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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Understanding Sickness and Diarrhoea with Mounjaro
Nausea and diarrhoea are among the most frequently reported side effects of Mounjaro (tirzepatide) in the UK. These digestive symptoms affect a significant proportion of people taking this prescription weight management injection, particularly when starting treatment or increasing doses. Most cases are mild to moderate and tend to improve over time as your body adjusts. This guide explains what you can expect, practical management strategies, and when to seek advice from your GP.
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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.
How Common Are These Side Effects?
Nausea is reported as "very common" by the MHRA, meaning it may affect more than 1 in 10 people taking Mounjaro. Diarrhoea affects a similar proportion, along with constipation and general stomach discomfort. The Mounjaro KwikPen delivers the medication in precise once-weekly doses, and symptoms often correlate with dose escalation during the titration schedule.
- Nausea: very common, especially in the first 4 weeks
- Diarrhoea: common, typically mild to moderate
- Constipation: also reported, sometimes alternating with diarrhoea
- Stomach pain: less common but can occur
In clinical practice, these symptoms are usually temporary. Most people find that nausea peaks around day 2–3 after injection and settles within a few days. Diarrhoea may persist slightly longer but typically reduces as your body adapts to the medication.
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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
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Managing Symptoms and When to Seek Help
Simple strategies can help manage nausea and diarrhoea whilst taking Mounjaro. Eat smaller, more frequent meals; avoid greasy, spicy, or high-fat foods; stay well hydrated; and take anti-nausea medication if recommended by your GP or pharmacist. Most symptoms settle within 1–2 weeks of starting or increasing your dose.
Contact your GP immediately if you experience severe or persistent stomach pain spreading to your back (possible pancreatitis), persistent vomiting lasting more than a few hours, severe diarrhoea with signs of dehydration, or symptoms that do not improve after 2 weeks. Use our free BMI calculator to track your overall health progress, and our GLP-1 weight loss calculator can help you understand expected outcomes alongside managing these temporary side effects.
- 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.
Will nausea and diarrhoea stop after a few weeks on Mounjaro?
Yes, for most people. Nausea typically peaks in the first few days after injection and settles within 1–2 weeks as your body adjusts. Diarrhoea may persist slightly longer but usually improves by week 2–3. However, individual timelines vary. If symptoms persist beyond 2 weeks or worsen, contact your GP.
Can I take anti-sickness medication alongside Mounjaro?
Yes. Over-the-counter anti-nausea remedies like ginger, peppermint tea, or antihistamine tablets (such as cyclizine) can help. Prescription anti-sickness medication is also available if needed. Always ask your GP or pharmacist before taking any new medication, as some may interact with Mounjaro or affect its absorption.
Does diarrhoea mean Mounjaro is working?
Not necessarily. Diarrhoea is a common side effect of how tirzepatide affects the gut, not a sign of effectiveness. Weight loss results depend on the dose, your diet, physical activity, and individual metabolism. Our GLP-1 weight loss calculator can give you realistic projections based on clinical trial data.
What foods help with nausea when taking Mounjaro?
Bland, easily digestible foods work best: plain rice, boiled potatoes, toast, crackers, chicken broth, and banana. Avoid high-fat, spicy, or greasy foods, caffeine, and alcohol. Eating smaller meals (every 2–3 hours) rather than three large meals also helps reduce nausea significantly.
Should I stop taking Mounjaro if the sickness is unbearable?
Do not stop without speaking to your GP. Stopping suddenly can affect your treatment plan. Instead, contact your GP to discuss your symptoms—they may suggest dose adjustment, temporary pause, or additional support. Severe symptoms are uncommon; most people manage with simple strategies and time.
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.