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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Mounjaro vs Wegovy: Digestive Side Effects Compared
Both Mounjaro and Wegovy can cause digestive changes including constipation, diarrhoea, and stomach discomfort — but the severity and frequency may differ between individuals. Understanding how these medications work differently, and which digestive effects are most common with each, can help you and your prescriber decide which is right for you. This guide compares both medications side-by-side based on clinical trial data and real-world experience from UK patients.
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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 and Wegovy Affect Digestion Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways in the gut. This dual action may explain why some patients report more pronounced initial digestive changes. In the SURMOUNT-1 clinical trial, nausea occurred in around 25% of participants on the highest dose (15 mg), whilst diarrhoea affected approximately 23%. These effects typically improve as the body adjusts over weeks.
Wegovy (semaglutide) activates the GLP-1 pathway only — a single mechanism. In the STEP-1 trial, nausea occurred in around 25% of patients on the maximum dose (2.4 mg), and diarrhoea in approximately 21%. The incidence is comparable, but the mechanism difference may mean the digestive pattern feels slightly different for some users.
Both medications slow how quickly food leaves your stomach (gastric emptying), which explains the nausea and constipation. Use our GLP-1 weight loss calculator to explore whether either medication might be suitable for your situation, and discuss digestive concerns directly with your prescriber before starting.
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Which Is More Likely to Cause Constipation or Diarrhoea?
Constipation is actually more common than diarrhoea with both medications, though diarrhoea does occur. The SURMOUNT and STEP trials show constipation rates of 20–30% on both Mounjaro and Wegovy, whilst diarrhoea affects 15–23%. Individual experience varies enormously — some patients never experience either, whilst others find one dominant issue.
Staying hydrated, increasing fibre gradually (not suddenly, as this can worsen nausea), and gentle movement may help manage constipation. If diarrhoea occurs, your GP can advise on dietary adjustments or temporary anti-diarrhoeal medication. Most digestive side effects settle within 4–12 weeks as your body adapts. Your prescriber will monitor you during the dose-titration phase and can adjust your dose schedule if digestive effects are severe.
- 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.
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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 cause more poop issues than Wegovy?
Clinical trials show comparable rates of constipation and diarrhoea between both medications. Mounjaro's dual mechanism may feel slightly different to some patients, but overall incidence is similar. Individual responses vary significantly — your experience will be unique to you.
How long do digestive side effects last on these medications?
Most digestive changes peak in the first 2–4 weeks after starting or increasing a dose, then gradually improve over 4–12 weeks as your body adjusts. Some patients experience minimal symptoms, whilst others take longer to adapt. Speak to your prescriber if effects are severe or persistent.
Which medication is better if I have IBS or a sensitive stomach?
Neither medication is specifically safer for IBS — both slow gastric emptying and can trigger digestive changes. Your prescriber will assess your medical history during the clinical assessment. If you have IBS, discuss this openly with your healthcare provider before starting any GLP-1 or GIP/GLP-1 medication.
Can I take constipation medication with Mounjaro or Wegovy?
Yes, many patients take mild laxatives or stool softeners alongside these medications. Always inform your prescriber of any other medications or supplements you're using, as some can interfere with absorption. Your GP can recommend suitable options for your situation.
Why do these medications affect digestion at all?
Both medications slow how quickly food leaves your stomach and activate gut hormone pathways that regulate appetite and digestion. This is how they help with weight management, but it also causes nausea, constipation, and other digestive effects. These typically reduce as your body adjusts to the medication.
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