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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Mounjaro Loose Stool — UK Information Guide
Loose stool and diarrhoea are among the most frequently reported side effects when starting Mounjaro (tirzepatide) in the UK. This happens because Mounjaro affects how your digestive system works, slowing gastric emptying and altering gut motility. While it can be uncomfortable, loose stool is usually temporary and often improves as your body adjusts to the medication. Understanding what to expect and how to manage it can help you decide if Mounjaro is right for you. Use our free GLP-1 weight loss calculator to explore potential outcomes with tirzepatide before discussing with your prescriber.
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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.
Why Does Mounjaro Cause Loose Stool?
Mounjaro works as a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways that regulate appetite and digestion. One key effect is that it slows how quickly food moves through your stomach and intestines — this is called delayed gastric emptying. When food moves more slowly through your digestive system, your gut bacteria and intestinal environment are disrupted, which can lead to loose stool or diarrhoea. Additionally, tirzepatide may increase gut sensitivity and alter the balance of digestive enzymes.
The gastrointestinal side effects are typically most pronounced in the first few weeks of treatment or when your dose increases. Many people find that loose stool improves significantly after 4–8 weeks as their body adapts to the medication. However, some people experience ongoing mild loose stool even at maintenance doses.
- Loose stool affects more than 1 in 10 people taking Mounjaro
- Diarrhoea is classified as a "very common" side effect in the product information
- Severity often depends on dose, individual metabolism, and diet
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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.
Managing Loose Stool While on Mounjaro
If you experience loose stool on Mounjaro, several practical steps may help. Stay well hydrated — loose stool increases fluid loss, so drinking plenty of water, electrolyte drinks, or herbal tea is important. Adjust your diet to include easily digestible foods like bananas, rice, toast, and cooked vegetables, whilst temporarily reducing high-fat and high-fibre foods that can worsen diarrhoea. Keep a food diary to identify which foods trigger your symptoms.
Your GP or prescriber may also recommend over-the-counter remedies like loperamide (Imodium) if needed, though check with them first as some anti-diarrhoea medications can interact with GLP-1 effects. If loose stool is severe, persistent, or accompanied by dehydration symptoms like dizziness or dark urine, contact your GP immediately.
- 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.
How long does Mounjaro loose stool last?
For most people, loose stool improves within 4–8 weeks as your body adjusts. However, some individuals experience mild loose stool for longer or as a persistent side effect. If it continues beyond 8 weeks or worsens, speak to your GP or prescriber about adjusting your dose or managing the symptom.
Is loose stool dangerous with Mounjaro?
Loose stool itself is not dangerous, but prolonged diarrhoea can lead to dehydration, which is a concern. Watch for signs of dehydration like extreme thirst, dark urine, dizziness, or fatigue. If you experience severe or persistent loose stool, contact your GP promptly to ensure you're managing fluids and electrolytes safely.
Can I take anti-diarrhoea medication with Mounjaro?
Over-the-counter remedies like loperamide may help, but check with your prescriber or GP first. Some anti-diarrhoea medications can interfere with how GLP-1 medications work or mask underlying issues. Your prescriber can recommend safe options based on your individual situation.
Does loose stool mean Mounjaro is working?
Not necessarily. Loose stool is a side effect, not a sign of effectiveness. Weight loss depends on the tirzepatide dose, diet, and activity level — not on experiencing gastrointestinal side effects. Some people lose weight with minimal side effects, whilst others have side effects without as much weight loss.
Should I stop Mounjaro if I have loose stool?
Do not stop without speaking to your prescriber. If loose stool is mild and improving, continuing treatment is often recommended as symptoms typically resolve. If it's severe or unbearable, your prescriber may adjust your dose, slow your dose titration schedule, or explore other options suited to your needs.
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.