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 Cause Colitis?
Colitis (inflammation of the colon) is not listed as a common side effect of Mounjaro (tirzepatide) in UK clinical trials or approved prescribing information. However, Mounjaro does cause gastrointestinal side effects like nausea, diarrhoea, and stomach pain in some users. If you experience severe abdominal symptoms while taking Mounjaro, you should contact your GP immediately to rule out colitis or other serious conditions. This guide explains what we know about Mounjaro's GI effects and when to seek medical advice.
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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.
What GI Side Effects Does Mounjaro Actually Cause?
Mounjaro's most common gastrointestinal side effects include:
- Nausea (very common — affects more than 1 in 10 users)
- Diarrhoea (very common)
- Vomiting (very common)
- Constipation (very common)
- Stomach (abdominal) pain (very common)
- Indigestion and acid reflux (common)
These side effects occur because Mounjaro, a dual GIP and GLP-1 receptor agonist, slows how quickly food moves through your digestive system and affects appetite regulation. For most users, GI symptoms are mild to moderate and improve as the body adjusts over the first few weeks. If diarrhoea or stomach pain is severe or persistent beyond 4 weeks, contact your GP — they may adjust your dose or consider alternative treatment.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
When to Contact Your GP About Mounjaro GI Symptoms
Seek immediate medical attention if you experience:
- Severe, persistent abdominal pain spreading to your back (possible pancreatitis)
- Bloody diarrhoea or signs of bleeding
- Fever and severe diarrhoea together (infection risk)
- Inability to keep down food or liquids for more than a few hours
- gradual weight management beyond expected rate (more than 1.5 kg per week)
While colitis specifically is rare with Mounjaro, these symptoms warrant urgent assessment. Your GP can investigate whether symptoms are dose-related, require dose adjustment, or indicate a separate medical condition. You can also use our GLP-1 weight loss calculator to discuss expected outcomes and timeline with your prescriber, which may help contextualise side effect management.
- 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.
Is colitis a known side effect of Mounjaro in the UK?
Colitis is not listed as a common or uncommon side effect of Mounjaro (tirzepatide) in UK prescribing information or clinical trials. However, severe gastrointestinal symptoms like persistent diarrhoea or abdominal pain should always be evaluated by your GP to rule out colitis or other conditions.
How does Mounjaro cause diarrhoea and stomach pain?
Mounjaro slows gastric emptying (how quickly food leaves the stomach) and affects digestive hormone signalling. This typically causes nausea, diarrhoea, and mild stomach discomfort, especially during dose increases. These effects often improve within 4 weeks as your body adjusts.
What's the difference between normal Mounjaro side effects and colitis symptoms?
Normal Mounjaro diarrhoea is usually mild to moderate, watery, and improves within days to weeks. Colitis causes bloody diarrhoea, mucus in stools, fever, and severe cramping pain. If you experience blood in stool or fever with diarrhoea, contact your GP immediately.
Can I continue Mounjaro if I have severe diarrhoea?
Severe diarrhoea may require a dose reduction or temporary pause while your body adjusts. Contact your prescriber before stopping Mounjaro — they can assess whether symptoms are dose-related and adjust your KwikPen dose accordingly or suggest management strategies.
Should I get tested for colitis if I have Mounjaro side effects?
If you experience only mild diarrhoea and stomach pain alongside nausea (typical Mounjaro symptoms), testing is usually not needed. However, if symptoms are severe, include blood in stool, or persist beyond 4 weeks despite dose stabilisation, your GP may recommend investigations to rule out inflammation or infection.
Ready to take control of your weight loss journey?
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