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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Diarrhoea with Mounjaro — How It Compares to Wegovy
Diarrhoea is a common side effect of both Mounjaro (tirzepatide) and Wegovy (semaglutide) weight management injections. However, the frequency and severity can differ between the two medications due to their different mechanisms of action. This guide explains what the clinical trial data shows, why diarrhoea occurs, and how the two prescription-only weight loss medications compare on this specific side effect. Understanding these differences can help you make an informed decision if your GP or prescriber offers either option during your clinical assessment.
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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.
Diarrhoea Rates — SURMOUNT vs STEP Trial Data
Clinical trial data provides direct comparison between Mounjaro and Wegovy on gastrointestinal side effects. In the SURMOUNT-1 trial (Mounjaro), diarrhoea affected up to 23% of participants at the maximum 15mg dose, whilst 18% experienced it at lower doses. In the STEP-1 trial (Wegovy), diarrhoea occurred in approximately 20–21% of participants at the maximum 2.4mg dose. This difference partly reflects the dual mechanism of Mounjaro — it activates both GIP and GLP-1 receptors, whereas Wegovy activates GLP-1 only. The GIP pathway affects gastric emptying differently, which can influence gastrointestinal symptoms. Importantly, diarrhoea typically appears in the first 4–8 weeks of starting either medication or increasing the dose, and often improves as the body adjusts. Individual experiences vary significantly.
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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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Why Does This Happen and Which Medication Might Suit You?
Both Mounjaro and Wegovy slow how quickly food leaves your stomach (gastric emptying), which is why gastrointestinal symptoms like diarrhoea occur. With Mounjaro's dual GIP+GLP-1 mechanism, the GIP activation adds an extra layer of metabolic effect, which may influence bowel transit slightly differently than Wegovy's single GLP-1 pathway alone. If you experience diarrhoea with one medication, it does not necessarily mean you will with the other — side effect profiles vary between individuals. Your prescriber can discuss your personal tolerance, medical history, and whether dose adjustment, timing of administration, or switching to a different medication might help. Using our free BMI calculator and weight loss projection tool can help you explore eligibility and expected outcomes before speaking to your GP or a licensed prescriber about which option suits you best.
- 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 diarrhoea more common with Mounjaro or Wegovy?
Clinical trial data shows similar rates — approximately 20–23% with Mounjaro at maximum dose and 20–21% with Wegovy. The difference is small and varies between individuals. Both medications slow stomach emptying, which causes gastrointestinal symptoms. Your individual experience depends on genetics, diet, and how your body responds to the medication's mechanism.
Why does diarrhoea happen with these weight loss injections?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) slow gastric emptying — how quickly food moves from your stomach into your intestines. This digestive effect is part of how they work. Diarrhoea or loose stools typically appear in the first 4–8 weeks and often improve as your body adapts. Staying hydrated and eating smaller, simpler meals can help.
Can I switch from Mounjaro to Wegovy if I get diarrhoea?
Switching medications requires a prescriber's decision. If diarrhoea is severe or persistent with one medication, your GP or prescriber may try dose adjustment first. Some people tolerate one medication better than the other due to individual differences. Always discuss side effects with your healthcare provider — do not stop taking your medication without guidance.
How long does diarrhoea last when starting these injections?
Most people experience gastrointestinal side effects in the first 4–8 weeks, especially after starting or increasing doses. Symptoms typically improve as the body adapts. If diarrhoea continues beyond 8 weeks, inform your prescriber — they may adjust your dose, suggest dietary changes, or discuss alternative options suited to your individual circumstances.
Are these medications prescription-only in the UK?
Yes. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only medicines in the UK. Access is through NHS specialist weight management services or private online prescribing following a clinical assessment. Your prescriber will review your medical history, eligibility, and potential side effects before prescribing either medication.
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