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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Gastroenteritis and Mounjaro — UK Health Guide
Mounjaro (tirzepatide) is a prescription-only weight management injection that can cause digestive side effects, including nausea, vomiting, diarrhoea, and constipation. Some patients report symptoms similar to gastroenteritis. This guide explains how Mounjaro affects digestion, which side effects are normal, when to seek medical attention, and how to manage gastrointestinal symptoms whilst taking this medication. Always contact your GP if symptoms persist or worsen.
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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 Mounjaro Affects Digestion
Mounjaro works by activating GIP and GLP-1 receptor pathways, which slow how quickly food moves through your stomach — a process called gastric emptying. This mechanism is intentional and helps create the feeling of fullness after eating smaller meals. However, this same action can trigger digestive symptoms.
Common gastrointestinal side effects include:
- Nausea (very common, particularly when starting or increasing doses)
- Diarrhoea or constipation (can alternate)
- Stomach pain or cramping
- Vomiting (less common but possible)
- Indigestion or acid reflux
- Loss of appetite
These symptoms typically improve over time as your body adjusts to the medication, usually within 2–4 weeks of starting or dose escalation. Most side effects are mild to moderate, though some patients report more severe symptoms during initial weeks.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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When to Seek Medical Attention
Whilst mild nausea and changes in bowel habits are expected with Mounjaro, contact your GP immediately if you experience severe, persistent stomach pain that spreads to your back (sign of possible pancreatitis), vomiting lasting more than a few hours, blood in vomit or faeces, signs of dehydration (extreme thirst, dark urine, dizziness), or symptoms that do not improve after 4 weeks. Do not stop taking Mounjaro without medical advice. If you suspect gastroenteritis from another cause (food poisoning, infection), inform your GP that you are taking Mounjaro, as this affects management. Use our free GLP-1 weight loss calculator to track your progress and discuss side effects with your prescriber at your next appointment.
- 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 gastroenteritis caused by Mounjaro or a real infection?
Mounjaro can cause digestive symptoms that feel like gastroenteritis (nausea, vomiting, diarrhoea), but these are usually direct side effects of the medication rather than an infection. True gastroenteritis involves fever, sudden onset, and usually resolves within days. Mounjaro-related symptoms develop during dose escalation and persist. Contact your GP if unsure — they can assess whether symptoms are medication-related or infectious.
How long do Mounjaro digestive side effects last?
Most nausea and digestive side effects improve within 2–4 weeks of starting or increasing your dose as your body adapts. Some patients experience ongoing mild symptoms, whilst others see resolution within days. Everyone is different. If side effects are severe or prolonged beyond 4 weeks, discuss this with your prescriber — dose adjustment or slower escalation may help.
Can I take anti-sickness medication with Mounjaro?
Yes, anti-sickness medications (antiemetics) like ondansetron can be used alongside Mounjaro if nausea is significant. Always inform your prescriber about any other medications you take, as some can interact with Mounjaro or affect how it works. Your GP or pharmacist can recommend safe options. Avoid over-the-counter remedies without checking first.
Should I stop taking Mounjaro if I feel unwell?
Never stop Mounjaro without medical advice, even if feeling unwell. Stopping suddenly can cause problems. Contact your GP or your prescriber instead — they can assess your symptoms, confirm whether they are medication-related, and advise whether dose adjustment or temporary pausing is needed. Most digestive side effects improve without stopping the medication.
What's the difference between Mounjaro side effects and actual food poisoning?
Mounjaro side effects develop predictably after starting or escalating dose, whereas food poisoning or viral gastroenteritis usually comes on suddenly with fever and abdominal cramping. Mounjaro nausea is usually worse in the first days after injection; food poisoning typically resolves within 24–72 hours. If you have sudden fever, severe cramps, or symptoms appearing unrelated to your injection timing, contact your GP to rule out infection.
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