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 known side effect of Mounjaro (tirzepatide), though it affects people differently. Mounjaro is a prescription-only GLP-1 receptor agonist and GIP receptor agonist weight loss injection available in the UK. Understanding which gastrointestinal side effects are most common, how long they typically last, and practical management strategies can help you make an informed decision about whether this weekly weight loss injection is right for you. This guide covers the facts about Mounjaro constipation in the UK context.
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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.
Constipation and Other Gastrointestinal Side Effects
According to clinical trial data, constipation is one of the most common side effects of Mounjaro, affecting a significant proportion of users. However, the frequency and severity vary between individuals. Constipation typically occurs because tirzepatide slows gastric emptying — how quickly food moves through your digestive system — which is part of how the medication helps with appetite control.
Other gastrointestinal side effects commonly reported include:
- Nausea — very common, especially in the first weeks
- Diarrhoea — often alternates with constipation
- Vomiting — less common but can occur
- Stomach pain or discomfort — usually mild to moderate
- Indigestion — affects some users regularly
Most gastrointestinal side effects improve after the first 4–8 weeks as your body adjusts to the medication. If constipation persists or worsens, your prescriber should be informed, as they may adjust your dose or suggest management strategies.
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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
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Managing Constipation While Taking Mounjaro
If you experience constipation while taking Mounjaro, practical lifestyle adjustments can help significantly. Increase water intake — aim for at least 2–3 litres daily, as dehydration worsens constipation. Eat more fibre gradually — soluble fibre from oats, fruits, and vegetables supports bowel regularity without causing bloating. Regular physical activity such as daily walks improves digestion and bowel function.
Speak to your GP or prescriber before taking over-the-counter laxatives, as some may interact with your medication or mask underlying issues. Your prescriber can recommend safe options if lifestyle changes alone aren't sufficient. Keeping a symptom diary can help your healthcare team understand your individual response to Mounjaro and adjust your treatment plan accordingly.
- 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?
For most people, gastrointestinal side effects including constipation improve within 4–8 weeks as your body adjusts. However, some users may experience ongoing symptoms. If constipation persists beyond 8 weeks, inform your prescriber — they can assess whether a dose adjustment or alternative management is needed.
Is constipation a serious side effect of Mounjaro?
Constipation is uncomfortable but not typically serious. However, severe or persistent stomach pain could indicate a more serious condition like pancreatitis or gallbladder problems — seek urgent medical attention if you experience severe abdominal pain spreading to your back.
Can I use laxatives while taking Mounjaro?
Always check with your GP or prescriber before using laxatives, as some may interact with tirzepatide or mask underlying issues. Your healthcare team can recommend safe, appropriate options if lifestyle changes aren't enough.
Does constipation mean Mounjaro isn't working?
No. Constipation is a side effect separate from how well the medication is helping with weight loss. If you're concerned about either the side effects or the weight loss results, discuss both with your prescriber — they can help adjust your approach.
Are there alternatives to Mounjaro if constipation is problematic?
Yes. Wegovy (semaglutide), a GLP-1 receptor agonist, is also licensed in the UK for weight management and may have a different side effect profile. Your prescriber can discuss whether switching to an alternative weight loss injection might be better suited to 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.