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
How Long Does Mounjaro Diarrhoea Last?
Diarrhoea is one of the most common side effects of Mounjaro (tirzepatide), affecting more than 1 in 10 people. Most cases resolve within the first few weeks as your body adjusts to the medication, though timing varies between individuals. Understanding what to expect, how to manage it, and where to access Mounjaro in the UK can help you make an informed decision about this prescription weight management injection.
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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.
Mounjaro Diarrhoea Timeline and Management
Diarrhoea typically begins within the first 1–2 weeks after starting Mounjaro or increasing your dose. Most people find it improves within 2–4 weeks as the body adjusts, though some experience it for longer. The severity often depends on your starting dose, how quickly you escalate, and individual factors like diet and hydration.
Management strategies include: increasing water and electrolyte intake, eating small frequent meals rather than large ones, avoiding high-fat and spicy foods, and keeping a food diary to identify triggers. If diarrhoea persists beyond 4 weeks or becomes severe, contact your GP or the prescriber — they may adjust your dose, delay escalation, or recommend over-the-counter remedies. Never stop the medication without medical guidance.
- Timeline: typically 1–4 weeks but varies individually
- Severity usually decreases as your body adapts
- Dose escalation speed affects symptom duration
- Hydration and dietary changes can help manage symptoms
Clinically BackedWeight Loss
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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.
NHS vs Private: Accessing Mounjaro in the UK
NHS access through tier 3 specialist weight management services is free but involves referral from your GP and can have waiting lists of months to years depending on your region. Private access is available through online prescribing services following a clinical assessment with a UK-licensed prescriber — typically faster but costs apply, usually between £150–£250 per month depending on your dose and provider, though specific pricing varies.
Whether diarrhoea concerns you about starting Mounjaro, our free GLP-1 weight loss calculator can help you explore potential outcomes and timelines. Both routes require a prescription and clinical assessment to confirm Mounjaro is suitable for you. Discuss side effect management options with your prescriber before starting, so you're prepared if diarrhoea occurs.
- 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 before Mounjaro diarrhoea goes away?
For most people, diarrhoea improves within 2–4 weeks as the body adjusts. Some experience it for longer, while others have minimal symptoms. If it persists beyond 4 weeks or worsens, contact your prescriber—they may adjust your dose or offer management strategies.
Why does Mounjaro cause diarrhoea?
Mounjaro (tirzepatide) slows how quickly food leaves the stomach and affects the gut hormones that regulate digestion. This can cause loose stools, particularly in the first few weeks. Most people adapt as their body becomes accustomed to the medication.
Can I reduce diarrhoea by changing my diet?
Yes. Eating smaller, more frequent meals, staying hydrated with water and electrolytes, and avoiding high-fat or spicy foods can help manage diarrhoea. Keep a food diary to identify triggers. Increasing fibre intake gradually (not suddenly) may also help—discuss dietary changes with your GP.
Is Mounjaro diarrhoea a reason to stop taking it?
Diarrhoea alone is not usually a reason to stop. It's a common, often temporary side effect that typically improves. However, if it's severe, prolonged, or affecting your quality of life significantly, speak to your prescriber—they can adjust your dose, delay dose escalation, or discuss alternatives.
How do I access Mounjaro in the UK if I'm concerned about side effects?
Both NHS and private routes require a clinical assessment. With NHS specialist services, discuss side effect concerns with your referrer or the specialist team. Via private prescribing, you'll have an online consultation where you can ask about diarrhoea management before starting. Either way, your prescriber can advise on managing or minimising this side effect.
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.