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 Belly Ache: What You Need to Know
Stomach pain and belly ache are among the most common side effects of Mounjaro (tirzepatide) in the UK. If you're considering this prescription-only weight management injection, or currently taking it, understanding why belly ache happens, when it's normal, and when to contact your GP is essential. This guide covers Mounjaro side effects in detail, plus the real costs and access routes for getting this medication in the UK through both NHS specialist services and private online prescribing.
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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.
Mounjaro Belly Ache: Causes, Severity & When to Seek Help
Abdominal pain and stomach discomfort are very common when starting Mounjaro or increasing your dose. In clinical trials, belly ache affected more than 1 in 10 people. The pain typically occurs because Mounjaro slows how quickly food moves through your digestive system (gastric emptying), and because it reduces appetite, meaning your stomach contains less food than usual.
Most belly ache is mild and improves within days to weeks as your body adjusts to the medication. However, you should seek immediate medical attention if you experience:
- Severe abdominal pain spreading to your back (possible pancreatitis)
- Pain that doesn't improve after several days
- Pain combined with vomiting or inability to keep food down
- Yellowing of skin or eyes (liver issues)
- Fever with severe stomach pain
Always contact your GP or NHS 111 if you're unsure. Mounjaro should be stopped immediately if pancreatitis is suspected.
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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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Accessing Mounjaro in the UK: NHS vs Private Cost & Eligibility
NHS access: Mounjaro is available through tier 3 specialist weight management services, but availability varies by region and waiting lists can be lengthy. Your GP can refer you, but you'll need a BMI of 30 or above (or 27+ with weight-related health conditions like type 2 diabetes). Use our free BMI calculator above to check if you may be eligible.
Private access: Available through GPhC-registered online prescribing services following a clinical consultation. Costs typically range from £129–£250 per month depending on your dose. A Mounjaro KwikPen contains 4 weekly doses. When comparing Mounjaro vs Wegovy or other GLP-1 options, consider both medication cost and GP follow-up fees. Many private providers include monitoring support.
- 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 belly ache normal when taking Mounjaro?
Yes, abdominal pain and stomach discomfort are very common side effects, affecting more than 1 in 10 people. It usually improves within a few weeks as your body adjusts. However, severe pain spreading to your back may indicate pancreatitis and requires immediate medical attention.
How much does Mounjaro cost privately in the UK?
Private Mounjaro typically costs £129–£250 per month depending on your dose and provider. A single KwikPen (4 weeks of injections) ranges from £130–£200. This is separate from any consultation or monitoring fees. NHS prescribing through specialist services is free but involves waiting lists.
What's the difference between Mounjaro side effects and serious conditions?
Common side effects like mild belly ache, nausea, and diarrhoea usually improve within days. Serious signs requiring urgent GP contact include severe abdominal pain radiating to the back, persistent vomiting, fever, or yellowing of skin—these may indicate pancreatitis or gallbladder issues.
Can I get Mounjaro on the NHS for belly ache or stomach pain?
You cannot get Mounjaro specifically for belly ache—it's licensed for weight management. However, if you experience severe side effects like persistent abdominal pain, your NHS specialist can review whether continuing is appropriate or suggest alternatives.
How do I access Mounjaro through the NHS in the UK?
Ask your GP for a referral to your local tier 3 specialist weight management service. You'll need a BMI of 30 or above (or 27+ with weight-related health conditions). Assessment and prescription happen through the specialist clinic. Availability and waiting times vary significantly by region.
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.