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, Diverticulitis, and Digestive Health
Diverticulitis is inflammation of small pouches in the colon, and it's important to understand how weight management medications like Mounjaro may affect digestive health. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1 based medications that slow gastric emptying and affect how your digestive system works. If you have a history of diverticulitis or digestive concerns, this is a critical discussion to have with your GP before starting any weight loss injection. This guide compares both medications in the context of gastrointestinal health and safety.
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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.
Mounjaro vs Wegovy: Mechanism and GI Effects
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways in the gut. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single pathway. Both medications slow gastric emptying—how quickly food leaves your stomach—which can contribute to constipation, nausea, and changes in bowel movements.
In clinical trials (SURMOUNT for Mounjaro, STEP for Wegovy), gastrointestinal side effects were common in both groups. Mounjaro users reported slightly higher rates of nausea and vomiting in early weeks, whilst Wegovy showed similar GI tolerability. Neither medication is contraindicated in diverticulitis specifically, but both require careful monitoring if you have a history of inflammatory bowel conditions.
The key difference: Mounjaro's dual mechanism may have a slightly different GI impact than Wegovy's single mechanism, though individual responses vary significantly. Your prescriber can assess which is more suitable based on your specific digestive history using the clinical assessment process.
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Diverticulitis Risk and Clinical Assessment
Diverticulitis develops when small pouches (diverticula) in the colon become infected or inflamed. Risk factors include constipation, low fibre intake, and certain genetic factors—not directly caused by weight loss medications. However, because both Mounjaro and Wegovy slow gastric emptying and can cause constipation, they may indirectly increase diverticulitis risk in people with existing diverticula.
If you have active diverticulitis, either medication should be avoided until inflammation resolves. If you have a history of diverticulitis, your GP or prescriber must assess whether the digestive effects of GLP-1 medication are safe for you. This may involve:
- Reviewing your bowel habit history and current digestive health
- Discussing strategies to maintain adequate fibre and hydration
- Monitoring you more closely during the dose titration phase
- Considering whether Wegovy (single mechanism) might be safer than Mounjaro (dual mechanism) in your case
Both medications are prescription-only and require a full clinical assessment before being prescribed.
- 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 I take Mounjaro if I have a history of diverticulitis?
Possibly, but only after full assessment by your GP or prescriber. Active diverticulitis is a contraindication. A history of diverticulitis requires careful evaluation of your current digestive health, bowel habits, and risk factors. Your prescriber may recommend Wegovy instead if they believe the dual GI effects of Mounjaro pose higher risk. Always disclose your full medical history.
Does Mounjaro cause constipation and diverticulitis?
Mounjaro slows gastric emptying, which can cause constipation in some users. Constipation is a risk factor for diverticulitis, but Mounjaro does not directly cause diverticulitis. However, if you're prone to constipation or have existing diverticula, the constipating effects could indirectly increase risk. Managing fibre and hydration is important.
Is Wegovy safer than Mounjaro for digestive health?
Not necessarily safer overall—both have similar GI side effect profiles. Wegovy's single GLP-1 mechanism may have a slightly different effect on gastric emptying than Mounjaro's dual mechanism, but evidence is limited. Individual suitability depends on your specific health history, not generalised comparisons. Your prescriber will assess which is appropriate for you.
What should I tell my GP about diverticulitis before starting Mounjaro?
Disclose any history of diverticulitis, frequency of episodes, current symptoms, bowel habits, and fibre intake. Mention any other gastrointestinal conditions (IBS, Crohn's, ulcerative colitis). Your GP can then assess risk and decide whether Mounjaro, Wegovy, or neither is suitable. Honest disclosure helps your prescriber make a safe decision.
What GI side effects should I watch for if I do take Mounjaro?
Common effects include nausea, constipation, diarrhoea, and stomach pain. If you develop persistent abdominal pain, especially pain radiating to your back (possible pancreatitis), or signs of infection (fever, severe cramping), seek urgent medical attention. Report any significant bowel habit changes to your GP promptly, especially if you have diverticulitis history.
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