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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Tummy Ache On Mounjaro — What's Normal and When to Worry
Stomach pain and discomfort are common side effects when starting Mounjaro (tirzepatide) in the UK. Most tummy aches are mild to moderate and tend to improve as your body adjusts to the medication over the first few weeks. However, it's important to understand the difference between expected gastrointestinal discomfort and signs that require urgent medical attention from your GP. This guide covers NHS guidance on managing Mounjaro-related stomach pain and what steps to take if you're concerned about your symptoms.
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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.
Understanding Stomach Pain On Mounjaro — NHS Context
Stomach discomfort is one of the most common side effects of Mounjaro, particularly when you first start the medication or increase your dose. In clinical trials, nausea and stomach pain affected more than 1 in 10 people taking the injection. The discomfort occurs because tirzepatide slows how quickly food moves through your digestive system (gastric emptying), which can cause bloating, cramping, and general tummy ache.
NHS guidance suggests that most people experience peak symptoms in the first 1–2 weeks after starting or increasing a dose, with gradual improvement by weeks 3–4. Your body typically adapts to the medication over time. Managing mild stomach pain at home includes eating smaller, more frequent meals; avoiding fatty or greasy foods; staying hydrated; and taking over-the-counter paracetamol if needed. If symptoms persist beyond 4 weeks or worsen significantly, contact your GP for advice.
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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.
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When To Seek Urgent Medical Help — Red Flags
Most tummy aches on Mounjaro are not emergencies, but severe stomach pain can signal a serious condition like pancreatitis or gallbladder problems, which require urgent medical attention. Seek immediate help from A&E or call 999 if you experience: severe, persistent abdominal pain spreading to your back; pain accompanied by persistent vomiting; yellowing of skin or eyes (possible gallbladder issue); or fever with abdominal pain.
Contact your GP urgently (same day) if stomach pain is severe but not emergency-level, you cannot keep food or fluids down for more than a few hours, or pain prevents you from taking your next Mounjaro dose. Your prescriber may need to pause treatment, adjust your dose, or investigate further to rule out other causes.
- 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 pain on Mounjaro normal?
Yes — stomach discomfort is very common (affecting more than 1 in 10 people in trials), especially when starting or increasing doses. Most mild to moderate pain improves within 3–4 weeks as your body adjusts. However, severe pain is not normal and requires GP assessment.
What can I do to reduce tummy ache when taking Mounjaro?
Eat smaller, frequent meals; avoid fatty, greasy, or spicy foods; drink plenty of water; and go slowly with dose increases. Paracetamol can help mild pain. Ginger tea and plain foods like toast, rice, and chicken may settle your stomach. If pain persists beyond a few weeks, ask your GP for additional support.
How long does Mounjaro stomach pain last?
Most people find stomach discomfort peaks in the first 1–2 weeks after starting or increasing their dose, then gradually improves by week 3–4. Some people experience mild ongoing discomfort, but severe pain should not persist. If it does, contact your GP.
When should I call my GP about Mounjaro stomach pain?
Contact your GP same-day if pain is severe, you cannot eat or drink for several hours, or pain prevents you from taking your dose. Call 999 or go to A&E if you have severe pain spreading to your back, persistent vomiting, yellowing of skin/eyes, or fever with abdominal pain.
Can I take paracetamol or ibuprofen with Mounjaro for stomach pain?
Paracetamol is generally safe with Mounjaro. Ibuprofen (NSAID) should be used cautiously — discuss with your GP first, as NSAIDs can sometimes irritate the stomach in people taking GLP-1 medications like tirzepatide. Your prescriber can advise on the safest pain relief option for you.
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