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
Does Mounjaro Cause Diarrhoea?
Yes, diarrhoea is one of the most common side effects reported with Mounjaro (tirzepatide) in the UK. In clinical trials, diarrhoea affected a significant proportion of people taking this prescription-only weight management injection. The good news is that for most people, diarrhoea typically improves over time as your body adjusts to the medication. However, it's important to understand why this happens, how to manage it, and when to seek advice from your GP. This guide explains what you need to know about Mounjaro and diarrhoea in the UK context.
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.
Why Does Mounjaro Cause Diarrhoea?
Mounjaro works by activating GIP and GLP-1 hormone pathways, which help regulate appetite and blood sugar. One effect of this activation is that it slows how quickly food moves through your digestive system — a process called gastric emptying. When food moves more slowly, your bowel can absorb more water, which often leads to constipation initially. However, as your body adjusts, the opposite can occur: your digestive system may process food more quickly than normal, resulting in looser stools or diarrhoea.
Diarrhoea is also linked to how tirzepatide affects your gut bacteria and the way your intestines process nutrients. Additionally, because Mounjaro reduces appetite, some people eat less or change their diet significantly when starting the medication, which can trigger digestive changes including diarrhoea. If you're taking Mounjaro via a Mounjaro KwikPen, the dose escalation schedule also plays a role — diarrhoea is more common during the first few weeks after each dose increase.
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.
Managing Diarrhoea While Taking Mounjaro
Most people find that diarrhoea improves naturally within 2–4 weeks as their body adjusts to each dose of tirzepatide. In the meantime, staying hydrated is essential — drink plenty of water, herbal tea, and electrolyte drinks to replace fluids lost through diarrhoea. Eating bland, easy-to-digest foods like rice, bananas, plain chicken, and toast can help. Avoid high-fat foods, spicy dishes, and excessive fibre initially, as these can worsen symptoms.
If diarrhoea persists beyond 4 weeks, is severe, or significantly affects your quality of life, contact your GP. They may recommend over-the-counter remedies like loperamide (Imodium) or suggest temporarily pausing your dose escalation. Never stop Mounjaro without medical guidance. Track any digestive changes using our free progress tracker alongside your weight loss journey, so you have clear information to share with your healthcare team.
- 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 diarrhoea a common side effect of Mounjaro in the UK?
Yes, diarrhoea is very common — affecting more than 1 in 10 people taking Mounjaro. It's one of the most frequently reported gastrointestinal side effects, along with nausea and constipation. Most people experience it during dose escalation phases and find it improves as their body adjusts to tirzepatide.
How long does Mounjaro-related diarrhoea last?
For most people, diarrhoea improves within 2–4 weeks of starting or increasing a dose. However, duration varies — some people experience it for longer. If diarrhoea persists beyond 4 weeks or is severe, contact your GP for personalised advice and management strategies.
Can I take anti-diarrhoea medication while on Mounjaro?
Yes, over-the-counter remedies like loperamide (Imodium) are generally safe to use alongside Mounjaro. However, always inform your GP or prescriber that you're taking them. Avoid using these medicines for prolonged periods without medical advice, as they can mask more serious issues.
Should I stop taking Mounjaro if I have diarrhoea?
No — do not stop Mounjaro without medical guidance. Diarrhoea usually improves naturally. Contact your GP if it's severe, persistent, or affecting your daily life. Your prescriber may suggest temporary adjustments rather than stopping the medication entirely.
Is diarrhoea worse with higher Mounjaro doses?
Gastrointestinal side effects, including diarrhoea, are often more pronounced when you first start Mounjaro or increase to a higher dose — such as moving from 5mg to 7.5mg via your KwikPen. Symptoms typically settle within weeks. If you're considering comparing treatment options, our GLP-1 weight loss calculator can help you understand different dosing approaches.
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.