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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How Long Does Diarrhoea Last With Mounjaro?
Diarrhoea is one of the most common side effects reported by people taking Mounjaro (tirzepatide) in the UK. For most people, it begins within the first few weeks of starting the injection or during dose increases, and typically resolves within 2–4 weeks as the body adjusts to the medication. However, the duration and severity vary significantly between individuals. Some people experience only mild, brief symptoms, whilst others may need management strategies to cope. This guide explains what NHS guidance and clinical evidence tell us about diarrhoea with Mounjaro, and practical steps you can take.
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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.
Duration, Severity, and What the Evidence Shows
Clinical trial data and real-world NHS experience show that most people experience diarrhoea for 1–4 weeks when starting Mounjaro or increasing their dose. The side effect typically peaks in the first 2 weeks and then gradually improves as your digestive system adapts to the medication.
Severity varies considerably. Some people report mild, occasional loose stools that barely affect daily life. Others experience more frequent symptoms that may require dietary adjustment. A small proportion experience persistent diarrhoea beyond 4 weeks — if this happens to you, it's important to contact your GP or the prescriber who manages your Mounjaro prescription.
Important context: diarrhoea is very common with GLP-1 and GIP/GLP-1 medications like Mounjaro and Wegovy. This is because these medications slow how quickly food leaves your stomach and affect how your gut processes nutrients. Your body usually adapts, which is why most people find symptoms improve over time. The Mounjaro KwikPen dose titration schedule — starting at 2.5 mg and increasing gradually every 4 weeks — is designed partly to allow your digestive system time to adjust.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Accessing Mounjaro: NHS vs Private, and Managing Costs
In the UK, you can access Mounjaro through two main routes: NHS specialist weight management services or private prescription. On the NHS, Mounjaro is available through tier 3 or tier 4 specialist services following GP referral — availability and waiting times vary significantly by region. Private online prescribing is faster and doesn't require referral, but you pay for the medication directly. Typical private costs range from approximately £129–£320 per month depending on your dose level (starting doses are cheaper than maximum doses).
When considering cost, remember that diarrhoea management is usually straightforward and low-cost: increasing dietary fibre, staying well hydrated, and temporary use of over-the-counter remedies often help. Your GP or online prescriber can advise on medications to manage symptoms if needed. Our free BMI calculator can help you understand whether Mounjaro eligibility applies to your situation — and you can use our GLP-1 weight loss calculator to explore projected outcomes before committing to treatment.
- 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 can I manage diarrhoea while taking Mounjaro?
Most people find relief through simple dietary changes: increase soluble fibre (oats, bananas), eat smaller, more frequent meals, stay well hydrated, and avoid high-fat or high-sugar foods. Over-the-counter remedies like loperamide may help temporarily, but check with your GP first. Symptoms usually improve as your body adjusts over 2–4 weeks.
Is diarrhoea a sign that Mounjaro isn't working for me?
No. Diarrhoea is a very common side effect that doesn't indicate the medication isn't effective for weight loss. In fact, it's a sign your body is responding to the medication. Most people lose weight despite experiencing diarrhoea in the early weeks.
Should I stop taking Mounjaro if diarrhoea lasts more than 4 weeks?
Contact your GP or prescriber if diarrhoea persists beyond 4 weeks or becomes severely bothersome. They may adjust your dose, suggest management strategies, or discuss whether continuing is right for you. Don't stop suddenly without medical guidance.
Does diarrhoea last longer with higher Mounjaro doses?
Diarrhoea often occurs again when you increase your dose (e.g., from 5 mg to 10 mg), but it typically resolves within 2–4 weeks again. The minimum 4-week gap between dose increases allows your digestive system time to adapt before the next change.
Can I get Mounjaro on the NHS if I'm concerned about side effects like diarrhoea?
Yes. Mounjaro is available on the NHS through specialist weight management services for eligible adults. Your GP can refer you. NHS specialists will discuss side effects, including diarrhoea management, during your assessment. You can also ask about private prescription if NHS waiting times are long in your area.
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.