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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Diarrhea On Mounjaro 5mg — Understanding This Common Side Effect
Diarrhoea is one of the most common side effects experienced by people taking Mounjaro (tirzepatide), particularly during the early dose adjustment phase. If you're on the 5mg maintenance dose, you may experience loose stools or increased bowel frequency as your body adapts to this prescription-only weight management injection. This guide explains why diarrhoea occurs, how long it typically lasts, and practical strategies to manage it whilst continuing your treatment under your prescriber's guidance.
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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 Occurs On Mounjaro 5mg
Mounjaro works as a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways that regulate appetite and digestion. One of the effects of GLP-1 activation is slowed gastric emptying — the rate at which food moves from your stomach into your small intestine. When this process is altered, it can trigger loose stools or diarrhoea in some patients.
Diarrhoea is particularly common during dose titration (the first 16–20 weeks when doses increase from 2.5mg to 5mg, 7.5mg, and beyond). Your gut bacteria and digestive system need time to adapt to these hormonal changes. Additionally, if Mounjaro causes you to eat differently — smaller portions, different food choices, or lower fibre intake — this dietary shift can also contribute to loose stools. The 5mg dose is the first standard maintenance dose, so some adjustment discomfort is normal at this stage.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Managing Diarrhoea Whilst On Mounjaro 5mg
Practical steps to reduce diarrhoea:
- Eat smaller, more frequent meals rather than large portions
- Stay well hydrated — drink water, herbal tea, and electrolyte drinks to replace lost fluids
- Increase soluble fibre gradually (oats, bananas, white rice) rather than insoluble fibre initially
- Avoid fatty, greasy, or spicy foods which may worsen loose stools
- Limit caffeine and alcohol, which can increase bowel frequency
Most people find diarrhoea improves significantly within 2–4 weeks as their body adjusts to the 5mg dose. If diarrhoea persists beyond this period, becomes severe, or is accompanied by stomach pain, contact your GP or the prescriber who issued your Mounjaro prescription for guidance. They may adjust your dose or recommend over-the-counter remedies.
- 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 diarrhoea last on Mounjaro 5mg?
For most people, diarrhoea improves within 2–4 weeks as the body adapts to the 5mg maintenance dose. Some experience improvement sooner. If it persists beyond 4 weeks or worsens, inform your prescriber or GP.
Should I stop taking Mounjaro if I have diarrhoea?
No — do not stop without discussing with your prescriber. Diarrhoea is a recognised side effect that usually resolves with time. Your GP or the healthcare professional who prescribed your KwikPen can suggest management strategies or, if necessary, dose adjustments.
Can diet changes help reduce diarrhoea on Mounjaro?
Yes. Eating smaller, frequent meals, staying hydrated, avoiding fatty foods, and gradually increasing soluble fibre can help. Many people find diarrhoea improves when they adjust their eating patterns alongside the medication's effects.
Is diarrhoea on Mounjaro 5mg a sign the medication isn't working?
No. Diarrhoea is a side effect, not an indicator of effectiveness. Mounjaro's weight management benefit and side effects are separate. Diarrhoea typically decreases with time, whilst the appetite-regulating effects continue.
When should I contact my GP about diarrhoea on Mounjaro?
Contact your GP or prescriber if diarrhoea is severe, persists beyond 4 weeks, is accompanied by severe stomach pain, or is significantly affecting your quality of life. They can assess whether dose adjustment or other support is needed.
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