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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Does Mounjaro Give You Diarrhea?
Yes, diarrhoea is a common side effect of Mounjaro (tirzepatide), affecting many people taking this weight loss injection. Both Mounjaro and Wegovy can cause digestive side effects, though they work slightly differently. This guide explains what the evidence shows, how the two medications compare, and what to expect if you're considering prescription weight loss treatment in the UK. Use our GLP-1 weight loss calculator to explore potential outcomes with different medications.
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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 vs Wegovy: Diarrhoea and Digestive Side Effects
Diarrhoea is one of the most common side effects of both Mounjaro and Wegovy. According to clinical trial data and real-world reports, approximately 20–40% of people taking these medications experience loose stools or diarrhoea at some point during treatment.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. In the SURMOUNT clinical trials, diarrhoea was reported in around 23% of people taking the 15 mg dose. Wegovy (semaglutide) is a GLP-1-only medication. In the STEP trials, diarrhoea affected roughly 20% of participants on the 2.4 mg maintenance dose.
The mechanisms are different—Mounjaro's dual action affects both appetite hormones and gut function slightly differently than Wegovy's single GLP-1 pathway—but both can slow gastric emptying and alter bowel habits. Diarrhoea typically appears during dose escalation and often improves as your body adjusts, usually within weeks.
- Mounjaro: digestive side effects more common during initial weeks and dose increases
- Wegovy: similar pattern, typically mild to moderate severity
- Both: often resolve without stopping treatment
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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.
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Managing Diarrhoea and Choosing the Right Medication for You
If you experience diarrhoea on either Mounjaro or Wegovy, practical steps include staying hydrated, eating smaller meals, avoiding high-fat foods, and discussing symptoms with your prescriber—they may adjust your dose or timing. A Mounjaro KwikPen is taken once weekly, so timing consistency can help your body adapt.
Neither medication is "better" for avoiding diarrhoea—individual responses vary significantly. Your prescriber will assess your medical history, current medications, and personal tolerance during the clinical assessment before recommending Mounjaro, Wegovy, or neither. If you have a history of inflammatory bowel disease or severe digestive conditions, this is important to disclose. Calculate your potential weight loss outcomes using our free GLP-1 calculator, then discuss the realistic benefits and side effects with your GP or specialist. Both medications require a prescription and ongoing clinical review.
- 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 trials show similar rates—roughly 20–23% across both medications. Individual experiences vary significantly. Some people have no diarrhoea, others experience it noticeably. Your prescriber can help predict likelihood based on your health profile and adjust dosing if needed.
How long does diarrhoea last when starting Mounjaro?
Most people find diarrhoea improves within 2–4 weeks as their body adjusts, especially if it appears during dose escalation. If it persists beyond 4 weeks or is severe, inform your prescriber—dose timing or adjustment may help.
Can I prevent diarrhoea while taking Mounjaro or Wegovy?
You cannot fully prevent it, but you can reduce severity: stay hydrated, eat smaller meals, avoid high-fat foods, and take the injection on the same day each week. Your prescriber may also recommend anti-diarrhoeal medication if symptoms are problematic.
Does diarrhoea mean I should stop taking Mounjaro?
Not necessarily. Most people continue treatment as diarrhoea typically improves. However, if it's severe or persistent, speak to your prescriber about dose timing, meal adjustments, or medication changes—stopping abruptly is not recommended.
How do I know if Mounjaro or Wegovy is right for me?
Both require a clinical assessment from a UK-licensed prescriber via NHS specialist services or private online consultation. They'll review your BMI, health conditions, medication interactions, and digestive history to recommend the most suitable option for you.
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.