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
Can Mounjaro Cause Diarrhoea in the UK?
Yes, diarrhoea is one of the most common side effects of Mounjaro (tirzepatide) reported by UK patients. It typically occurs during the dose adjustment phase and often improves as your body adapts to the medication. Understanding this side effect, how to manage it, and what access options are available in the UK—both private and NHS routes—can help you make an informed decision about whether Mounjaro is right for you.
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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.
Mounjaro Diarrhoea: What Happens and How to Manage It
Diarrhoea affects more than 1 in 10 people taking Mounjaro during clinical trials. It's classified as a very common side effect and typically emerges during the dose titration phase—the first 8–12 weeks when your dose increases gradually from 2.5 mg to your maintenance dose. The underlying cause is that tirzepatide slows how quickly food moves through your stomach, which can trigger loose stools or increased bowel frequency.
- Most people experience diarrhoea for a few days to a few weeks after a dose increase
- Symptoms usually settle within 1–2 weeks as your digestive system adapts
- Staying hydrated is essential—aim for 2–3 litres of water daily
- Eat smaller, frequent meals rather than large portions
- Avoid high-fat and high-fibre foods temporarily
- Some patients find over-the-counter rehydration solutions helpful
If diarrhoea persists beyond 2 weeks or becomes severe, contact your prescriber or GP. In rare cases, dose adjustment or medication review may be needed. Try our GLP-1 weight loss calculator to see projected results and understand whether Mounjaro's benefits align with managing potential side effects.
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.
UK Access Routes: NHS vs Private Prescription Costs
NHS access: Mounjaro is approved by NICE for use in NHS specialist weight management services (tier 3/4), but availability varies significantly by region and waiting lists can be 6–18 months in many areas. You'll need a GP referral to a specialist service.
Private access: GPhC-registered online pharmacies prescribe Mounjaro following an online clinical assessment. Private prescriptions typically cost £180–£320 per month depending on your dose. The Mounjaro KwikPen supplies 4 weeks of injections. You'll complete a simple health questionnaire, a licensed prescriber reviews your eligibility, and medication is dispensed if approved. This route bypasses NHS waiting lists and offers flexibility, though costs are out-of-pocket.
Both routes require you to be over 18, have a BMI of 30+ (or 27+ with weight-related health conditions), and be willing to combine treatment with diet and exercise changes.
- 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.
How long does Mounjaro diarrhoea last?
Diarrhoea usually appears within a few days of a dose increase and settles within 1–2 weeks as your body adapts. If it persists beyond 2 weeks or worsens, contact your prescriber for advice. Most people find it manageable with hydration and dietary adjustments.
Can I take anti-diarrhoea medication while on Mounjaro?
Over-the-counter remedies like loperamide are generally safe, but check with your GP or pharmacist first, as some medications may interact with Mounjaro. Hydration and small meals are usually the first-line approach.
How much does Mounjaro cost privately in the UK?
Private prescriptions typically range from £180–£320 per month depending on your dose (2.5 mg to 15 mg). Each KwikPen contains 4 weeks of once-weekly injections. Costs vary slightly between online pharmacies—compare providers for best value.
Can I get Mounjaro on the NHS quickly?
NHS availability depends on your region and local specialist weight management services. Waiting times vary from a few months to 18+ months in some areas. Your GP can refer you to your local tier 3 service to check eligibility and waiting times.
Is there a difference between Mounjaro diarrhoea and Wegovy diarrhoea?
Both are GLP-1-based medications and diarrhoea is common with both. Mounjaro uses dual GIP+GLP-1 action while Wegovy uses GLP-1 only, but side effect patterns are similar. Individual tolerance varies—some find one easier to manage than the other.
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.