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
Why Does Mounjaro Cause Diarrhoea?
Diarrhoea is one of the most common side effects of Mounjaro (tirzepatide), affecting many people during the early weeks of treatment. It occurs because of how tirzepatide works in your body — specifically by slowing the speed at which food moves through your digestive system and changing how your gut responds to meals. Understanding why this happens can help you manage it effectively and know when to seek advice from 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.
How Mounjaro Affects Your Digestion
Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways in your body. One key effect is that it slows gastric emptying, which means food moves more slowly from your stomach into your small intestine. This slower movement can alter normal bowel function and cause looser stools or diarrhoea, especially in the first few weeks after starting tirzepatide or increasing your dose.
Additionally, Mounjaro changes how your gut responds to food and fluid, and may affect the balance of bacteria in your digestive system. These combined effects on your gastrointestinal tract are why diarrhoea is listed as a very common side effect — affecting more than 1 in 10 people during clinical trials. The effect is usually temporary as your body adjusts to the medication over time.
It's worth noting that other GLP-1 injections like Wegovy (semaglutide) work through a single mechanism and can also cause diarrhoea, though Mounjaro's dual action may affect digestion differently for some people.
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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.
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Managing Diarrhoea and When to Seek Help
Most people find that diarrhoea improves within the first 2–4 weeks as their body adjusts to Mounjaro. In the meantime, staying well hydrated is essential — drink plenty of water and consider electrolyte drinks if diarrhoea is frequent. Eating smaller, more regular meals rather than large meals, and avoiding fatty or high-fibre foods initially, may help reduce symptoms.
Contact your GP if diarrhoea is severe, lasts longer than a few weeks, or is accompanied by abdominal pain, blood in your stool, or signs of dehydration (dizziness, dark urine, extreme thirst). Your GP can assess whether the dose needs adjusting or whether continuing treatment is appropriate for you. Use our free BMI calculator and progress tracker to monitor your overall health journey while managing side effects.
- 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 diarrhoea usually last?
Most people experience diarrhoea for 1–4 weeks after starting tirzepatide or increasing their dose. It typically improves as your body adapts to the medication. If it persists beyond 4 weeks or worsens, contact your GP for advice about dose adjustment or continuation.
Is diarrhoea dangerous, and should I stop taking Mounjaro?
Occasional diarrhoea is a common side effect and not dangerous for most people. However, severe or persistent diarrhoea can lead to dehydration. Never stop taking Mounjaro without consulting your GP — they can assess your symptoms and decide whether to adjust your dose or continue treatment.
Can I reduce diarrhoea by changing my diet while taking Mounjaro?
Yes. Eat smaller, more frequent meals; stay well hydrated; and temporarily reduce high-fat and high-fibre foods. Include bland, easy-to-digest options like plain rice, banana, and toast. Keep a food diary to identify triggers. Your GP or a dietitian can offer personalised dietary guidance.
Is Mounjaro diarrhoea the same as diarrhoea from other weight loss injections?
GLP-1 injections like Wegovy also commonly cause diarrhoea through similar digestive mechanisms. Mounjaro's dual GIP and GLP-1 action may affect some people differently. Individual experiences vary — discuss any concerns about side effects with your prescriber.
When should I contact my GP about Mounjaro diarrhoea?
Contact your GP if diarrhoea is severe, persists beyond 4 weeks, includes blood or mucus, or is accompanied by abdominal pain, fever, or signs of dehydration. Your GP can ensure the diarrhoea isn't caused by another condition and adjust your Mounjaro treatment if needed.
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.