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) in the UK. Understanding these effects, how long they typically last, and your options for accessing Mounjaro through NHS or private prescription is essential for making an informed decision. This guide covers what causes these side effects, cost context, and how to get Mounjaro in the UK.
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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.
Stomach Pain And Diarrhoea With Mounjaro — NHS And Private Access
Stomach pain and diarrhoea affect many people starting Mounjaro, particularly during the dose titration phase when the body is adjusting to the medication. These are classified as "very common" side effects, occurring in more than 1 in 10 people. Stomach pain is often mild to moderate, though some people experience more severe abdominal discomfort. Diarrhoea typically improves as your body adjusts, usually within 4–8 weeks, though it can persist longer in some cases.
Why this happens: Mounjaro slows gastric emptying (how quickly food leaves your stomach) and alters digestive function. This is part of how the medication works to reduce appetite, but it causes temporary digestive disturbance. Staying hydrated, eating smaller meals, and avoiding high-fat foods can help manage symptoms whilst your body adjusts.
If stomach pain is severe or persistent beyond 8 weeks, contact your GP — this may indicate pancreatitis or gallbladder issues requiring urgent assessment.
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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 — Cost And Route Options
In the UK, you can access Mounjaro through two main routes: NHS specialist services or private prescription. NHS access is through tier 3 weight management services following GP referral, though waiting lists vary significantly by region and can be several months. NICE has approved Mounjaro for NHS use, but availability depends on your local integrated care board (ICB).
Private prescription involves an online consultation with a UK-licensed prescriber, typically costing £120–£180 for the consultation. Mounjaro itself varies by dose and provider but generally ranges from £150–£280 per pen (4-week supply). The Mounjaro KwikPen is prescribed at increasing doses over time, so costs build as you titrate upwards. Both routes require clinical assessment to confirm eligibility.
Use our free GLP-1 weight loss calculator to estimate potential outcomes with Mounjaro at different doses, and our BMI calculator to check your starting point. If you experience severe stomach pain or diarrhoea lasting more than a week, report this to your prescriber — dose adjustment or temporary pausing may help.
- 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 last with Mounjaro?
For most people, these side effects peak in the first 2–4 weeks and improve significantly by week 8. However, some experience symptoms for 8–12 weeks. If diarrhoea or pain persists beyond 12 weeks, inform your prescriber — dose adjustment or additional support may help. Individual experiences vary widely.
What's the difference between NHS and private Mounjaro access in the UK?
NHS access is free but requires referral to a specialist weight management service and involves waiting lists (often several months). Private prescription is faster (usually within 1–2 weeks) but costs £120–£180 for consultation plus £150–£280 per pen depending on dose. Both require clinical assessment; eligibility criteria are the same.
How does Mounjaro compare to Wegovy for side effects?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) cause similar very common side effects including stomach pain and diarrhoea. Mounjaro's dual GIP and GLP-1 mechanism may cause slightly different symptom profiles in some people, but the incidence of gastrointestinal effects is comparable. Individual tolerance varies significantly.
When should I seek medical help for Mounjaro stomach pain?
Seek urgent medical attention if you experience severe, persistent upper abdominal pain radiating to the back (possible pancreatitis), severe constipation, or signs of gallbladder problems (pain in the upper right abdomen). Mild to moderate stomach pain typically improves within 2–8 weeks and is expected, but severe pain requires GP assessment.
Can I get Mounjaro on the NHS if I'm experiencing side effects?
Yes. If you're prescribed Mounjaro through NHS specialist services, you can report side effects at any time. Your prescriber may adjust your dose, pause treatment temporarily, or recommend lifestyle modifications to manage symptoms. You should not stop taking Mounjaro suddenly without medical guidance.
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