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 With Mounjaro — UK Costs and How to Access It
Diarrhoea is one of the most common side effects reported by people taking Mounjaro (tirzepatide) in the UK, particularly during the dose titration phase. If you're considering this weight management injection or are already taking it, understanding both the cost of accessing Mounjaro and how to manage digestive symptoms is important. This guide covers UK prescription costs, NHS vs private access routes, and what to expect when starting treatment. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses.
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Calculate Your BMI Instantly
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 in the UK typically range from approximately £129 to £320 per month, depending on the dose and pharmacy provider. Starting doses (2.5 mg) are usually at the lower end, while higher maintenance doses (15 mg) cost more. Each KwikPen contains four weekly injections, so monthly costs reflect a full month's supply.
- 2.5 mg to 5 mg doses: approximately £129–£180/month
- 7.5 mg to 10 mg doses: approximately £180–£240/month
- 12.5 mg to 15 mg doses: approximately £240–£320/month
Costs vary between GPhC-registered online pharmacies, so it's worth comparing. NHS access is free at the point of use if prescribed through a specialist tier 3/4 weight management service, but availability varies significantly by region and waiting lists can be lengthy. Private online consultations typically cost £50–£150 and are required before a prescription can be issued.
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No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 Access — Which Route Is Right for You?
NHS route: Referral from your GP to a specialist weight management service (tier 3/4). Free once prescribed, but access is regional and waiting lists vary from months to years. NICE has approved tirzepatide for NHS use within specialist services.
Private route: Online consultation with a UK-licensed prescriber (no NHS referral needed), followed by prescription through a GPhC-registered pharmacy. Faster access (often within days) but costs apply. You can start at any dose level your prescriber assesses as suitable, and manage your own dose titration timeline. Our BMI calculator can help you understand your eligibility before booking a consultation.
- 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 diarrhoea worse at higher Mounjaro doses?
Diarrhoea can occur at any dose but is often most frequent during dose escalation phases. Many people report that symptoms settle after 1–2 weeks on the same dose as their body adjusts. Higher doses may carry greater risk, but individual responses vary significantly. Your prescriber can advise on managing this symptom.
Does diarrhoea mean I should stop Mounjaro?
Not necessarily. Diarrhoea usually improves over time and can often be managed through dietary changes (eating smaller meals, avoiding high-fat foods, staying hydrated). If severe or persistent beyond a few weeks, contact your prescriber. They may adjust your dose, delay your next injection, or suggest additional support.
Can I access Mounjaro through the NHS if I have private health insurance?
Access through NHS specialist services and private insurance are separate routes. If you have private health insurance, check your policy to see if weight management injections are covered. Some private plans do cover Mounjaro, but you'd need to follow their approval process. NHS access requires GP referral to a specialist service.
How long does a private online consultation take before I get a prescription?
Private online consultations typically take 15–30 minutes. If approved, your prescription is usually issued the same day or within 24 hours. The pharmacy then processes and dispatches your first KwikPen, which typically arrives within 2–5 working days, depending on the provider.
Will my GP support me if I choose private Mounjaro?
Some GPs will provide ongoing monitoring and support for privately prescribed Mounjaro; others may not. It's worth discussing with your GP practice before starting. They can help monitor your progress, check for complications, and manage any interactions with other medications you're taking, regardless of where the prescription comes from.
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.