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 Diarrhoea?
Yes, diarrhoea is one of the most common side effects of Mounjaro (tirzepatide) in the UK. Clinical trials show diarrhoea affects more than 1 in 10 people taking this prescription weight loss injection. It typically occurs during dose titration when your body is adjusting to the medication. While often mild and temporary, understanding how diarrhoea compares to other GLP-1 medications like Wegovy can help you decide which treatment might suit you best. Always discuss side effect concerns with your GP before starting any weight loss medication.
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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.
Mounjaro Diarrhoea vs Wegovy: Clinical Evidence
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) cause diarrhoea as a very common side effect, affecting more than 1 in 10 users. However, the mechanisms differ slightly. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. This dual action in Mounjaro may explain why gastrointestinal side effects, including diarrhoea, are slightly more prevalent during the SURMOUNT clinical trials compared to Wegovy's STEP trials.
- Mounjaro (SURMOUNT data): Diarrhoea reported in clinical trials as "very common"
- Wegovy (STEP data): Diarrhoea also "very common" but frequency patterns differ slightly
- Onset: Usually appears within the first 4 weeks of starting or during dose titration
- Duration: Often improves as your body adjusts, typically within 2–4 weeks
Your prescriber will monitor your individual response and adjust your Mounjaro KwikPen dose if needed.
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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.
Managing Diarrhoea and When to Seek Help
Most diarrhoea linked to Mounjaro uk prescription use is mild and resolves without stopping treatment. However, practical steps can help: stay well hydrated, avoid fatty or high-fibre foods temporarily, and eat smaller, more frequent meals. If diarrhoea is severe, persistent, or accompanied by stomach pain, contact your GP immediately—this could indicate pancreatitis or other complications requiring urgent attention.
Individual suitability for Mounjaro versus Wegovy depends on your medical history, other medications, and how your body tolerates dose titration. A licensed prescriber will assess your eligibility and side effect risk during your initial clinical assessment. Neither medication is suitable during pregnancy or breastfeeding.
- 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 from Mounjaro permanent?
No. Diarrhoea from Mounjaro is usually temporary, occurring mainly during dose titration or the first few weeks. Most people find it improves as their body adjusts to the medication, typically within 2–4 weeks. If it persists, your prescriber may adjust your Mounjaro dose or recommend dietary changes.
Does Wegovy cause less diarrhoea than Mounjaro?
Both medications cause diarrhoea as a very common side effect. Clinical trial data shows comparable rates, though individual experiences vary. Wegovy's STEP trials and Mounjaro's SURMOUNT trials both reported diarrhoea in more than 1 in 10 participants. Your prescriber can help decide which is more suitable based on your medical history.
What foods help reduce diarrhoea during Mounjaro treatment?
Eat bland, low-fat foods: white rice, plain chicken, toast, and boiled vegetables. Avoid fatty, spicy, and high-fibre foods temporarily. Stay hydrated with water and electrolyte drinks. Small, frequent meals are easier to digest than large ones. If symptoms persist, ask your GP or prescriber for dietary advice specific to your situation.
When should I stop Mounjaro due to diarrhoea?
Do not stop Mounjaro without your prescriber's guidance. Contact your GP if diarrhoea is severe, persistent, or accompanied by severe stomach pain, vomiting, or signs of dehydration. These could indicate complications requiring medical attention. Your prescriber may adjust your dose or recommend temporary dietary changes instead.
How do Mounjaro side effects compare to other weight loss injections?
Mounjaro and Wegovy both cause diarrhoea, nausea, and stomach pain as very common side effects. Mounjaro's dual GIP+GLP-1 mechanism may cause slightly different side effect patterns than Wegovy's GLP-1-only action. Individual tolerance varies significantly. Use our free GLP-1 weight loss calculator to explore how different medications might affect your projected weight loss, and discuss side effect concerns with your prescriber.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.