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 Causing Diarrhoea — Cost and Access in the UK
Diarrhoea is one of the most common side effects when starting Mounjaro (tirzepatide), particularly during the dose titration phase. If you're experiencing this or considering Mounjaro, understanding how to access it in the UK — whether through the NHS or private prescription — is essential. This guide covers costs, eligibility, and what to expect when starting this GLP-1 medication, plus practical support for managing digestive side effects.
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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.
How Much Does Mounjaro Cost in the UK?
Private prescription costs for Mounjaro vary depending on the dose and provider, but typically range from £129 to £320 per month. A full KwikPen contains four weekly doses, so costs reflect a four-week supply. Starting doses (2.5 mg) are usually at the lower end of this range, while maintenance doses (15 mg) cost more. NHS access is free but is typically only available through specialist tier 3 weight management services following a referral from your GP — and waiting times can be many months depending on your local Integrated Care Board. Some NHS areas have very limited availability. Private prescription costs do not include delivery or consultation fees, which GPhC-registered online prescribers may charge separately (typically £15–£50 for an online assessment). The KwikPen device itself is included in the medication cost and cannot be reused.
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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 Routes — Which Is Right for You?
NHS route: Free medication but requires GP referral to specialist weight management services, long waiting lists (sometimes 12+ months), and varies by region. Private route: Faster access (often within days), no referral needed, but you pay full cost per prescription. Both routes require a clinical assessment — your prescriber will check if you're eligible based on BMI, health conditions, and medication interactions. If you experience diarrhoea on Mounjaro, either route provides the same medication; side effect support depends on your prescriber, not the access route. Our GLP-1 weight loss calculator can help you understand potential outcomes at different doses, while your prescriber will assess suitability for your individual circumstances.
- 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.
Is Mounjaro available on the NHS in the UK?
Yes, NICE has approved tirzepatide (Mounjaro) for NHS use, but access is through specialist tier 3 weight management services, not standard GP prescription. You need a referral from your GP. Availability and waiting times vary significantly by region — some areas have waiting lists of 12+ months, while others have limited services.
How long does diarrhoea usually last when starting Mounjaro?
Diarrhoea is very common in the first 4–8 weeks, particularly during dose increases. For most people, it improves as the body adjusts to the medication. If it persists beyond 12 weeks or is severe, speak to your prescriber — they may advise slowing the dose titration or trying dietary adjustments.
Can I get Mounjaro without waiting for the NHS?
Yes, private prescription is available through online consultations with UK-licensed prescribers. Following a clinical assessment, if you're eligible, a prescription is issued and dispensed by a GPhC-registered pharmacy. Private access is faster but costs approximately £129–£320 per month depending on dose.
What should I eat if Mounjaro is causing diarrhoea?
Eat small, frequent meals; avoid high-fat and high-fibre foods initially; stay hydrated; try bland foods like rice, toast, and plain chicken. Your GP or prescriber can refer you to a dietitian for personalised advice. Side effects often settle as your body adjusts.
Do I need a GP's approval to get Mounjaro privately?
No, you do not need GP approval for private prescription. However, informing your GP that you're taking Mounjaro is recommended so they can monitor your health and adjust other medications if needed. Your private prescriber will take a full medical history during the online assessment.
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.