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 IBS? Mounjaro and Wegovy Compared
Mounjaro (tirzepatide) and Wegovy (semaglutide) can both cause digestive side effects including nausea, diarrhoea, and constipation — but neither directly causes irritable bowel syndrome (IBS). However, if you have pre-existing IBS, these medications may trigger or worsen symptoms. This guide compares digestive effects between the two weight loss injections to help you understand what to expect. Always discuss your medical history with your prescriber before starting either medication, as individual responses vary significantly.
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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.
Digestive Side Effects: Mounjaro vs Wegovy Clinical Data
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) work by activating gut hormone receptors that slow digestion and increase feelings of fullness. This mechanism naturally affects the digestive system, causing common side effects.
Mounjaro's digestive side effects (SURMOUNT-1 trial):
- Nausea — very common (affects more than 1 in 10 people)
- Diarrhoea — very common
- Constipation — very common
- Stomach pain — very common
- Indigestion — common (up to 1 in 10)
Wegovy's digestive side effects (STEP-1 trial):
- Nausea — very common
- Diarrhoea — very common
- Constipation — very common
- Stomach pain — common
- Indigestion — common
Both medications show similar frequency and severity of gastrointestinal symptoms. The main difference lies in their mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 receptor agonist only. Mounjaro's additional GIP pathway may theoretically affect digestion differently, but clinical data does not show substantially different digestive outcomes between the two.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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IBS Risk and Individual Suitability — When to Seek Prescriber Advice
IBS is not a reported side effect of either Mounjaro or Wegovy. However, if you have pre-existing IBS, these medications' digestive effects — particularly slowed gastric emptying and changes in bowel habits — may trigger IBS flares or worsen existing symptoms. This is why disclosing your full medical history, including any history of IBS or inflammatory bowel disease, is essential before starting either medication.
Your prescriber will assess whether a weight loss injection is suitable for you based on your individual health profile. Some people with IBS tolerate these medications well; others experience significant symptom exacerbation. There is no clinical trial data directly comparing IBS outcomes between Mounjaro and Wegovy, so prescriber judgement and individual monitoring are crucial. Starting at the lowest dose and titrating gradually (as both medications require) may help minimise digestive disruption. Use our GLP-1 weight loss calculator to explore medication options, and always discuss IBS concerns with your GP or specialist before proceeding.
- 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.
Does Mounjaro directly cause IBS?
No. Mounjaro does not directly cause irritable bowel syndrome. However, it causes common digestive side effects (nausea, diarrhoea, constipation) that may trigger or worsen existing IBS symptoms. If you have a history of IBS, inform your prescriber before starting treatment.
Is Mounjaro or Wegovy safer for someone with IBS?
Neither medication has proven superiority for IBS sufferers — clinical trials do not directly compare IBS outcomes. Individual responses vary significantly. Your prescriber will assess your specific situation. Starting at the lowest dose and titrating slowly may help. Discuss your IBS history with your GP before treatment.
How long do digestive side effects last on Mounjaro?
Most digestive side effects improve within 2–4 weeks as your body adjusts. However, some persist longer. Dose titration (starting at 2.5 mg and increasing weekly) helps minimise severity. If severe symptoms continue beyond 4 weeks, contact your prescriber — dose adjustment or switching medications may be considered.
Can I take Mounjaro if I have IBS?
Mounjaro can be prescribed to people with IBS, but requires careful prescriber assessment. Your GP will weigh the potential benefits of weight loss against the risk of digestive symptom flares. Pre-existing digestive conditions are not absolute contraindications, but they require informed discussion and monitoring.
What's the difference between Mounjaro and Wegovy for digestive side effects?
Both cause similar digestive side effects at similar frequencies. Mounjaro (dual GIP+GLP-1) and Wegovy (GLP-1 only) show comparable nausea, diarrhoea, and constipation rates in clinical trials. No clinical data shows one is gentler on the gut than the other — prescriber choice depends on overall health fit, not digestive symptom profile alone.
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