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 and Diarrhoea — Understanding Costs and UK Access
Diarrhoea is one of the most commonly reported side effects of Mounjaro (tirzepatide) in the UK, affecting many people during the first few weeks of treatment. If you're considering this prescription weight management injection, understanding both the potential side effects and how to access it — whether through the NHS or privately — is essential. This guide covers UK costs, eligibility, and what to expect when starting Mounjaro.
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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 Cost and Pricing in the UK
Mounjaro is available through two main routes in the UK: NHS specialist services and private prescription. Private costs typically range from £120 to £320 per month depending on your dose and the provider, though prices vary. Each KwikPen contains four weekly injections, so costs reflect a four-week supply.
NHS access is free at point of use but is limited to tier 3 specialist weight management services. Not all areas offer NHS Mounjaro prescriptions, and waiting lists can be lengthy — sometimes 6–12 months or longer depending on your region and ICB (Integrated Care Board). Dose titration means starting at 2.5 mg and increasing gradually every four weeks, so your monthly cost will increase as you progress through doses. At maximum dose (15 mg), private costs are typically at the higher end of the range. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses before committing.
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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: How to Access Mounjaro in the UK
NHS route: Ask your GP for a referral to your local tier 3 weight management service. You'll need a BMI of 30+ or 27+ with weight-related health conditions. Assessment and prescription happen through the specialist service, but availability varies significantly by region.
Private route: Complete an online clinical assessment with a UK-licensed prescriber. If approved, your prescription is dispensed by a GPhC-registered pharmacy. This route doesn't require NHS referral and is faster — typically 3–7 days from assessment to first delivery. Mounjaro vs Wegovy comparisons often centre on cost; Wegovy is sometimes cheaper at lower doses but Mounjaro may offer better results for some people. Results vary individually. Both require ongoing prescriber support and monitoring.
- 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.
Does diarrhoea with Mounjaro go away?
For most people, nausea and diarrhoea improve within the first 4–8 weeks as your body adjusts. However, some people experience ongoing digestive changes, particularly at higher doses. If diarrhoea persists or is severe, contact your prescriber — they may adjust your dose or suggest dietary strategies. Staying hydrated and eating smaller, lower-fat meals often helps.
Can I switch from Mounjaro to Wegovy if I get diarrhoea?
Both Mounjaro and Wegovy can cause diarrhoea because both are GLP-1 or GLP-1/GIP agonists. However, side effects vary between individuals and medications. A prescriber can discuss switching options with you based on your tolerance. Any medication change should be managed by your prescriber, not attempted independently.
Is Mounjaro cheaper on the NHS or private prescription?
Mounjaro is free on the NHS, but access is limited to tier 3 specialist services with long waiting lists in many areas. Private costs range from £120–£320/month depending on dose. If you can access NHS Mounjaro quickly, it's cost-free; otherwise, private prescription offers faster access at a cost. Check your local ICB's policies.
How long do I need to stay on Mounjaro?
This varies individually. Some people stay on Mounjaro long-term for ongoing weight management; others use it for 1–2 years then transition to lifestyle maintenance. Your prescriber will discuss duration with you based on your weight loss goals and health. Stopping suddenly isn't recommended — doses are typically stepped down gradually.
What's the difference between Mounjaro (tirzepatide) and other weight loss injections?
Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1-only agonist. The dual mechanism may produce slightly greater average weight loss in trials, but individual results vary significantly. Cost, availability, and personal tolerance all influence which medication is right for you. A prescriber can advise on the best option.
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.