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 and Diarrhoea vs Wegovy — What's the Difference?
Diarrhoea is a common side effect of both Mounjaro (tirzepatide) and Wegovy (semaglutide), but the frequency and severity can differ between the two medications. Both are GLP-1-based weight loss injections licensed in the UK, though Mounjaro uses a dual GIP and GLP-1 mechanism which may account for some differences in gastrointestinal side effects. This guide compares the two based on clinical trial data to help you understand what prescribers consider when choosing between them for your individual circumstances.
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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.
Clinical Trial Data: Diarrhoea Rates Compared
Diarrhoea was reported in clinical trials for both medications, though the frequency varied by dose and trial population. In the SURMOUNT-1 trial for Mounjaro, diarrhoea was reported as a very common side effect (affecting more than 1 in 10 participants), with rates highest during the initial dose escalation phase and often improving over time as the body adapted. In the STEP trials for Wegovy, diarrhoea was similarly very common, though some trial data suggested slightly different patterns depending on the dose and individual response.
It's important to note that individual experience varies significantly — some people experience no diarrhoea, while others find it more problematic. The mechanism difference between the medications may play a role: Mounjaro activates both GIP and GLP-1 receptors, whilst Wegovy activates GLP-1 only. This dual action in Mounjaro can affect gastric function differently, potentially influencing digestive side effects. Both medications slow how quickly food leaves the stomach, which can trigger diarrhoea, constipation, or nausea as your digestive system adapts.
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Which Is More Likely to Cause Diarrhoea — Individual Factors
Neither medication is assured to cause diarrhoea, and neither is clearly "better" or "worse" in this respect based on published trial data. What matters more is your individual health profile and how your body responds to the medication type. Your prescriber will assess whether Mounjaro or Wegovy is more suitable based on your medical history, other medications, and any previous gastrointestinal conditions. If you have a history of inflammatory bowel disease, irritable bowel syndrome, or gastroparesis, your prescriber may lean towards one medication over the other.
If diarrhoea does occur, it's usually manageable through dietary adjustments (eating smaller, lower-fat meals), staying hydrated, and allowing your body time to adapt as you progress through dose escalation. Most people find gastrointestinal side effects ease after the first few weeks. Your prescriber can adjust your dose schedule or provide guidance if diarrhoea becomes problematic.
- 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 worse with Mounjaro than Wegovy?
Clinical trial data shows both medications cause diarrhoea at similar rates. Individual experience varies significantly — some people experience no diarrhoea on either medication, whilst others find it more problematic. The dual GIP and GLP-1 mechanism in Mounjaro may affect digestion differently than Wegovy's GLP-1-only action, but this doesn't consistently make one "worse" than the other.
How long does diarrhoea last when starting Mounjaro or Wegovy?
Diarrhoea typically appears during the initial dose escalation phase and often improves within 2–4 weeks as your body adapts. By the time you reach your maintenance dose, many people find gastrointestinal symptoms have settled significantly. If diarrhoea persists beyond this, discuss it with your prescriber.
Can I prevent diarrhoea when taking Mounjaro or Wegovy?
You can't prevent it entirely, but you can reduce severity by eating smaller meals, avoiding high-fat foods, staying well hydrated, and eating slowly. Soluble fibre and staying active may also help. Your prescriber can provide personalised dietary guidance when you start the medication.
What other side effects should I expect besides diarrhoea?
Both medications commonly cause nausea, constipation, vomiting, and stomach pain — sometimes alongside or instead of diarrhoea. Other common side effects include fatigue, headache, dizziness, and injection site reactions. These are usually temporary and improve over time as your body adjusts.
How do I decide between Mounjaro and Wegovy if diarrhoea is a concern?
Your UK prescriber will assess your individual health profile, medical history, and suitability for each medication. Neither medication is contraindicated solely because of diarrhoea risk. Use our free GLP-1 weight loss calculator to explore projected outcomes with either medication, then discuss your concerns and preferences with your prescriber who can make the best recommendation for you.
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