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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Why Does Mounjaro Cause Diarrhoea?
Diarrhoea is one of the most common side effects of Mounjaro (tirzepatide), affecting many people during the first few weeks of treatment. It happens because Mounjaro slows how quickly food moves through your stomach and changes how your gut processes nutrients. This is usually temporary and often improves as your body adjusts to the medication. Understanding why it occurs and how to manage it can help you stay comfortable while using this prescription-only weight management injection.
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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 works as a dual GIP and GLP-1 receptor agonist, which means it activates two natural hormone pathways in your body. One key effect is that it slows gastric emptying — the rate at which food leaves your stomach and enters your small intestine. This delayed digestion can lead to looser stools or diarrhoea, particularly in the first 4–8 weeks as your body adjusts.
The medication also affects how your gut absorbs water and electrolytes, which contributes to softer stools. Additionally, as your appetite decreases on Mounjaro, you may eat different foods or smaller portions, which can change your bowel habits temporarily. This side effect is listed as "very common" in the UK prescribing information — meaning it may affect more than 1 in 10 people taking the medication.
- Slowed stomach emptying is a temporary effect that usually improves over time
- Changes in appetite may alter the types and amounts of food you eat
- Most people experience improvement within 4–12 weeks as their body adapts
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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 While on Mounjaro
If you experience diarrhoea on Mounjaro, several practical steps can help. Stay well hydrated by drinking plenty of water — diarrhoea increases fluid loss. Eat smaller, more frequent meals rather than large portions, and focus on bland, easy-to-digest foods like rice, toast, bananas, and lean proteins. Avoid fatty, spicy, or high-fibre foods that may worsen symptoms.
Your prescriber may suggest you slow your dose escalation if diarrhoea is severe — doses typically increase every 4 weeks, but timing can be adjusted. Keep a food and symptom diary to identify any trigger foods. If diarrhoea persists beyond 12 weeks, worsens significantly, or is accompanied by severe abdominal pain, contact your GP immediately as this could indicate a different issue. You can also track your symptoms and weight progress using our free progress tracker to monitor improvements as your body adapts.
When to seek medical advice: contact your GP if diarrhoea is severe, persistent beyond 12 weeks, or accompanied by abdominal pain radiating to your back (which could indicate pancreatitis). Your prescriber should know about any ongoing digestive issues so they can adjust your treatment plan if needed. Over-the-counter anti-diarrhoea medications like loperamide may provide temporary relief, but check with your pharmacist first as they can interact with Mounjaro's effects on digestion. Most people find that as their Mounjaro dose stabilises and their body adjusts, diarrhoea improves significantly — usually within 8–12 weeks of starting treatment.
- 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 last?
For most people, diarrhoea improves within 4–12 weeks as your body adjusts to the medication. It's most common during the first few weeks and during dose increases. If it persists beyond 12 weeks or worsens, contact your GP.
Can I take anti-diarrhoea medication with Mounjaro?
Over-the-counter products like loperamide may help temporarily, but speak to your pharmacist first. Because Mounjaro naturally slows digestion, anti-diarrhoea medications need careful use. Your GP can advise on what's safe for you.
Is Mounjaro diarrhoea a sign the medication isn't working?
No. Diarrhoea is a common side effect and doesn't mean Mounjaro isn't working for weight loss. Many people experience diarrhoea while still losing weight effectively. The two are separate effects of how the medication affects your gut.
What foods should I avoid to reduce Mounjaro diarrhoea?
Avoid fatty, spicy, high-fibre, and greasy foods initially. Stick to bland options like rice, toast, lean proteins, and plain vegetables. Once symptoms improve, gradually reintroduce other foods. Smaller, more frequent meals often work better than large portions.
How does Mounjaro diarrhoea differ from Wegovy side effects?
Both Mounjaro and Wegovy (semaglutide) can cause diarrhoea as a very common side effect. However, Mounjaro is a dual GIP/GLP-1 agonist while Wegovy is GLP-1-only, so individual tolerance may vary. Your GP can help determine which is better for you.
Ready to take control of your weight loss journey?
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