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 IBS: What You Need to Know
If you have IBS and are considering Mounjaro (tirzepatide) for weight management, it's important to understand how they interact and whether this prescription medication is suitable for you. Mounjaro can affect digestion and may influence IBS symptoms, so a clinical assessment is essential. This guide covers what happens when you combine these conditions, how to access Mounjaro in the UK, what it costs, and when to seek GP advice. Always consult your healthcare provider before starting any new medication.
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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.
How Mounjaro Affects Digestion and IBS Symptoms
Mounjaro works by activating GIP and GLP-1 receptors, which slow how quickly food moves through your stomach. This is part of how it helps reduce appetite and support weight management. However, this same mechanism—called delayed gastric emptying—can initially cause nausea, constipation, diarrhoea, and stomach discomfort in clinical trials.
For people with IBS, this is particularly important: Mounjaro may temporarily worsen or alter IBS symptoms during the early weeks of treatment, especially at lower doses like 2.5 mg or 5 mg. Some people experience increased diarrhoea (particularly if they have IBS-D), whilst others develop constipation (relevant for IBS-C). These effects often improve as your body adjusts, typically within 4–8 weeks.
- GI side effects are most common in weeks 1–4 and may ease over time
- IBS triggers and Mounjaro side effects can overlap, making symptoms harder to predict
- Dose titration is gradual—your prescriber starts at 2.5 mg and increases weekly, giving time to assess tolerance
Because of this complexity, anyone with IBS should have a thorough clinical assessment before starting Mounjaro to ensure it's safe and appropriate for their individual situation.
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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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Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Accessing and Paying for Mounjaro in the UK
NHS access to Mounjaro is limited and often involves waiting lists. NICE approved tirzepatide (the active ingredient in Mounjaro) for NHS specialist weight management services, but availability depends on your local ICB (Integrated Care Board). Many areas have long waiting lists—sometimes 6 months to over a year. You'll need a GP referral to a tier 3 or tier 4 specialist weight management service.
Private prescription access is faster. Through registered online prescribing services, you can have a clinical assessment within days, and if approved, receive Mounjaro from a GPhC-registered pharmacy. Costs typically range from £129 to £320 per month depending on your dose (starting at 2.5 mg, escalating to a maximum of 15 mg). Your prescriber will assess your IBS history and suitability before issuing a prescription. A Mounjaro KwikPen contains 4 weeks of doses and requires cold storage.
Both routes require you to declare IBS and any gastric conditions during your clinical assessment.
Whichever route you choose—NHS or private—your prescriber must understand your IBS type (IBS-D, IBS-C, or IBS-M) and any triggers you've identified. They may recommend starting at the lowest dose and titrating slowly to monitor how your gut responds. Keep a symptom diary during the first 4–8 weeks; this helps distinguish Mounjaro adjustment effects from underlying IBS flares. Use our free BMI calculator to understand your starting point, and track any weight changes alongside GI symptoms using our progress tracker. If you're considering medication, our GLP-1 weight loss calculator can help you estimate potential results at different doses so you can discuss realistic expectations with your prescriber.
- 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 IBS?
Yes, but with caution. Mounjaro can affect digestion and may initially worsen or change IBS symptoms because it slows gastric emptying. A clinical assessment is essential—inform your prescriber of your IBS type and triggers. Your doctor will assess whether Mounjaro is suitable and may recommend a slower dose titration.
Will Mounjaro make my IBS worse?
Mounjaro may temporarily alter IBS symptoms during the first 4–8 weeks. People with IBS-D may experience increased diarrhoea; those with IBS-C may develop constipation. These effects often ease as your body adjusts. Close monitoring with your GP or prescriber is important.
How much does Mounjaro cost in the UK?
Private Mounjaro costs typically range from £129 to £320 per month, depending on your dose. Starting dose is 2.5 mg, increasing weekly to a maximum of 15 mg. NHS access is free but often involves waiting lists. Costs vary by provider; always confirm pricing with your chosen service.
What's the difference between Mounjaro and Wegovy for people with IBS?
Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist; Wegovy (semaglutide) is a single GLP-1 agonist. Both slow digestion and can affect IBS. Mounjaro may have a stronger GI effect initially due to its dual mechanism. Individual tolerance varies—your prescriber will assess which suits you better.
How do I access Mounjaro in the UK with IBS?
Via NHS: referral to specialist weight management services (long waiting lists). Via private: online clinical assessment with a UK-licensed prescriber; faster access (days to weeks), costs £129–£320/month. Both require disclosure of your IBS history. Your prescriber will ensure it's safe for your condition before issuing a UK prescription.
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.