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 and IBD — Important Safety Information for UK Patients
If you have inflammatory bowel disease (IBD) such as Crohn's disease or ulcerative colitis and you're considering Mounjaro (tirzepatide) for weight management, it's essential to understand how this medication may affect your condition. Mounjaro affects how your digestive system works, and people with IBD need careful medical assessment before starting this medication. This guide covers what you need to know about Mounjaro, tirzepatide, and IBD safety in the UK healthcare context.
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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.
How Mounjaro Works and Why IBD Matters
Mounjaro is a dual GIP and GLP-1 receptor agonist injection that works by slowing gastric emptying — the rate at which food moves from your stomach into your intestines. For most people, this effect supports weight loss by promoting a feeling of fullness. However, in people with inflammatory bowel disease, slowing gut movement can be problematic.
Key concerns for IBD patients:
- Mounjaro may worsen constipation, which is already common in IBD management
- Slowed gastric emptying could trigger or exacerbate abdominal symptoms in people with active inflammation
- Nausea and vomiting — common Mounjaro side effects — can be harder to distinguish from IBD flares
- The medication's dose titration schedule (starting at 2.5 mg weekly and increasing up to 15 mg) means effects accumulate over weeks
Clinical trials for Mounjaro did not include large numbers of participants with active IBD, so real-world safety data in this population is still emerging. Your GP or gastroenterologist needs to assess your individual IBD severity, current medications, and whether weight loss medication is appropriate for you.
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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.
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NHS vs Private Access — What's Important for IBD Patients
NHS route: If you're eligible for weight loss treatment through the NHS, you'll be referred to a specialist tier 3 weight management service. This is important for IBD patients because NHS specialists can access your full medical history, liaise with your gastroenterology team, and monitor your condition closely. NHS access is free but waiting lists can be lengthy.
Private route: Private online prescribing services offer faster access, but you must disclose your IBD diagnosis during the online consultation. A responsible prescriber will likely require evidence that your IBD is well-controlled before prescribing. Some may decline to prescribe if your IBD is active or unstable. Always mention current IBD medications, recent flares, and whether you're under active gastroenterology care. Never attempt to access Mounjaro privately without full disclosure of your IBD status.
Medication interactions: IBD medications like mesalamine, azathioprine, or biologics (infliximab, vedolizumab) may interact with Mounjaro. Additionally, Mounjaro may reduce absorption of oral medications, which is critical if you're taking orally administered IBD treatments. Your prescriber or GP must review all your current medications before starting tirzepatide.
The cost of private Mounjaro prescription varies, typically ranging from £129–£320 per month depending on the dose you're prescribed. However, cost should not be the primary decision factor for IBD patients — safety assessment by a healthcare professional who understands both IBD and GLP-1 medications is far more important.
- 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 Mounjaro safe if I have Crohn's disease or ulcerative colitis?
Mounjaro may not be safe for people with active or severe IBD because it slows gastric emptying and can worsen digestive symptoms. If your IBD is well-controlled and stable, your GP or gastroenterologist may consider it, but this requires careful individual assessment. Never start Mounjaro without discussing your IBD with your healthcare provider.
What should I tell a private prescriber about my IBD?
Tell them your IBD diagnosis (Crohn's or ulcerative colitis), current disease status (active, remission, or recently flared), all current medications including biologics or immunosuppressants, and whether you're under active gastroenterology care. Responsible prescribers will use this information to determine whether Mounjaro is appropriate for you.
Can Mounjaro interact with my IBD medications?
Yes. Mounjaro may interact with some IBD medications and can reduce the absorption of oral medications taken at the same time. Your GP needs to review your full medication list before prescribing. Some combinations may require dose adjustments or monitoring.
How does tirzepatide affect constipation in IBD?
Tirzepatide slows gastric emptying, which can worsen constipation. People with IBD already experience altered bowel function, so this side effect may be more problematic. If you have a history of IBD-related constipation, discuss this explicitly with your prescriber before starting.
Should I access Mounjaro through the NHS or privately if I have IBD?
The NHS route is generally safer for IBD patients because specialist weight management services can coordinate care with your gastroenterology team and access your full medical history. However, NHS waiting times can be very long. If you choose private access, ensure the prescriber fully assesses your IBD status and is willing to take on the additional monitoring responsibility.
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