mounjaro vs Wegovy
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both GLP-1 receptor agonists approved for weight management in the UK, and both commonly cause gastrointestinal side effects. Mounjaro is a dual GIP and GLP-1 agonist, whilst Wegovy activates GLP-1 alone — this dual mechanism in Mounjaro may contribute to slightly different digestion patterns.
Very common side effects for both include:
- Nausea and vomiting
- Diarrhoea and constipation
- Stomach pain and indigestion
Unusual stool colour, including green poop, is not formally reported in clinical trial data for either medication. However, changes in bowel habits — including altered stool colour — can occur as a secondary effect of slowed gastric emptying and changes to bile flow during digestion.
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.
When to Contact Your GP or Prescriber
Green stools are usually not a cause for alarm, especially if you feel otherwise well. However, you should contact your GP or your prescriber if:
- Green poop persists for more than a few days
- You also have severe abdominal pain, fever, or blood in stool
- Digestive symptoms are significantly affecting your quality of life
- You're unsure whether your symptoms are related to Mounjaro, Wegovy, or another cause
Your GP can assess whether your symptoms are a normal adjustment response, related to your diet, or require investigation. Many digestive side effects improve within 4–8 weeks as your body adjusts to the medication dose. Never stop taking a prescribed injection without speaking to your prescriber first — they can advise on dose adjustments or switching to an alternative if needed.
- 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?
Green or unusually coloured stools aren't listed as a common side effect in Mounjaro's safety data. However, diarrhoea and constipation are very common, and these changes to bowel habits can occasionally alter stool colour. If you're experiencing green poop alongside other digestive symptoms, it's likely related to how the medication affects your digestion rather than a sign of serious harm.
Does Wegovy cause green poop like Mounjaro does?
Wegovy (semaglutide) can cause similar digestive changes to Mounjaro because both are GLP-1 medications. Diarrhoea is very common with Wegovy, and changes to digestion speed may occasionally result in unusual stool colour. The frequency and severity of digestive side effects vary between individuals and between the two medications.
What's the difference between Mounjaro and Wegovy for digestive side effects?
Mounjaro is a dual GIP + GLP-1 agonist, whilst Wegovy is GLP-1 only. Clinical trial data shows both cause similar rates of nausea, diarrhoea, and constipation. Some people report different tolerability between the two, but this is individual. Your prescriber can help you decide which is most suitable for you based on your medical history and previous medication responses.
How long do digestive side effects last on Mounjaro or Wegovy?
Most people find digestive side effects — including unusual stool colour — improve significantly within 4–8 weeks as their body adapts. This is why both medications involve a gradual dose titration schedule. If side effects persist or worsen, speak to your prescriber about dose adjustment or alternative options. Always consult your healthcare provider before making any changes.
Should I be worried if my poop is green on these injections?
Green poop alone is usually not concerning if you feel well otherwise. It's often a sign of faster digestion or dietary changes. However, if you also have fever, severe pain, blood in stool, or the symptom persists beyond a week, contact your GP. Your prescriber needs to know about any persistent changes so they can rule out other causes and adjust your treatment if needed.
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.