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 Ache — What You Need to Know
Stomach ache and abdominal pain are among the most frequently reported side effects of Mounjaro (tirzepatide) in UK users. If you're taking this prescription injection or considering it through NHS tier 3 services or private prescribing, understanding what causes this symptom, how long it typically lasts, and when to contact your GP is essential. Most stomach discomfort improves with time as your body adjusts to the medication, but severe or persistent pain requires urgent medical attention.
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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.
Why Mounjaro Causes Stomach Ache and How to Manage It
Mounjaro works by slowing how quickly food moves through your stomach (delayed gastric emptying) and activating appetite-regulating hormones. This dual GIP and GLP-1 mechanism means your digestive system is working differently than before, which commonly causes mild to moderate stomach discomfort, nausea, and cramping.
- Week 1–4 (2.5 mg starting dose): Stomach ache may be more pronounced as your body adjusts
- Week 5–8 (5 mg escalation): Many users report improvement, though some experience discomfort with each dose increase
- After week 12: For most people, stomach symptoms settle significantly as the body adapts
Management strategies include eating smaller, more frequent meals; choosing bland, easy-to-digest foods; staying well-hydrated; and avoiding fatty or spicy foods. Your prescriber may recommend taking the injection at a different time of day. Using your MyWeightCheck BMI calculator can help track whether your weight loss progress justifies any temporary discomfort as your body adjusts.
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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 Contact Your GP — Serious Stomach Symptoms
While mild stomach ache is expected, severe or worsening abdominal pain requires urgent medical attention. Contact your GP immediately or call 111 if you experience severe pain spreading to your back (possible pancreatitis), persistent vomiting preventing food or fluid intake, signs of gallbladder problems (pain in the upper right stomach area), or pain lasting more than a few days without improvement.
If accessing Mounjaro through NHS specialist services, your tier 3 weight management clinic will have provided emergency contact details. Private prescribers must be informed of any serious symptoms. Never delay seeking medical advice if you're unsure — your GP can assess whether the symptom is expected medication adjustment or requires investigation.
- 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. Stomach discomfort, nausea, and cramping are very common side effects affecting more than 1 in 10 users in clinical trials. Most people find symptoms improve within 2–4 weeks as their body adjusts. However, symptoms can recur with each dose increase during the titration phase. If symptoms persist beyond a few weeks or worsen significantly, contact your GP.
How long does Mounjaro stomach ache last?
Most users report stomach discomfort settling within 2–4 weeks of starting or increasing their dose. However, individual timelines vary. Some people experience minimal symptoms, while others have ongoing mild discomfort. Eating smaller meals, staying hydrated, and avoiding fatty foods can help. If pain persists beyond 4 weeks, discuss this with your prescriber.
Can I stop taking Mounjaro if the stomach ache is too severe?
Do not stop taking Mounjaro without speaking to your prescriber. Stopping abruptly can affect your weight loss progress and underlying health conditions. If stomach pain is severe or unbearable, contact your GP or NHS tier 3 service (if NHS-prescribed) immediately. Your prescriber can discuss whether to pause treatment, reduce your dose, or switch to an alternative medication like Wegovy.
What's the difference between normal Mounjaro stomach ache and a serious problem?
Normal symptoms: mild to moderate cramping, nausea, and discomfort that improve over days or weeks. Serious symptoms requiring urgent attention: severe pain spreading to your back, persistent vomiting preventing food intake, upper right stomach pain (gallbladder), or pain lasting many days without improvement. When in doubt, contact your GP or call 111.
Are there foods that help reduce Mounjaro stomach ache?
Yes. Eat smaller, frequent meals; choose bland foods (rice, plain chicken, toast, bananas); avoid fatty, spicy, or greasy foods; stay hydrated with water or electrolyte drinks; and avoid carbonated beverages. Some users find ginger tea helpful. Your prescriber or GP can refer you to a dietitian for personalised guidance, particularly if accessing treatment through NHS services.
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.