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 Sickness and Diarrhoea: What You Need to Know
Nausea and diarrhoea are among the most commonly reported side effects of Mounjaro (tirzepatide) in the UK. These symptoms affect how your digestive system processes food and typically occur early in treatment, particularly when starting the injection or increasing your dose. Understanding why they happen and how to manage them can help you tolerate treatment more comfortably. If you're considering Mounjaro or currently taking it, this guide explains what to expect and when to seek medical advice.
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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.
Why Does Mounjaro Cause Sickness and Diarrhoea?
Mounjaro is a dual GIP and GLP-1 receptor agonist — it works by activating two natural hormone pathways in your body. One key effect is slowing how quickly food leaves your stomach (gastric emptying). This slower digestion can lead to nausea, especially after eating larger meals. Diarrhoea occurs because the medication alters how your gut processes and absorbs nutrients, and this effect is particularly noticeable in the first few weeks of treatment.
These side effects are very common but usually temporary. Clinical data shows nausea and diarrhoea affect more than 1 in 10 people taking Mounjaro. The good news is that many people find symptoms improve as their body adjusts to the medication, often within 2–4 weeks of starting or after dose increases. However, individual experiences vary significantly — some people experience minimal symptoms, while others find them more persistent.
Dose titration (gradually increasing your dose every 4 weeks) is designed to allow your body to adapt. This is why your prescriber starts you on a lower 2.5 mg dose rather than jumping straight to a higher strength.
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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 Sickness and Diarrhoea While Taking Mounjaro
Dietary adjustments are often the most effective first step. Eat smaller, more frequent meals rather than large portions. Avoid greasy, spicy, or high-fat foods — these trigger nausea more easily. Stay hydrated by drinking small sips of water or sugar-free fluids throughout the day, especially if you're experiencing diarrhoea. Ginger tea, crackers, and bland foods can help settle your stomach.
If symptoms persist beyond the first few weeks or become severe, contact your prescriber. They may suggest temporarily pausing dose escalation, adjusting your diet further, or in some cases, prescribing anti-nausea medication. Never stop taking Mounjaro without medical guidance. Keep a food and symptom diary — this helps identify specific triggers and provides useful information for your GP or weight management service when discussing your experience.
- 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.
How long does Mounjaro nausea last?
Nausea typically improves within 2–4 weeks as your body adjusts. However, it may return temporarily when your dose is increased. If nausea persists beyond 4 weeks or is severe, speak to your prescriber — they may adjust your treatment plan or recommend medication to help manage symptoms.
Is it normal to have diarrhoea on Mounjaro?
Yes. Diarrhoea is very common with Mounjaro and affects more than 1 in 10 people. It occurs because the medication slows gastric emptying and alters how your gut absorbs nutrients. Usually it improves within 2–4 weeks, but staying hydrated and eating smaller meals helps manage it.
Can you take anti-nausea medication with Mounjaro?
Possibly, but always check with your prescriber first. Some anti-nausea medications are suitable to take alongside Mounjaro, while others may interact. Your GP or weight management service can recommend safe options if over-the-counter remedies aren't helping.
Should you stop Mounjaro if you have severe diarrhoea or vomiting?
No — do not stop without medical guidance. Contact your prescriber immediately if you experience severe or persistent diarrhoea or vomiting, as you may be at risk of dehydration. They can assess your symptoms and decide whether to pause your dose, adjust timing, or try management strategies.
Does everyone get sickness and diarrhoea on Mounjaro?
No. While these are very common side effects, they're not inevitable. Some people experience only mild symptoms or none at all. Individual tolerance varies based on metabolism, diet, dose, and overall health. Starting at a lower dose and increasing gradually helps your body adapt.
Ready to take control of your weight loss journey?
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