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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Can Mounjaro Cause Diarrhoea in the UK?
Yes, diarrhoea is one of the most common side effects of Mounjaro in the UK. Clinical trials and real-world prescribing data show that diarrhoea affects a significant proportion of people taking tirzepatide. The good news is that for most people, this side effect is temporary and improves as your body adjusts to the medication. This guide explains why Mounjaro causes diarrhoea, what to expect, and when to speak to your GP.
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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 (tirzepatide) is a dual GIP and GLP-1 receptor agonist that works by slowing how quickly food moves through your digestive system. This mechanism, called delayed gastric emptying, is essential for appetite regulation — but it can cause digestive side effects including diarrhoea, constipation, and nausea.
Diarrhoea occurs because the changes to your digestive transit time, combined with the medication's effects on gut hormone signalling, can alter how your body processes food and absorbs water. In clinical trials and real-world NHS prescribing experience, diarrhoea is classified as a very common side effect — meaning it may affect more than 1 in 10 people taking Mounjaro.
Important: diarrhoea is not a sign that Mounjaro is harming you. It is a known, expected side effect that typically improves as your body adjusts during the dose titration phase. Most people find that diarrhoea peaks in the first few weeks and then resolves as they stabilise on their maintenance dose.
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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
If you experience diarrhoea while taking Mounjaro, keep a food diary to identify any dietary triggers — spicy foods, high-fat meals, and lactose can worsen symptoms. Stay well hydrated by drinking extra water, and consider eating smaller, more frequent meals rather than large portions.
Most cases of Mounjaro-related diarrhoea improve within 2–4 weeks as your body adjusts. However, contact your GP if diarrhoea is severe, persistent beyond 4 weeks, accompanied by bloody stools, severe stomach pain, or signs of dehydration (dizziness, dark urine, extreme thirst). Your GP can assess whether the diarrhoea is medication-related or due to another cause, and may suggest dose adjustment or alternative management strategies.
- 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 peaks in the first 1–2 weeks after starting Mounjaro or increasing your dose, then improves over the following 2–4 weeks as your digestive system adjusts. Some people experience mild, intermittent diarrhoea throughout treatment. If diarrhoea persists beyond 4 weeks, speak to your GP.
Is Mounjaro diarrhoea dangerous?
Occasional or mild diarrhoea is not dangerous — it's an expected side effect. However, severe or prolonged diarrhoea can lead to dehydration and electrolyte imbalance. If you have severe diarrhoea, bloody stools, or signs of dehydration, contact your GP or call NHS 111 for advice.
Can I stop Mounjaro if I have diarrhoea?
Do not stop Mounjaro without speaking to your GP first. Stopping abruptly can cause other problems. Your GP can help manage the diarrhoea through dietary changes, hydration, or medication adjustments. If the side effect is truly unbearable, your prescriber may discuss reducing your dose or switching to an alternative.
Does diarrhoea mean Mounjaro is working?
No — diarrhoea is a side effect, not a sign of effectiveness. Weight loss with Mounjaro is determined by the medication's appetite-suppressing action combined with your dietary choices and physical activity. Diarrhoea and weight loss are independent. Some people lose weight with minimal diarrhoea; others experience diarrhoea without significant weight loss.
Can dietary changes help with Mounjaro diarrhoea?
Yes. Avoid high-fat, spicy, and lactose-rich foods which commonly trigger diarrhoea. Eat smaller, frequent meals; stay hydrated; include soluble fibre (oats, bananas); and eat slowly. If diarrhoea persists despite dietary changes, ask your GP about anti-diarrhoeal medication or further dose management. Use our GLP-1 weight loss calculator to track your progress while managing side effects.
Ready to take control of your weight loss journey?
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