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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Managing Diarrhoea While Using Mounjaro in the UK
Diarrhoea is one of the most common side effects of Mounjaro (tirzepatide), affecting more than 1 in 10 people using this weight management injection. If you're experiencing digestive changes while on Mounjaro, you're not alone — and there are practical steps you can take to manage it. This guide explains why diarrhoea occurs with Mounjaro, when it typically improves, and how to access this prescription medication through NHS and private routes in the UK.
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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.
Why Diarrhoea Happens with Mounjaro and Managing It
Mounjaro works as a dual GIP and GLP-1 receptor agonist, which means it activates two hormone pathways in your gut. This affects how your digestive system processes food, slowing gastric emptying and altering bowel movements — causing diarrhoea in many users. The good news is that this side effect often improves over time as your body adapts to the medication.
Managing diarrhoea while on Mounjaro:
- Stay hydrated — drink plenty of water throughout the day to replace lost fluids
- Eat smaller, more frequent meals rather than large meals
- Avoid high-fat, high-fibre foods initially, which can worsen symptoms
- Keep a food diary to identify triggers specific to your body
- Consider over-the-counter remedies like loperamide if needed, but consult your prescriber first
- Allow at least 4 weeks on each dose before expecting improvement — your body may need time to adjust
Most people find diarrhoea peaks in the first few weeks and gradually improves. If it persists beyond 8 weeks or is severe, contact your GP or prescriber immediately, as this may indicate a need for dose adjustment.
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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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Accessing Mounjaro in the UK — NHS vs Private Routes and Costs
NHS route: Mounjaro is available on the NHS through specialist tier 3 weight management services. You'll need a GP referral and will be assessed by a multidisciplinary team. NHS access is free at the point of use, but availability varies significantly by region — waiting lists can be months to years depending on your ICB area. Contact your GP to ask if your local service has capacity.
Private route: You can access Mounjaro privately through a UK-licensed prescriber via online consultation (typically £15–£40 per assessment). If approved, your prescription is dispensed by a GPhC-registered pharmacy. Private Mounjaro costs range from approximately £129 to £320 per month depending on your dose — higher doses cost more. An online consultation assesses your eligibility based on BMI and health conditions, similar to NHS criteria. This route offers faster access without NHS waiting lists.
Both routes require a prescription and individual clinical assessment — Mounjaro is never available without one.
- 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.
What dose of Mounjaro typically causes diarrhoea?
Diarrhoea can occur at any dose, though it's more common when starting or increasing doses. The starting dose of 2.5 mg may cause mild symptoms; some people experience worsening when escalating to 5 mg, 7.5 mg, or higher. Individual responses vary significantly. If diarrhoea is severe, your prescriber may pause dose escalation until symptoms settle.
How long does diarrhoea last when taking Mounjaro?
For most people, diarrhoea is worst in the first 1–2 weeks of starting or increasing a dose, then improves as your body adapts. By weeks 3–4, many users notice significant improvement. If diarrhoea persists beyond 8 weeks or worsens, contact your prescriber — this may indicate your current dose isn't right for you.
Can I use anti-diarrhoea medication like loperamide with Mounjaro?
Loperamide can be used occasionally for mild diarrhoea, but check with your prescriber first — some anti-diarrhoea medications can slow gastric emptying further, which Mounjaro already does. For ongoing diarrhoea, dietary adjustments and hydration are usually recommended first. Never use anti-diarrhoea medication regularly without medical guidance.
Is diarrhoea a sign that Mounjaro isn't working or is unsafe?
No. Diarrhoea is a known, common side effect and does not mean Mounjaro is unsafe or ineffective. It's your body responding to the medication's effects on your digestive system. Most people who experience diarrhoea still achieve significant weight loss. If you're concerned, discuss it with your prescriber — they may adjust your dose or offer additional support.
How much does Mounjaro cost privately in the UK, and how often do I pay?
Private Mounjaro costs approximately £129–£320 per month depending on your dose (higher doses cost more). Costs vary slightly between pharmacies. You typically pay monthly when you order your next KwikPen. An online consultation fee (usually £15–£40) is a one-time cost. NHS Mounjaro is free, but access requires a GP referral to specialist services.
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