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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Managing Sickness and Diarrhoea When Taking Mounjaro
Nausea and diarrhoea are among the most frequently reported side effects of Mounjaro (tirzepatide) in the UK. These digestive symptoms are very common, especially when starting treatment or increasing your dose, but they typically improve over time as your body adjusts. Understanding what to expect and how to manage these side effects can help you stay on your medication safely and effectively.
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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.
Why Mounjaro Causes Nausea and Diarrhoea
Mounjaro works by slowing gastric emptying — the rate at which food moves from your stomach into your small intestine. This deliberate slowdown signals fullness to your brain, helping reduce appetite and support weight loss. However, this slower digestion can cause nausea, diarrhoea, constipation, and stomach discomfort, particularly during the initial weeks of treatment or when your dose increases.
In clinical trials, nausea occurred in more than 1 in 10 people taking Mounjaro, and diarrhoea was equally common. The dose titration schedule — where you start at 2.5 mg and gradually increase to your maintenance dose over several weeks — is specifically designed to allow your digestive system time to adapt. Most people find these symptoms reduce significantly after 4–6 weeks on the same dose.
- Nausea often peaks in the first 1–2 weeks after starting or increasing
- Diarrhoea typically improves as your body adjusts to the medication
- Constipation can alternate with diarrhoea as digestion stabilises
- Stomach pain or indigestion may also occur temporarily
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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 Digestive Side Effects — Practical Strategies
Dietary changes are your first line of defence. Eat smaller, more frequent meals rather than large portions — your stomach cannot handle typical meal sizes while on Mounjaro. Choose bland, easy-to-digest foods: plain rice, boiled vegetables, lean protein, crackers, and plain bread. Avoid fatty, greasy, spicy, and high-fibre foods initially, as these worsen nausea and diarrhoea.
Stay well hydrated — aim for at least 6–8 glasses of water daily, especially if experiencing diarrhoea. Sip small amounts frequently rather than drinking large volumes at once. If nausea persists beyond a few weeks or becomes severe, contact your prescriber or GP — they may recommend anti-nausea medication, adjust your dose timing, or temporarily reduce your dose. Never stop taking Mounjaro without guidance. Your healthcare team can help find strategies tailored to your individual needs and ensure you're using your KwikPen correctly.
- 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 nausea last when starting Mounjaro?
Nausea typically peaks in the first 1–2 weeks after starting or increasing your dose, then gradually improves over 3–6 weeks as your body adjusts. Most people find it manageable with dietary changes and smaller meals. If nausea persists severely beyond 4 weeks, contact your GP.
What foods should I eat to reduce diarrhoea on Mounjaro?
Choose bland, easy-to-digest foods: boiled rice, plain chicken, toast, crackers, and cooked vegetables without skin. Avoid fatty, spicy, high-fibre, and dairy-rich foods temporarily. Eat smaller, frequent meals instead of large portions. Stay hydrated with water and electrolyte drinks if diarrhoea is significant.
Can I take anti-nausea medication while on Mounjaro?
Yes, some anti-nausea medications can be used alongside Mounjaro, but always ask your GP or prescriber first. Ginger tablets, peppermint tea, and small, frequent meals are natural first steps. Your prescriber may recommend medication if nausea is severe or interferes with eating.
What's the difference between Mounjaro and Wegovy side effects?
Both are GLP-1-related medications with similar digestive side effects (nausea, diarrhoea, constipation). Mounjaro is a dual GIP+GLP-1 agonist, while Wegovy is GLP-1 only. Individual response varies — some people tolerate one better than the other. Your prescriber can discuss which suits you best.
When should I contact my GP about Mounjaro side effects?
Contact your GP if nausea or diarrhoea is severe, persistent beyond 4–6 weeks, prevents you eating or drinking, or if you develop severe abdominal pain. These could indicate pancreatitis or another serious condition. Never stop your medication without guidance.
Ready to take control of your weight loss journey?
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