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 Stomach Pain and Diarrhoea — What You Need to Know
Stomach pain and diarrhoea are among the most common side effects of Mounjaro (tirzepatide), the prescription-only weight management injection. These digestive symptoms affect many people starting treatment, but they're usually manageable and often improve over time. Understanding what to expect and when to seek medical help is essential for safe use. Our free GLP-1 weight loss calculator can help you estimate potential outcomes while considering these important safety factors.
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Calculate Your BMI Instantly
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.
How Common Are These Side Effects with Mounjaro?
Stomach pain and diarrhoea are classified as "very common" side effects of Mounjaro, meaning they may affect more than 1 in 10 people taking the medication. Nausea, vomiting, and constipation are also frequently reported. These symptoms typically occur during the initial weeks of treatment or when the dose is increased, as your body adjusts to how tirzepatide works on your digestive system.
The medication slows gastric emptying—how quickly food moves from your stomach into your small intestine. This mechanism helps with appetite control but can temporarily disrupt normal digestion. Most people find that symptoms become less severe within a few weeks as their body adapts. However, timing and severity vary significantly between individuals, so your experience may differ from others taking Mounjaro.
- Stomach pain typically described as mild to moderate discomfort
- Diarrhoea often resolves within 2–4 weeks of starting or adjusting dose
- Symptoms may recur briefly when increasing to a new dose strength
- Keeping a symptom diary can help your prescriber understand your individual response
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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.
When to Seek Urgent Medical Help
While mild stomach discomfort is expected, severe or persistent pain can signal a serious condition and requires immediate medical attention. Seek emergency care if you experience severe, sudden stomach pain that radiates to your back—this can indicate pancreatitis, a potentially serious inflammation of the pancreas. Other warning signs include persistent vomiting preventing you from keeping food or fluids down, signs of severe dehydration (extreme dizziness, confusion, rapid heartbeat), or abdominal pain lasting more than a few hours without improvement.
Contact your GP or prescriber immediately if you develop new digestive symptoms that don't fit the typical pattern, such as bloody diarrhoea, severe bloating, or pain that significantly interferes with daily life. Your healthcare provider can assess whether the symptoms are expected side effects or require dose adjustment or medication review. Never discontinue Mounjaro suddenly without medical guidance.
- 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 do stomach pain and diarrhoea typically last when taking Mounjaro?
Most people experience these symptoms most intensely in the first 1–2 weeks after starting or increasing their dose. For many, symptoms improve within 2–4 weeks as the body adjusts. However, individual timelines vary. If symptoms persist beyond 4 weeks, discuss this with your prescriber, as dose adjustment or other strategies might help.
What can I do to manage diarrhoea while taking Mounjaro?
Stay well hydrated by drinking water throughout the day. Eat bland, easily digestible foods (rice, toast, bananas, plain chicken). Avoid high-fat, spicy, or fibre-rich foods temporarily. Over-the-counter treatments like loperamide may help, but always check with your GP or pharmacist before using them alongside Mounjaro, as they could affect medication absorption.
Is stomach pain a reason to stop taking Mounjaro?
Mild stomach discomfort is expected and usually not a reason to stop. However, severe pain requires immediate medical assessment. Your prescriber may recommend dose adjustment, timing changes, or dietary modifications before considering stopping. Always contact your healthcare provider rather than stopping suddenly, as this requires medical supervision.
Can I prevent stomach pain and diarrhoea with Mounjaro?
You can reduce severity by eating smaller, more frequent meals; avoiding high-fat foods; staying hydrated; and eating slowly. Some people find symptoms are worse on an empty stomach. Ginger tea or peppermint may offer mild relief. However, these are adaptations to manage symptoms—some digestive effect is typical with the medication.
How does stomach pain from Mounjaro compare to side effects from other weight loss injections like Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) cause similar digestive side effects including stomach pain and diarrhoea, as both activate GLP-1 pathways. Mounjaro also activates GIP receptors, which may affect digestion differently. Individual tolerance varies. Your prescriber can discuss which medication might suit you better based on your health history and previous medication responses.
Ready to take control of your weight loss journey?
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