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
Free tracker included with every download this week
Does Mounjaro Cause Constipation? NHS Guide
Yes, constipation is a common side effect of Mounjaro (tirzepatide) in the UK. It affects many people taking this prescription weight management injection, particularly during the first weeks of treatment or when doses are increased. The good news is that constipation typically improves over time as your body adjusts, and there are several practical steps you can take to manage it. This guide explains why Mounjaro causes constipation, what the NHS recommends, and when you should contact your GP for advice.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
Why Mounjaro Causes Constipation and How to Manage It
Mounjaro works as a dual GIP and GLP-1 receptor agonist, which means it activates two natural hormone pathways in your body. One effect of this activation is that it slows how quickly food moves through your stomach and digestive system—a process called gastric emptying. This slower digestion can lead to constipation, especially in the first few weeks of treatment or when your dose titration increases from one Mounjaro kwikpen strength to the next.
According to MHRA safety data and clinical trials, constipation is classified as a very common side effect (affecting more than 1 in 10 people). The NHS recommends several practical management strategies:
- Drink at least 6–8 glasses of water or sugar-free fluids daily—dehydration makes constipation worse
- Gradually increase your fibre intake through fruits, vegetables, wholegrain bread, and oats
- Move more—regular walking, stretching, or light exercise helps stimulate bowel movement
- Eat smaller, more frequent meals rather than large meals
- Avoid foods that commonly trigger constipation, such as processed foods and those high in refined sugar
- Over-the-counter remedies like fibre supplements or osmotic laxatives (e.g. lactulose) may help—ask your pharmacist
Most people find that constipation improves after 4–8 weeks as their body adjusts to the medication. If constipation persists beyond this, contact your GP or the prescriber who issued your Mounjaro UK prescription.
Clinically BackedWeight Loss
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.
When to Contact Your GP and Comparing with Wegovy
You should contact your GP or prescriber if constipation is severe, persists for more than 4 weeks, or is accompanied by abdominal pain, bloating, or nausea. These could indicate a more serious digestive issue that needs medical assessment.
If you're comparing weight management options, Mounjaro vs Wegovy shows that both medications can cause constipation as a side effect, though individual experiences vary. Wegovy (semaglutide) is a GLP-1-only injection, while Mounjaro is a dual GIP/GLP-1 agonist. Some people on Wegovy also experience constipation, but it may be less frequent than with Mounjaro in some cases. Your prescriber can help you choose which medication suits your health profile. Use our GLP-1 weight loss calculator to compare potential outcomes with either medication, and our BMI calculator to track your progress alongside bowel management.
- 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 common with Mounjaro in the UK?
Yes. Clinical trial data and MHRA monitoring shows constipation affects more than 1 in 10 people taking Mounjaro. It is classified as a "very common" side effect. However, it typically improves within 4–8 weeks as your body adjusts to the medication, especially as your dose titration stabilises.
Why does Mounjaro slow digestion?
Mounjaro's dual GIP and GLP-1 action slows gastric emptying—how quickly food leaves your stomach. This effect, combined with reduced appetite, supports weight loss but can temporarily slow bowel movement. This effect often reduces over time as your body adjusts.
What can I do to relieve constipation while taking Mounjaro?
Drink plenty of water, increase fibre gradually from fruits and vegetables, exercise regularly, eat smaller meals, and consider over-the-counter remedies like fibre supplements or osmotic laxatives. Always consult your pharmacist before using laxatives alongside Mounjaro.
Should I stop taking Mounjaro if I have constipation?
No. Constipation is a known side effect that usually improves within weeks. Do not stop taking Mounjaro without medical advice. If constipation is severe or persistent beyond 4 weeks, contact your GP or prescriber for guidance.
Is Mounjaro constipation worse than with Wegovy?
Mounjaro and Wegovy both can cause constipation, but individual experiences vary. Mounjaro's dual mechanism may cause constipation in slightly more people compared to Wegovy's single GLP-1 action, though this is not universal. Your prescriber can advise which is more suitable for you.
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.