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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Why Am I Not Pooping on Mounjaro?
Constipation is one of the most common side effects of Mounjaro (tirzepatide), affecting many people taking this GLP-1 and GIP receptor agonist for weight management. Understanding why it happens and how to manage it can help you decide whether Mounjaro is right for you. This guide explains the causes, practical solutions, and how to access support through the UK healthcare system, whether through NHS specialist services or private prescription 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.
Why Mounjaro Causes Constipation and How to Manage It
Mounjaro works by slowing gastric emptying — the rate at which food moves through your digestive system. This effect helps you feel fuller for longer, supporting weight loss. However, this slowdown can also slow movement through the entire digestive tract, leading to constipation. Additionally, when people lose weight rapidly on Mounjaro, reduced food intake overall means less fibre and bulk moving through the system, which compounds the problem.
Practical management strategies include:
- Drinking significantly more water — at least 2–3 litres daily, often more
- Increasing dietary fibre gradually through vegetables, fruits, and whole grains
- Staying physically active — even light daily walks support digestive movement
- Taking over-the-counter fibre supplements or stool softeners (discuss with your prescriber first)
- Avoiding dehydration, which worsens constipation
- Eating smaller, more frequent meals rather than large meals
Most people find that constipation improves as their body adjusts to Mounjaro, typically within 2–4 weeks. However, if constipation is severe or persistent, inform your prescriber — they may adjust your dose or recommend additional support.
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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 and Costs
NHS access: Available through specialist tier 3 weight management services, but access varies significantly by region and waiting lists can be lengthy. Your GP can refer you, but you'll need a BMI of 30 or above (or 27+ with weight-related health conditions) and may face months of waiting.
Private prescription route: You can obtain Mounjaro through UK-licensed online prescribers following a clinical assessment, typically costing £180–£280 per month depending on your dose. Each KwikPen contains four weekly injections. This route bypasses NHS waiting lists but requires you to pay privately. A prescriber will assess whether tirzepatide is suitable before issuing a prescription to a GPhC-registered pharmacy.
If constipation is a significant concern, discuss it with your prescriber during your assessment — they can advise whether Mounjaro or an alternative like Wegovy might suit you better, or suggest preventative strategies before you start.
- 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 constipation on Mounjaro permanent?
No. Most people experience constipation as a temporary side effect that typically improves within 2–4 weeks as the body adjusts. However, some individuals continue to experience mild constipation throughout treatment. It's manageable with increased water intake, fibre, and physical activity. If it remains severe, speak to your prescriber about dose adjustment.
What's the difference between Mounjaro and Wegovy regarding constipation?
Both are GLP-1-type medications and cause constipation through similar mechanisms. Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy is GLP-1 only. Clinical trial data suggests comparable rates of constipation between the two, though individual experiences vary. Your prescriber can discuss which might suit you best based on your medical history.
Can I take laxatives while on Mounjaro?
Yes, over-the-counter stool softeners and fibre supplements are generally safe to use alongside Mounjaro. However, avoid strong laxatives without checking with your prescriber first, as they can interact with how quickly Mounjaro moves through your system. Discuss any medication additions with the person who prescribed your Mounjaro.
Does the Mounjaro dose affect how much constipation I'll experience?
Constipation tends to be more noticeable at higher doses (10 mg, 15 mg) since the medication's effects on gastric emptying are stronger. Some people find that starting at lower doses and titrating slowly (increasing gradually) helps minimise constipation. Your prescriber will discuss dose progression during your clinical assessment.
How do I access Mounjaro if I'm struggling with side effects like constipation?
If you're on NHS Mounjaro through a specialist service, discuss side effects directly with that service — they can adjust your dose or provide additional support. If using private prescription Mounjaro, contact your prescriber to discuss ongoing management. You can also consult your GP for general constipation advice and fibre/hydration support regardless of how you access the medication.
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.