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
Free tracker included with every download this week
Mounjaro and Ulcerative Colitis — What You Need to Know
If you have ulcerative colitis (UC) and are considering Mounjaro for weight management, it's important to understand how this medication may interact with your condition. Mounjaro (tirzepatide) is a prescription-only, once-weekly injection licensed in the UK for weight management in adults. However, because it affects how your digestive system works, it requires careful consideration if you have inflammatory bowel disease. This guide explains what you need to know before discussing it with your GP.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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.
How Mounjaro Works and Why Digestive Health Matters
Mounjaro is a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways that help regulate appetite and blood sugar. Part of how it works involves slowing gastric emptying (the rate at which food leaves your stomach), which can contribute to feeling fuller after meals.
For people with ulcerative colitis, this mechanism is particularly important to consider. Ulcerative colitis is an inflammatory bowel disease (IBD) that affects the colon and rectum, causing inflammation, ulceration, and symptoms including diarrhoea, abdominal pain, and urgency. Because Mounjaro interacts with stomach and intestinal function, it may potentially worsen digestive symptoms or inflammation in people with active UC.
Key concerns:
- Slowed gastric emptying could increase bloating or discomfort in people with UC
- Common Mounjaro side effects (nausea, diarrhoea, constipation, stomach pain) overlap with UC symptoms, making it harder to monitor your condition
- The medication's effects on gut motility may not be suitable during active inflammation
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
What Your GP Needs to Know — Assessment and Prescribing
Before any prescription weight loss medication is prescribed in the UK, your prescriber must conduct a thorough clinical assessment. If you have ulcerative colitis, you must disclose this during your assessment — whether your UC is currently in remission or active.
Your GP or specialist weight management service will need to:
- Review your UC history, current medications (including immunosuppressants), and symptom control
- Consider whether Mounjaro is safe for you, or whether an alternative approach might be better
- Discuss your weight management goals and other options (dietary support, exercise, other medications)
- Monitor you closely if they decide to prescribe — you'll need to report any changes in UC symptoms immediately
Use our free GLP-1 weight loss calculator to explore potential outcomes, but always remember that individual results vary, and your personal health circumstances must guide any decision.
- 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.
Can you take Mounjaro if you have ulcerative colitis?
Possibly, but not without careful assessment. If your UC is in stable remission and well-controlled, a prescriber may consider Mounjaro after weighing the risks and benefits. However, if your UC is active or poorly controlled, Mounjaro is likely not suitable due to potential worsening of digestive symptoms. Your GP or specialist must assess your individual situation — this is not a yes or no decision, it's personalised.
What is the difference between Mounjaro and Wegovy for someone with IBD?
Mounjaro is a dual GIP/GLP-1 agonist (tirzepatide), while Wegovy is a GLP-1-only agonist (semaglutide). Both slow stomach emptying and may cause digestive side effects. Wegovy is a single-mechanism medication, which some specialists believe may be slightly safer to assess in IBD patients, but both require the same careful clinical evaluation. Neither is automatically safer — it depends on your individual UC status.
Will Mounjaro trigger a UC flare-up?
Mounjaro hasn't been specifically studied in people with active ulcerative colitis, so there's no definitive answer. However, because it affects gut motility and common side effects include diarrhoea and stomach pain (which overlap with UC symptoms), there is a theoretical risk of triggering or worsening inflammation. This is why disclosure and specialist assessment are essential before prescribing.
What should I tell my GP about my ulcerative colitis before asking for Mounjaro?
Tell your GP: your UC diagnosis, how long you've had it, whether you're currently in remission or having symptoms, what medications you take for UC (biologics, mesalamine, corticosteroids, etc.), how often you have flare-ups, and any recent changes in your symptoms. This information helps your GP assess whether Mounjaro is safe for you or whether lifestyle changes, dietitian support, or other options might be better first.
Are there safer weight loss options if you have ulcerative colitis?
Yes. Before considering prescription medications, discuss with your GP: dietitian referral (specialising in IBD if possible), structured exercise programmes, NHS weight management support groups, and psychological support if emotional eating is a factor. These approaches don't interact with your UC. If medication is needed after these are explored, your prescriber may consider alternatives based on your specific health picture.
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.