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
Can Mounjaro Give You Diarrhoea?
Yes, diarrhoea is one of the most common side effects of Mounjaro (tirzepatide) in the UK. Clinical trials show it affects many people taking this prescription weight management injection, particularly during the dose titration phase when your body is adjusting to the medication. The good news is that for most people, gastrointestinal side effects like diarrhoea tend to improve over time as your body adapts. Understanding what to expect, how to manage it, and when to seek help from your GP is important if you're considering or already taking Mounjaro through NHS specialist services or private prescribing.
10,000+ calculations today ยท NHS-aligned thresholds ยท 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/mยฒ).
While BMI isn't a perfect measure โ it doesn't account for muscle mass, bone density, or fat distribution โ 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 โ 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 receptors in your gut, which affects how your digestive system functions. This dual mechanism slows gastric emptying โ how quickly food moves from your stomach into your intestines โ and can alter normal bowel movements. Diarrhoea is particularly common during dose titration, when you're gradually increasing your Mounjaro KwikPen dose from 2.5 mg to your maintenance level over several weeks.
- Very common side effect โ affects more than 1 in 10 people in clinical trials
- Usually appears within the first few weeks of starting or increasing your dose
- Often improves as your body adjusts, typically after 4โ8 weeks on a stable dose
- More frequent if you increase doses too quickly or consume high-fat foods
- Related to how the medication affects gut motility and water absorption
Your GP or prescriber should have discussed this risk before issuing your Mounjaro prescription. If diarrhoea is severe, persistent, or accompanied by stomach pain, contact your GP promptly.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care โ 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 โ 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 โ completely free.
Managing Diarrhoea While Taking Mounjaro
Practical management strategies can help reduce diarrhoea whilst taking Mounjaro. Stay well hydrated by drinking water throughout the day to replace lost fluids and electrolytes. Eat smaller, more frequent meals rather than large ones, and avoid high-fat foods which can worsen symptoms. Keep a food diary to identify trigger foods โ common culprits include fried foods, dairy, and high-fibre foods in excess. If diarrhoea persists beyond 8 weeks on a stable dose, contact your GP; they may adjust your dose or suggest over-the-counter remedies like loperamide. Never stop taking Mounjaro without medical advice โ your prescriber needs to manage any dose changes safely.
- 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 โ 5mg โ 7.5mg โ 10mg โ 12.5mg โ 15mg
- GLP-1 receptor agonist
- Up to 15โ17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg โ 0.5mg โ 1mg โ 1.7mg โ 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 | 15โ22.5% | 12โ17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20โ32 weeks | 16โ20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | ยฃ150โยฃ250/month | ยฃ150โยฃ300/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 โ 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 diarrhoea usually last?
For most people, diarrhoea peaks during dose titration and improves within 4โ8 weeks once you reach a stable maintenance dose. Some people experience mild symptoms throughout treatment, while others resolve completely. If diarrhoea persists beyond 8 weeks, contact your GP for advice.
Can I take anti-diarrhoeal medication with Mounjaro?
Over-the-counter remedies like loperamide are generally safe to use alongside Mounjaro if needed, but always inform your GP or prescriber first. They may suggest dietary changes before recommending medication. Never self-treat severe diarrhoea without medical guidance.
Is diarrhoea a reason to stop taking Mounjaro?
Mild to moderate diarrhoea is not usually a reason to stop treatment, as it often improves with time and dietary adjustment. However, if diarrhoea is severe, persistent, or affecting your quality of life significantly, discuss this with your GP or prescriber โ they may adjust your dose or explore alternatives.
Does diarrhoea mean Mounjaro isn't working?
No. Diarrhoea is a side effect of how Mounjaro works on your digestive system, but it's separate from its weight management effects. Many people experience mild diarrhoea whilst still losing weight successfully. Track your progress using MyWeightCheck's free progress tracker to monitor your results.
What's the difference between Mounjaro and Wegovy side effects?
Both are GLP-1 pathway medications with similar gastrointestinal side effects, but Mounjaro has a dual GIP+GLP-1 mechanism. Some people tolerate one better than the other. Your prescriber will assess which is suitable after a clinical assessment โ compare the two using our free GLP-1 weight loss calculator to understand potential outcomes.
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.