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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Mounjaro Sickness and Diarrhoea — What You Need to Know
Sickness and diarrhoea are among the most common side effects reported by people taking Mounjaro (tirzepatide) in the UK. These digestive symptoms typically occur when starting the medication or increasing your dose, and for many people they improve over time. This guide explains what causes these side effects, how long they usually last, and practical steps you can take to manage them while taking Mounjaro under NHS or private prescription.
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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 Mounjaro Causes Sickness and Diarrhoea
Mounjaro is a dual GIP and GLP-1 receptor agonist — it works by activating two natural hormone pathways that help regulate appetite and blood sugar. One of the ways these hormones work is by slowing how quickly food moves through your stomach, a process called gastric emptying. This slower digestion can trigger feelings of nausea and stomach upset, especially when you first start the medication or increase your dose. Additionally, the medication affects gut motility — the muscle contractions that move food through your digestive system — which can cause diarrhoea or constipation in some people. These side effects are dose-dependent, meaning they tend to be more noticeable at higher doses. The good news is that for most people, sickness and diarrhoea improve significantly within 4–8 weeks as your body adjusts to the tirzepatide. Your GP or prescriber can discuss what to expect and when symptoms typically ease.
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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.
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Managing Sickness and Diarrhoea While Taking Mounjaro
Dietary adjustments: Eat smaller, more frequent meals rather than large portions. Focus on bland, easy-to-digest foods such as plain rice, toast, boiled vegetables, and lean proteins. Avoid fatty, spicy, or high-fibre foods until symptoms improve. Stay hydrated by drinking water, herbal tea, or electrolyte drinks — diarrhoea can lead to fluid loss.
Timing and lifestyle: Take your weekly injection on a day when you can rest if needed, as side effects often peak in the first 24–48 hours. Gentle movement such as walking may help ease stomach discomfort. If sickness is severe or persistent beyond 8 weeks, contact your GP or prescriber — they may adjust your dose or explore other options. Our GLP-1 weight loss calculator can help you understand expected side effect timelines at different doses.
- Medications: Over-the-counter anti-nausea or antidiarrhoeal medications may provide relief, but check with your pharmacist or GP first as some can interact with Mounjaro or mask underlying issues.
- When to seek help: Seek urgent medical attention if you experience severe, persistent stomach pain (possible pancreatitis), bloody diarrhoea, or signs of severe dehydration such as extreme thirst, dark urine, or dizziness.
- Long-term outlook: Clinical trial data shows that side effects diminish significantly for most people by weeks 8–12 as the body adapts to tirzepatide. Weight loss benefits typically continue even after digestive symptoms ease.
- 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 do sickness and diarrhoea usually last when taking Mounjaro?
For most people, these side effects peak in the first week after injection or dose increase, then gradually improve over 4–8 weeks. By weeks 8–12, many people report that sickness and diarrhoea have largely resolved. Individual timelines vary — some people experience relief sooner, others take longer to adjust.
Is it normal to feel sick every time I inject Mounjaro?
Yes, it's very common to feel some nausea in the 24–48 hours after your weekly Mounjaro KwikPen injection, especially during the first month or when increasing doses. This doesn't mean the medication isn't working. If sickness is severe or doesn't improve after 8 weeks, contact your GP.
What foods should I avoid while taking Mounjaro?
Avoid fatty, greasy, spicy, and high-fibre foods initially. Stick to bland options: white rice, plain bread, boiled vegetables, skinless chicken, plain pasta. Gradually reintroduce normal foods as your stomach adjusts. Staying hydrated is equally important.
Can I take anti-nausea or anti-diarrhoeal medication with Mounjaro?
Some over-the-counter options may help, but always check with your pharmacist or GP first — certain medications can interact with tirzepatide or delay digestion in problematic ways. Your healthcare provider can recommend safe options for your situation.
Should I stop taking Mounjaro if sickness and diarrhoea don't improve?
Do not stop without speaking to your GP or prescriber first. Most people find these side effects resolve by weeks 8–12. Your prescriber may adjust your dose, suggest dietary changes, or explore alternatives. Stopping abruptly isn't recommended — any changes should be managed medically.
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.