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, Wegovy, and Colitis Risk — What You Need to Know
Colitis is not a commonly reported side effect of either Mounjaro or Wegovy in UK clinical trials, but both medications can cause gastrointestinal side effects including nausea, diarrhoea, and constipation. This guide compares how Mounjaro (tirzepatide) and Wegovy (semaglutide) differ in their mechanisms and gastrointestinal tolerability, and explains what symptoms warrant medical attention. Both are prescription-only medications that require clinical assessment before prescribing.
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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 and Wegovy Work — Mechanism Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two separate hormone pathways in the body. This dual action regulates appetite, slows gastric emptying (food movement through the stomach), and helps regulate fat utilisation. Wegovy (semaglutide) is a GLP-1 receptor agonist only — activating a single pathway to reduce appetite and slow digestion.
In the SURMOUNT clinical trial for Mounjaro, gastrointestinal side effects including nausea, diarrhoea, and vomiting were very common (affecting over 1 in 10 participants), particularly during dose escalation. Similarly, in STEP trials for Wegovy, nausea and diarrhoea were the most frequently reported side effects. Neither trial identified colitis as a primary safety concern, though serious gastrointestinal events require immediate medical attention and should be reported to your GP.
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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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Gastrointestinal Side Effects — Comparison and When to Seek Help
Both medications share similar common gastrointestinal side effects: nausea, diarrhoea, vomiting, constipation, and abdominal pain. The frequency and severity may vary between individuals. Serious symptoms requiring urgent GP contact include:
- Severe, persistent abdominal pain spreading to the back (possible pancreatitis)
- Bloody diarrhoea or signs of intestinal inflammation
- High fever with stomach symptoms
- Severe vomiting preventing food or fluid intake
Your prescriber will assess which medication is more suitable based on your individual health history, including any previous gastrointestinal conditions. Both require a UK prescription and clinical assessment — neither is available without this evaluation.
- 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 Mounjaro cause colitis?
Colitis (inflammation of the colon) is not a commonly reported side effect in Mounjaro clinical trials or post-marketing surveillance. However, Mounjaro can cause gastrointestinal symptoms including diarrhoea, nausea, and constipation. Serious intestinal symptoms require immediate GP attention.
Is Wegovy safer than Mounjaro for stomach problems?
Both medications have similar rates of gastrointestinal side effects in clinical trials. Neither has colitis as a primary safety concern. Individual tolerability varies. Your prescriber will assess which is more suitable based on your personal medical history and any previous stomach issues.
What gastrointestinal symptoms are normal with these injections?
Nausea, diarrhoea, and constipation are very common during the first weeks of treatment with both Mounjaro and Wegovy. These often improve as your body adjusts. Symptoms typically ease after 4–8 weeks. Persistent or severe symptoms should be discussed with your GP.
How do I know if I have a serious intestinal problem on these medications?
Seek urgent medical attention if you experience severe abdominal pain spreading to your back, bloody stools, high fever with stomach symptoms, or inability to keep food or fluids down. These warrant immediate GP evaluation to rule out serious conditions like pancreatitis or intestinal obstruction.
Which medication is better if I have a sensitive stomach?
Neither medication is inherently "better" for sensitive stomachs — both carry similar gastrointestinal risks. Your prescriber will review your medical history, previous medication tolerability, and current health conditions to determine which is more appropriate for you following a clinical assessment.
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