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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Yellow Diarrhoea on Mounjaro — What You Need to Know
Yellow diarrhoea can occur as a side effect when taking Mounjaro (tirzepatide), a prescription-only weight management injection used by UK adults. While diarrhoea is a common side effect of this GLP-1 and GIP receptor agonist, yellow or pale stools may indicate changes in how your digestive system is processing food and bile. Understanding what's normal, when to contact your GP, and how to manage digestive symptoms is important for safe treatment. Most cases resolve as your body adjusts, but persistent symptoms need professional guidance.
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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 Diarrhoea Occurs on Mounjaro and What Yellow Stools Mean
Mounjaro works by slowing gastric emptying — how quickly food moves from your stomach through your digestive tract. This can cause diarrhoea, constipation, and changes in stool colour. Yellow or pale diarrhoea specifically may suggest:
- Reduced bile processing as food moves faster through your system
- Fat malabsorption, which can give stools a yellow or greasy appearance
- Increased bowel motility (faster movement) combined with reduced water absorption
- Changes in gut bacteria as your digestive system adjusts to the medication
Yellow diarrhoea is not uncommon in the first 4–8 weeks of Mounjaro treatment or after dose increases. Most people find symptoms improve as their body adapts. However, if stools remain yellow, pale, or clay-coloured for more than a few days, or if you experience severe abdominal pain, you should contact your GP. Pale stools combined with yellowing skin or eyes can signal gallbladder or liver issues, which require urgent assessment.
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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 Seek Medical Help and How to Manage Symptoms
Contact your GP immediately if you experience: severe, persistent stomach pain spreading to your back (possible pancreatitis), yellow skin or eyes alongside yellow stools (possible gallbladder or liver problems), diarrhoea lasting more than 7–10 days, or signs of dehydration (dizziness, dark urine, extreme thirst). For mild diarrhoea, stay well hydrated with water and electrolyte drinks, eat bland foods (rice, toast, bananas), and avoid high-fat or high-fibre foods temporarily. If yellow diarrhoea persists but you have no other warning signs, mention it at your next appointment — your prescriber may adjust your dose or timing. Never stop 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 yellow diarrhoea a normal side effect of Mounjaro?
Yes, diarrhoea is very common on Mounjaro, affecting more than 1 in 10 people. Yellow or pale stools can occur as your digestive system adjusts to the medication's effects on gastric emptying and bile processing. Most cases improve within 4–8 weeks as your body adapts, but persistent symptoms should be reported to your GP.
What's the difference between normal diarrhoea and yellow diarrhoea on Mounjaro?
Normal diarrhoea is frequent loose stools; yellow diarrhoea suggests changes in bile processing or fat absorption. Pale or clay-coloured stools are more concerning and may indicate gallbladder issues, especially if combined with abdominal pain or yellowing skin. This needs GP assessment.
Can I take Mounjaro if I'm worried about diarrhoea side effects?
Many people experience diarrhoea on Mounjaro but manage it successfully. Discuss your concerns with your prescriber before starting. They can advise on dose timing, diet adjustments, and when symptoms typically improve. If diarrhoea becomes unmanageable, your prescriber may adjust your dose or explore alternatives like Wegovy or other weight loss options.
How long does yellow diarrhoea last on Mounjaro?
For most people, diarrhoea improves within 4–8 weeks as the body adapts. Yellow stools may persist slightly longer. However, if symptoms continue beyond 2–3 weeks or worsen, contact your GP. Persistent yellow diarrhoea may need investigation to rule out other causes.
What should I eat if I have yellow diarrhoea on Mounjaro?
Eat bland, easily digestible foods: white rice, plain toast, bananas, boiled potatoes, and lean chicken. Avoid high-fat foods, spicy dishes, dairy, and high-fibre foods temporarily. Stay hydrated with water and electrolyte drinks. Ginger tea or peppermint tea may help settle your stomach. Gradually reintroduce normal foods as symptoms improve.
Ready to take control of your weight loss journey?
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