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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Does Mounjaro Cause Constipation?
Yes, constipation is a common side effect of Mounjaro (tirzepatide) in the UK. It affects a significant number of people taking this prescription weight management injection. Constipation occurs because Mounjaro slows how quickly food moves through your digestive system — a mechanism called delayed gastric emptying. While this effect typically reduces over time as your body adjusts, understanding why it happens and how to manage it is important for your treatment journey. This guide covers what you need to know about Mounjaro-related constipation and practical steps to help.
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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 is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways in your body. One of the effects of this activation is delayed gastric emptying, which means food passes through your stomach more slowly than usual. This slower digestion is actually part of how Mounjaro helps with weight management — it keeps you feeling fuller for longer. However, this same slowing effect can lead to constipation, particularly in the first few weeks of treatment or when your dose increases.
The good news is that your digestive system often adapts over time. Many people find constipation becomes less problematic as they continue treatment. Additionally, constipation is typically manageable with lifestyle adjustments — you don't necessarily need to stop your medication if you experience this side effect. Your prescriber can also discuss alternative management strategies or dose timing with you if constipation persists.
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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.
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Managing Mounjaro Constipation — Practical Steps
Increase your fluid intake: Drink at least 2–3 litres of water daily. Dehydration worsens constipation, and Mounjaro may increase this risk. Keep water accessible throughout the day.
Increase dietary fibre gradually: Eat more vegetables, fruits, beans, and whole grains. Add fibre slowly to avoid bloating or stomach discomfort — sudden increases can worsen nausea that some people experience with Mounjaro.
Move more: Physical activity stimulates bowel movements. Even a 20-minute daily walk can help. Track your activity and progress using our free progress tracker alongside your Mounjaro treatment.
Consider stool softeners: Over-the-counter options like docusate sodium are safe alongside Mounjaro. Speak to your pharmacist or GP before starting any new supplements or medicines.
Timing: Some people find taking their weekly Mounjaro injection at a time when constipation is least disruptive (e.g., before a weekend) helpful for planning.
- 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 common is constipation with Mounjaro in the UK?
Constipation is very common — clinical data shows it affects more than 1 in 10 people taking Mounjaro. It's one of the most frequently reported side effects. However, severity varies widely. Some people experience mild constipation that resolves quickly; others find it more persistent. Your GP or prescriber can discuss whether this risk is acceptable for you personally.
Does constipation happen with every Mounjaro dose increase?
Not necessarily, but constipation risk can increase when you move to a higher dose. This is because your body is adjusting to a stronger medication effect. The 2.5 mg starting dose typically causes less constipation than the 5 mg or 10 mg doses. Most people find symptoms settle after 1–2 weeks at each new dose level.
Is constipation with Mounjaro the same as with Wegovy?
Both Mounjaro and Wegovy can cause constipation because both affect digestion, but Mounjaro works via a dual mechanism (GIP plus GLP-1) whereas Wegovy is a GLP-1 agonist only. Clinical trial data suggests constipation rates are similar, though individual experiences vary significantly. Mounjaro's tirzepatide mechanism may cause slightly different symptoms for some people.
Should I stop Mounjaro if I get constipation?
Not immediately. Constipation is manageable in most cases through the lifestyle strategies outlined above — increased water, fibre, activity, and sometimes stool softeners. Many people find it improves as their body adjusts. If constipation is severe or doesn't improve after 2–3 weeks, contact your GP or prescriber. They can discuss whether dose adjustment or alternative support is needed.
Does the Mounjaro KwikPen dose affect how much constipation I'll have?
Constipation risk generally increases with higher doses. The starting 2.5 mg KwikPen dose causes less constipation than the 15 mg maximum dose. Your prescriber will escalate your dose gradually (every 4 weeks) partly to allow your body to adjust, which helps minimise side effects like constipation. This is called dose titration and is a key part of Mounjaro treatment in the UK.
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