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 vs Wegovy — What's the Difference?
Green or discoloured stools can occur with Mounjaro (tirzepatide), though it's not one of the most frequently reported side effects. Both Mounjaro and Wegovy can affect digestion, but they work through different mechanisms—Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. This page compares digestive side effects between the two medications based on clinical trial data and UK prescribing guidance. Individual experiences vary significantly, and any unusual bowel changes should be discussed with your GP or prescriber.
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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 and Wegovy Affect Digestion Differently
Mounjaro (tirzepatide) activates two hormone pathways—GIP and GLP-1—which may explain why some users experience more pronounced gastrointestinal effects, including changes in stool colour or consistency. In the SURMOUNT-1 clinical trial, diarrhoea was very common (affecting more than 1 in 10 participants), whilst constipation also occurred. Green or dark stools can result from rapid transit through the bowel (diarrhoea) or from bile pigments, especially if fat absorption is altered.
Wegovy (semaglutide) activates only the GLP-1 pathway, which may cause a slightly different digestive pattern. In the STEP-1 trial, diarrhoea and constipation were both common, but the mechanism differs slightly due to the single-pathway activation. Neither medication directly causes the green colour—rather, changes in how quickly food moves through your system affect stool appearance.
Both medications slow gastric emptying, meaning food stays in your stomach longer initially. This effect typically reduces over time as your body adjusts.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Individual Suitability and What Your Prescriber Will Consider
Whether green stools or other digestive changes occur depends on individual factors: your baseline gut health, how your body responds to the medication, and your starting dose. Most digestive side effects are temporary and improve within the first few weeks as your body adapts. If you experience persistent green stools, severe diarrhoea, or abdominal pain, contact your GP or prescriber immediately—these can occasionally signal gallbladder issues or pancreatitis, both of which require urgent assessment.
A UK-licensed prescriber will assess whether Mounjaro or Wegovy is more suitable based on your overall health profile, existing digestive conditions (such as IBS or gastroparesis), and other medications. Neither medication is "better" than the other—suitability is individual. Use our free BMI calculator to check your eligibility, then discuss your options with your GP or a licensed prescriber.
- 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 discoloured stools are not among the most commonly reported side effects of Mounjaro, but digestive changes (diarrhoea, constipation) are very common. Green stools usually result from faster bowel transit or changes in bile pigment exposure, rather than from Mounjaro itself. Most cases resolve within weeks as your body adjusts.
Does Wegovy cause green poop more or less than Mounjaro?
Both medications can alter stool colour through similar mechanisms—changes in digestion speed and gut transit. Wegovy's single GLP-1 pathway may cause slightly different digestive effects than Mounjaro's dual GIP+GLP-1 mechanism, but individual variation is significant. Clinical trials don't specifically track stool colour, so direct comparison is limited.
When should I contact my GP about green stools on Mounjaro?
Contact your GP if green stools persist beyond a few weeks, are accompanied by severe abdominal pain, or come with signs of dehydration. Whilst colour changes alone are usually harmless, persistent symptoms deserve assessment to rule out other causes or medication adjustment.
Can I switch from Mounjaro to Wegovy if I'm experiencing digestive side effects?
Switching between medications is possible, but must be managed by your prescriber. Wegovy's different mechanism may result in different digestive effects, though there's no assures. Your prescriber will assess whether a switch is appropriate or whether dose adjustment might help instead.
Do digestive side effects of Mounjaro improve over time?
Yes, most GI side effects improve significantly within 4–8 weeks as your body adapts. Dose titration (gradually increasing the dose over weeks) helps minimise digestive disruption. However, if symptoms persist or worsen, discuss with your prescriber—they may adjust your dose or consider alternative options.
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