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 Colitis — UK Health Information
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK, but it's not suitable for everyone. If you have a history of colitis or inflammatory bowel disease, it's important to understand the risks before starting treatment. This guide explains the connection between Mounjaro and colitis, who should avoid it, and how to access weight loss support safely through the NHS or private prescription routes in the UK.
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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, Colitis, And Inflammatory Bowel Disease
Mounjaro is contraindicated (meaning it should not be used) in people with a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia syndrome type 2. However, colitis and inflammatory bowel disease (IBD) require careful assessment before starting any GLP-1 or GIP/GLP-1 medication.
GLP-1 and GIP/GLP-1 receptor agonists, including the tirzepatide in Mounjaro, can affect how the gut works. In people with existing colitis or Crohn's disease, these changes may potentially trigger flares or worsen inflammation. Additionally, Mounjaro commonly causes gastrointestinal side effects—nausea, diarrhoea, vomiting, and stomach pain are very common, affecting more than 1 in 10 users. In someone with colitis, distinguishing between medication side effects and disease symptoms becomes difficult.
- Colitis causes inflammation of the colon
- IBD involves chronic digestive inflammation
- GLP-1 medications slow gastric emptying
- Gut changes may trigger IBD flares in susceptible people
If you have colitis or IBD, your GP or prescriber must assess your individual risk before Mounjaro is prescribed. In many cases, alternative weight management approaches may be safer.
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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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Accessing Weight Management Support Safely In The UK
If you have colitis or IBD and need weight management support, the NHS offers safer routes. Ask your GP to refer you to a tier 3 specialist weight management service, where a multidisciplinary team—including gastroenterologists if needed—can assess your individual case and recommend appropriate treatment.
Private online prescribing is also available in the UK, but only a prescriber with access to your full medical history (including IBD status) can determine suitability. Never start Mounjaro without a clinical assessment. Always disclose colitis, Crohn's disease, ulcerative colitis, or any history of digestive inflammation to your prescriber. Use our BMI calculator to check your current health baseline, and discuss the results with your GP as a starting point for personalised weight loss guidance suited to your digestive health.
- 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 colitis?
Mounjaro is not recommended for people with active colitis or a history of severe inflammatory bowel disease. GLP-1 medications can cause gastrointestinal side effects that may trigger flares. If you have colitis, discuss alternative weight management options with your GP or a specialist gastroenterologist before starting any weight loss medication.
Can Mounjaro cause colitis?
Mounjaro doesn't directly cause colitis, but it can cause significant gastrointestinal side effects (nausea, diarrhoea, vomiting, stomach pain) that may worsen existing colitis or trigger flares in people with inflammatory bowel disease. In people without IBD, these effects are usually temporary and manageable.
What should I tell my prescriber about my digestive health?
Always disclose any history of colitis, Crohn's disease, ulcerative colitis, irritable bowel syndrome, or other chronic digestive conditions. Mention if you've had surgery affecting your digestive system or if you take medications for IBD. Your prescriber uses this information to assess whether Mounjaro is safe for you personally.
What are safer weight loss options if I have colitis?
Discuss with your GP or gastroenterologist: structured dietary support, referral to a specialist weight management service, gentle physical activity suitable for IBD, and in some cases, alternative medications that don't affect gut motility. NHS tier 3 services can provide personalised guidance tailored to your digestive health.
How do I access weight management support on the NHS with colitis?
Ask your GP for a referral to a specialist tier 3 weight management service. These services assess your individual needs, including any medical conditions like colitis. Assessment and treatment are free on the NHS, though waiting lists vary by region. Online private consultations are also available but require a full health assessment including IBD status.
Ready to take control of your weight loss journey?
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