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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Managing Constipation While Taking Mounjaro in the UK
Constipation is one of the most frequently reported side effects when starting Mounjaro (tirzepatide), affecting many people in the first weeks of treatment. This guide explains why constipation occurs with Mounjaro, practical strategies to manage it, and when to seek advice from your GP. Understanding what to expect and how to respond helps you stay comfortable whilst using this prescription weight management injection.
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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 Does Mounjaro Cause Constipation?
Mounjaro slows gastric emptying — the rate at which food moves from your stomach into your small intestine. This delayed digestion is part of how the medication reduces appetite and supports weight loss. However, this slower movement through the digestive system often leads to constipation, particularly during the first 4–8 weeks as your body adjusts to the drug. The KwikPen delivery of tirzepatide affects both appetite hormones and digestive processes, making this side effect very common. Constipation may improve as your body adapts, but managing it actively during early treatment helps significantly.
- Reduced food intake also means less fibre and bulk moving through your digestive system
- Dehydration is common when eating less — this worsens constipation
- Physical activity levels may temporarily decrease, slowing digestion further
- Individual sensitivity to tirzepatide varies — some people experience severe constipation whilst others have mild symptoms
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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.
How to Manage Constipation: Practical Strategies
Start with simple dietary and lifestyle adjustments: drink significantly more water (aim for 2–3 litres daily), increase soluble fibre gradually through oats, beans, and fruits, and move more — even short walks after meals help. Over-the-counter laxatives like docusate or macrogol (polyethylene glycol) are safe options most people try first. If constipation is severe or doesn't improve within a week, contact your GP. They may recommend prescription laxatives or pause your dose titration whilst your bowel adjusts. Never stop Mounjaro without medical guidance. Keep a food and symptom diary to identify patterns — this helps your GP understand whether constipation is improving as you continue treatment.
- Gradual fibre increase prevents bloating and gas
- Stool softeners are gentler than stimulant laxatives for long-term use
Access to Mounjaro in the UK depends on whether you're using NHS specialist services or a private prescription route. NHS access is through tier 3 weight management services following a GP referral, though waiting times vary significantly by region (ICB). Private access is through an online clinical assessment with a UK-licensed prescriber — usually faster, with consultations completed within days. Costs differ substantially: NHS is free at point of use for eligible patients, whilst private Mounjaro costs range from approximately £150–£300 per month depending on dose (starting at 2.5 mg, titrating to maximum 15 mg). An initial online consultation typically costs £50–£150, then ongoing prescription fees apply. Some private providers bundle consultation and dispensing; others charge separately. Always verify that any service is GPhC-registered and that prescriptions come from UK-licensed doctors. Compare your options — NHS offers no cost but longer waiting, whilst private offers faster access at a cost.
- 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 long does Mounjaro constipation usually last?
Most people experience the worst constipation in weeks 1–4, with improvement by week 8–12 as your body adjusts. Some experience it longer. If it persists beyond 12 weeks, contact your GP — it may warrant further investigation or a pause in dose increases.
Can I take laxatives whilst on Mounjaro?
Yes, over-the-counter laxatives like docusate, macrogol, or senna are safe with Mounjaro. Avoid stimulant laxatives long-term. Always mention any laxatives to your GP to rule out other causes, especially if constipation is severe.
Is constipation a reason to stop Mounjaro?
No — constipation alone is not a reason to stop. It's a manageable side effect. However, if it's severe or accompanied by stomach pain, contact your GP immediately as this could indicate a more serious issue like pancreatitis.
Does diet affect Mounjaro constipation?
Yes significantly. Eating less food naturally reduces digestive bulk. Gradually increasing soluble fibre, drinking more water, and maintaining regular meals helps considerably. Avoid high-fat foods which slow digestion further.
How does Mounjaro constipation compare to Wegovy side effects?
Both Mounjaro and Wegovy cause constipation via the same mechanism (slowed gastric emptying). Mounjaro users report similar or slightly higher rates because it's a dual GIP/GLP-1 agonist. Individual experience varies — some find one easier to tolerate than the other.
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.