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: UK Cost and Access Information
Sickness and diarrhoea are among the most common side effects of Mounjaro (tirzepatide), affecting more than 1 in 10 people taking the medication. Understanding these effects, how they're managed, and what costs and access options are available in the UK will help you make an informed decision about whether this prescription weight management injection is right for you. This guide covers everything from symptom management to NHS and private access routes.
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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.
Understanding Mounjaro Sickness and Diarrhoea
When taking Mounjaro, nausea and diarrhoea occur because the medication slows how quickly food moves through your stomach and affects the hormones controlling appetite and digestion. These effects are typically dose-dependent — meaning they're more likely at higher doses and often improve over time as your body adjusts.
- Nausea: Reported in more than 1 in 10 people, usually mild to moderate, often improves after 2–4 weeks on the same dose
- Diarrhoea: May affect up to 1 in 10 people, often resolves as the dose stabilises
- Vomiting: Less common than nausea, may occur during dose increases
- Constipation: Some people experience constipation instead of diarrhoea, depending on individual response
Most side effects are manageable with dietary adjustments, staying hydrated, and discussing options with your prescriber. In clinical trials, side effects were rarely severe enough to stop treatment.
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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 in the UK: NHS vs Private Routes
NHS route: Mounjaro is approved by NICE for use in specialist weight management services. Access is through tier 3/4 services via GP referral, with no direct cost to you, but waiting lists can be lengthy — often months to years depending on your region. Availability varies significantly by integrated care board (ICB).
Private route: You can obtain a prescription through a UK-licensed prescriber via online consultation with a GPhC-registered pharmacy. This bypasses NHS waiting lists and is available within days. Costs typically range from £129–£320 monthly depending on your dose. An online assessment (usually £15–£50) confirms you meet eligibility criteria before a prescription is issued. You'll need a BMI of 30+ or 27+ with weight-related health conditions to qualify.
- 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 Mounjaro sickness and diarrhoea usually last?
Most people find nausea and diarrhoea improve within 2–4 weeks on the same dose as their body adjusts. If you increase your dose, symptoms may temporarily return. Eating smaller, bland meals and staying well hydrated helps. If symptoms persist beyond 4 weeks or are severe, contact your prescriber — they may slow your dose progression.
Can I take medication to stop the diarrhoea while on Mounjaro?
Always discuss this with your GP or prescriber first. Some people use over-the-counter remedies like loperamide (Imodium), but these aren't recommended as a first-line approach with Mounjaro. Dietary changes — eating low-fat, smaller meals and increasing fibre gradually — are usually more effective. Your prescriber can advise what's safe alongside your Mounjaro dose.
What's the cost difference between NHS and private Mounjaro in the UK?
NHS access is free but involves waiting lists. Private prescription costs range from approximately £129–£320 per month depending on your dose (higher doses cost more). This covers the prescription only — the actual medication is dispensed by a GPhC-registered pharmacy. An initial online consultation typically costs £15–£50. Use our GLP-1 weight loss calculator to estimate your potential dose and see projected costs.
Do I need to see my GP before getting private Mounjaro?
You don't need a GP referral for private access. A licensed prescriber conducts an online clinical assessment, asking about your medical history, current medications, and weight management goals. If you're eligible (BMI 30+ or 27+ with health conditions), a prescription is issued and dispensed by a GPhC-registered pharmacy. Your GP can still monitor your progress if you inform them.
Will my GP manage my Mounjaro side effects if I get it privately?
Yes. Even though you obtain Mounjaro privately, your GP can support symptom management and monitor your overall health. It's worth informing your GP you're taking Mounjaro so they can note it in your records and discuss dietary advice, hydration, and any interactions with other medications. Your prescriber is also available for dose-related questions.
Ready to take control of your weight loss journey?
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