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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Diarrhea and Mounjaro — Costs and Access in the UK
Diarrhea is one of the most common side effects of Mounjaro (tirzepatide), affecting many UK adults using this weight management injection. If you're considering Mounjaro or already taking it, understanding both the cost implications and how to access it through NHS or private routes is essential. This guide covers UK pricing, eligibility, and what to expect when managing diarrhea whilst using this prescription medication.
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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.
UK Access Routes and Mounjaro Costs
In the UK, Mounjaro is available through two main routes: NHS specialist services or private prescription. NHS access requires referral to a tier 3 or tier 4 weight management service through your GP, though availability and waiting times vary significantly by region and ICB (Integrated Care Board). NICE has approved tirzepatide for use within NHS weight management services, but access is not automatic.
Private prescription costs typically range from £129 to £320 per month depending on your dose. A Mounjaro KwikPen contains four weekly injections (one month's supply). Starting doses (2.5 mg) are usually less expensive than maintenance or escalated doses (15 mg maximum). Private prescriptions are issued following an online clinical assessment by a UK-licensed prescriber and dispensed by a GPhC-registered pharmacy. This route avoids NHS waiting lists but involves out-of-pocket costs.
- NHS: Free at point of use (if prescribed), but may involve waiting lists of months to years in some areas
- Private: £129–£320/month depending on dose escalation stage
- Additional costs: Any private GP or prescriber consultation fees (usually £20–£50)
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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.
Managing Diarrhea and Accessing Support
Diarrhea typically develops in the first few weeks of Mounjaro and often improves as your body adjusts, though it remains common throughout treatment. Managing diarrhea involves dietary changes — increase fibre gradually, stay hydrated, and eat smaller, frequent meals. Avoid high-fat foods and dairy if they trigger symptoms. Speak to your prescriber or GP about over-the-counter remedies like loperamide if diarrhea persists and significantly impacts daily life.
Whether accessing Mounjaro through NHS or private routes, your prescriber should provide ongoing support for side effect management. NHS patients have free follow-up appointments; private patients may need to pay for additional consultations. Use our free GLP-1 weight loss calculator to understand potential outcomes at different doses, and our progress tracker to log both weight loss and any side effects you experience.
- 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 much does Mounjaro cost privately in the UK?
Private Mounjaro costs typically range from £129 to £320 per month depending on your dose. Starting doses (2.5 mg) are usually around £129–£160, while maintenance and escalated doses (10–15 mg) cost £240–£320. Each KwikPen contains four weekly injections, so costs reflect a month's supply. Prices vary between providers.
Can I get Mounjaro on the NHS in the UK?
Yes, NICE has approved tirzepatide (Mounjaro) for use within NHS weight management services. However, NHS access requires referral from your GP to a specialist tier 3 or tier 4 weight management service. Availability and waiting times vary significantly by region. Many areas have lengthy waiting lists. Your GP can advise whether you're eligible and which services are available in your area.
Does diarrhea from Mounjaro usually improve over time?
Diarrhea is common in the first few weeks but often improves as your body adjusts to the medication. However, it remains a common side effect throughout treatment for some people. Managing it involves staying hydrated, eating smaller meals, increasing fibre gradually, and avoiding high-fat foods. Speak to your GP or prescriber if diarrhea persists or significantly impacts daily life.
How does the online prescription process work for Mounjaro in the UK?
You complete an online health questionnaire with a UK-licensed prescriber. They assess whether Mounjaro is suitable based on your BMI, weight-related health conditions, and medical history. If approved, a prescription is issued and sent to a GPhC-registered pharmacy, which dispenses your KwikPen. The process typically takes 1–3 working days. You'll need to pay for the medication upfront.
What's the difference between Mounjaro and Wegovy in terms of UK access and cost?
Both are available through NHS specialist services or private prescription. Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist; Wegovy (semaglutide) is a GLP-1 agonist only. Private costs are similar (£129–£320/month). Mounjaro has higher maximum doses (15 mg vs 2.4 mg) and may show slightly higher average weight loss in trials, but individual results vary. Your prescriber will recommend which suits you best.
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.