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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Diarrhea on Mounjaro — NHS UK Guide
Diarrhea is one of the most common side effects reported by people taking Mounjaro (tirzepatide) in the UK. Clinical trials showed that gastrointestinal effects, including diarrhoea, affected more than 1 in 10 people using this prescription-only weight management injection. This guide explains why diarrhea occurs, what to expect, and when you should contact your GP. Understanding this side effect can help you manage it effectively during your treatment.
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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.
Why Does Mounjaro Cause Diarrhea?
Mounjaro works by activating GIP and GLP-1 receptors — natural hormone pathways in your gut that regulate appetite and digestion. This dual mechanism helps reduce food cravings and supports weight loss, but it also affects how your stomach and intestines process food.
The tirzepatide in Mounjaro slows gastric emptying (how quickly food moves from your stomach into your intestines). This can initially cause constipation, but as your body adjusts, the opposite effect — diarrhoea — is more commonly reported. Additionally, GLP-1 activation increases secretions in your bowel, which can loosen stools.
- Diarrhea typically occurs within the first few weeks of starting Mounjaro or after dose increases
- It usually improves as your body adapts to the medication
- Diet changes and reduced food intake during weight loss can also contribute
- Individual responses vary significantly — not everyone experiences diarrhea
If diarrhea persists beyond a few weeks or is severe, discuss this with your prescriber or GP, as they may need to adjust your dose or suggest additional support.
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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 Diarrhea and When to Seek Help
Most people find diarrhea improves within 2–4 weeks as their body adjusts to Mounjaro. In the meantime, you can manage symptoms by staying well-hydrated, eating bland foods, and avoiding high-fat or spicy meals that may worsen symptoms.
Contact your GP or prescriber urgently if you experience severe stomach pain, blood in your stools, signs of dehydration, or diarrhea lasting more than a week without improvement. These may indicate a more serious condition requiring medical assessment. Your GP can rule out other causes like infection or food intolerance, and can advise whether continuing Mounjaro is appropriate or if a dose adjustment is needed.
Use our free BMI calculator to track your overall health progress while managing side effects, and our GLP-1 weight loss calculator to understand how Mounjaro may help with your weight management goals alongside managing any gastrointestinal 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 diarrhea last when taking Mounjaro?
Diarrhea typically improves within 2–4 weeks as your body adjusts to Mounjaro. However, individual timelines vary. If it persists beyond this, contact your GP. Some people experience intermittent diarrhea throughout treatment, particularly after dose increases on the KwikPen.
Is diarrhea a serious side effect of Mounjaro?
Diarrhea is very common but usually not serious — it's listed as affecting more than 1 in 10 people in clinical trials. However, severe or prolonged diarrhea can lead to dehydration and electrolyte imbalance. Contact your GP if symptoms are severe or don't improve within a week.
Can I stop taking Mounjaro if diarrhea is too uncomfortable?
Do not stop Mounjaro without speaking to your prescriber or GP first. They can discuss whether to pause, reduce your dose, or manage symptoms differently. Stopping suddenly isn't medically necessary for diarrhea alone, but your healthcare team can advise what's best for you.
What's the difference between diarrhea on Mounjaro and Wegovy?
Both are GLP-1-based medications, but Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy is GLP-1 only. Diarrhea rates are similar in clinical trials. The specific tirzepatide dose titration on Mounjaro may affect gastrointestinal symptoms differently than semaglutide (Wegovy), but both require individual assessment.
Should I change my diet to help with Mounjaro diarrhea?
Yes. Eat smaller, bland meals and avoid high-fat, spicy, or very fibrous foods initially. Stay hydrated with water and electrolyte drinks. As symptoms improve, gradually reintroduce normal foods. Your GP or a dietitian can provide personalised guidance based on your mounjaro nhs uk prescription and individual needs.
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