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 — What You Need to Know
Loose stool (diarrhoea) is one of the most frequently reported side effects of Mounjaro (tirzepatide) in the UK. While it can be uncomfortable, it's usually temporary and often improves as your body adjusts to the medication. Understanding why it happens, how long it typically lasts, and practical ways to manage it can help you decide whether Mounjaro is right for you. This guide covers everything UK adults need to know about loose stool on Mounjaro, including NHS guidance and when to contact your GP.
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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.
Why Does Mounjaro Cause Loose Stool?
Mounjaro (tirzepatide) works as a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways in your body. One of these effects is slowing down gastric emptying—how quickly food moves from your stomach into your small intestine. This slower movement, combined with increased gut sensitivity and changes to how your digestive system processes food, can lead to loose stool or diarrhoea.
Loose stool typically appears in the first few weeks of starting Mounjaro or when increasing to a higher dose. The good news is that most people find it improves significantly within 2–4 weeks as their digestive system adapts. It's classified as a "very common" side effect by the UK medicines regulator (MHRA), meaning it affects more than 1 in 10 people.
- More common in the first 4 weeks after starting or dose escalation
- Usually mild to moderate, not severe
- Often accompanied by other digestive symptoms (nausea, constipation, indigestion)
- Individual severity varies widely
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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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Managing Loose Stool While Taking Mounjaro
If you're experiencing loose stool on Mounjaro, there are practical steps you can take to manage it safely:
- Stay hydrated—Drink plenty of water and electrolyte drinks (coconut water, diluted sports drinks) to replace fluids lost through loose stool
- Adjust your diet—Eat bland, easily digestible foods (white rice, bananas, plain chicken, toast). Avoid high-fat, spicy, or high-fibre foods temporarily
- Take anti-diarrhoeal medication carefully—Over-the-counter options like loperamide (Imodium) can help, but avoid using them long-term without consulting your GP
- Contact your GP if loose stool persists beyond 4 weeks, becomes severe, or is accompanied by abdominal pain, blood in stools, or signs of dehydration
Your GP or prescriber may suggest adjusting your Mounjaro dose schedule or temporarily pausing dose escalation if loose stool is severe. Never stop taking Mounjaro without professional guidance.
If you're experiencing loose stool on Mounjaro, there are practical steps you can take to manage it safely:
- Stay hydrated—Drink plenty of water and electrolyte drinks (coconut water, diluted sports drinks) to replace fluids lost through loose stool
- Adjust your diet—Eat bland, easily digestible foods (white rice, bananas, plain chicken, toast). Avoid high-fat, spicy, or high-fibre foods temporarily
- Take anti-diarrhoeal medication carefully—Over-the-counter options like loperamide (Imodium) can help, but avoid using them long-term without consulting your GP
- Contact your GP if loose stool persists beyond 4 weeks, becomes severe, or is accompanied by abdominal pain, blood in stools, or signs of dehydration
Your GP or prescriber may suggest adjusting your Mounjaro dose schedule or temporarily pausing dose escalation if loose stool is severe. Never stop taking Mounjaro without professional 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.
How long does loose stool last on Mounjaro in the UK?
Most people experience loose stool for 2–4 weeks after starting Mounjaro or increasing their dose. It often resolves as your digestive system adapts. If it persists beyond 4 weeks or worsens, contact your GP for personalised advice.
Is loose stool a sign that Mounjaro is working?
No, loose stool is a side effect of how Mounjaro affects your digestive system, not evidence that it's working for weight loss. Weight loss results from the medication's appetite-suppressing effects, not from loose stool. Individual experiences vary widely.
Can I take anti-diarrhoeal medication like Imodium with Mounjaro?
Over-the-counter anti-diarrhoeal medicines like loperamide can help in the short term, but avoid regular use without consulting your GP. Some anti-diarrhoeals can interact with medications or mask more serious issues. Always mention Mounjaro when discussing over-the-counter options with your pharmacist.
Should I stop taking Mounjaro if I have loose stool?
No—never stop Mounjaro without professional guidance. Contact your GP or prescriber instead. They may recommend dietary adjustments, temporary dose pausing, or slower dose escalation. Loose stool often improves naturally within a few weeks.
When should I contact my GP about loose stool on Mounjaro?
Contact your GP immediately if loose stool is accompanied by severe abdominal pain, blood in stools, signs of dehydration (extreme thirst, dark urine, dizziness), or if it persists beyond 4 weeks despite dietary management.
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