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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Stomach Ache and Mounjaro — What You Need to Know
Stomach pain is one of the most common side effects of Mounjaro (tirzepatide), affecting many people during the first few weeks of treatment. While mild abdominal discomfort is often temporary and tends to improve as your body adjusts, it's important to understand what's typical, what may signal a more serious issue, and when to contact your GP. This guide explains how Mounjaro works, why stomach ache occurs, and what steps you can take to manage it safely throughout your treatment.
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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.
How Mounjaro Causes Stomach Discomfort
Mounjaro works as a dual GIP and GLP-1 receptor agonist, which means it activates two natural hormone pathways in your body. One of its key effects is slowing gastric emptying—how quickly food moves from your stomach into your small intestine. This slower digestion can cause stomach cramping, bloating, and abdominal discomfort, particularly in the first 4–8 weeks of treatment.
Additionally, Mounjaro significantly reduces appetite and may cause nausea, which can lead to eating very little. An empty stomach combined with slowed digestion can trigger cramping and pain. As your body adjusts to the medication—usually after 4–12 weeks—these symptoms often settle. The tirzepatide KwikPen delivers the same medication each week, so side effects typically follow a predictable pattern during dose escalation phases.
- Slowed stomach emptying causes bloating and cramping
- Reduced appetite may mean eating smaller meals irregularly
- Nausea can contribute to digestive discomfort
- Symptoms usually improve after the adjustment period
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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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When Stomach Pain Needs Urgent Attention
Most stomach ache on Mounjaro is mild to moderate and temporary. However, severe, persistent pain spreading to your back can signal pancreatitis (inflammation of the pancreas), which is a serious medical emergency. Seek immediate medical attention if you experience severe upper abdominal pain, especially if accompanied by vomiting, fever, or yellowing of the skin.
Other warning signs requiring urgent GP contact include: pain that worsens despite small meals, signs of gallbladder problems (pain in the upper right abdomen after eating fatty foods), or digestive symptoms lasting more than 3 weeks without improvement. Your GP can assess whether the pain is a normal side effect or requires further investigation. Always report new or worsening symptoms at your next check-in, particularly if they affect your ability to eat or stay hydrated.
- 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 stomach ache normal when starting Mounjaro?
Yes, abdominal discomfort is very common—affecting more than 1 in 10 people in clinical trials. Mild to moderate cramping and bloating typically occur during the first 4–8 weeks as your body adjusts to the medication. These symptoms usually improve significantly or resolve completely over time, especially after reaching maintenance dose.
What can I do to ease stomach pain from Mounjaro?
Eat small, frequent meals of bland, low-fat foods (toast, rice, boiled vegetables). Stay well hydrated with water and electrolyte drinks. Avoid fatty, greasy, or spicy foods that can worsen discomfort. Walk gently after meals to aid digestion. If nausea is significant, take antiemetics as recommended by your GP. Most people find symptoms improve within 4–12 weeks as tolerance builds.
Should I stop taking Mounjaro if I have stomach pain?
Do not stop Mounjaro without discussing it with your prescriber, even if you have stomach discomfort. Stopping abruptly is not recommended and may cause other health effects. Contact your GP or prescriber to discuss your symptoms—they may adjust your dose, manage symptoms with additional medication, or provide dietary guidance to help you continue treatment safely.
How is Mounjaro stomach ache different from serious pancreatitis?
Normal Mounjaro stomach ache is mild to moderate, predictable, and typically improves with small meals and rest. Pancreatitis pain is severe, persistent, and radiates to the back; it's often accompanied by vomiting, fever, or rapid heart rate. If you experience severe pain or these warning signs, seek immediate medical attention. Your GP can run blood tests to rule out pancreatitis if concerned.
Can I use our GLP-1 weight loss calculator if I'm experiencing stomach ache on Mounjaro?
Yes. Our weight loss calculator can help you track projected outcomes and understand realistic expectations for your dose, which may help contextualise your current experience. Knowing that stomach discomfort is a temporary side effect often experienced by others taking Mounjaro can help you stay motivated during the adjustment phase. Use the tool alongside your prescriber's guidance.
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.