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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What to Do If You're Constipated on Mounjaro
Constipation is one of the most common side effects reported by people taking Mounjaro (tirzepatide) in the UK. It happens because Mounjaro slows how quickly food moves through your digestive system, and this effect can take time to settle. The good news is that constipation often improves as your body adjusts to the medication, though managing it proactively can help you feel more comfortable. If you're experiencing constipation whilst on Mounjaro, there are practical steps you can take, and it's important to let your prescriber know if it becomes severe or doesn't improve.
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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
Mounjaro is a dual GIP and GLP-1 receptor agonist — it works by activating two hormone pathways that help regulate appetite and blood sugar. One side effect of this mechanism is delayed gastric emptying, meaning food moves more slowly from your stomach into your intestines. This slowed digestion can lead to constipation, particularly in the first few weeks of treatment or when your dose increases.
Constipation is listed as a very common side effect of Mounjaro — it may affect more than 1 in 10 people taking the medication. The severity varies: some people experience mild, manageable constipation, whilst others find it more bothersome. The good news is that for most people, constipation tends to improve within 2–4 weeks as the body adjusts to the tirzepatide dose. However, this doesn't happen for everyone, and some people need ongoing management strategies.
Other digestive side effects of Mounjaro include nausea, diarrhoea, and stomach pain — interestingly, some people experience diarrhoea later in their treatment cycle, which can alternate with periods of constipation.
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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.
Practical Steps to Manage Constipation on Mounjaro
Increase water intake: Drink at least 2–3 litres of water per day. Dehydration makes constipation worse, and Mounjaro can reduce your appetite for fluids alongside food.
Add fibre gradually: Increase fruit, vegetables, wholegrains, and legumes — but do this slowly to avoid bloating or cramping. Aim for 25–30g of fibre daily.
Move your body: Even light activity like a 20-minute walk after meals can stimulate digestion and bowel movement.
Stool softeners: Over-the-counter options like docusate sodium or osmotic laxatives (e.g. lactulose) are generally safe to use alongside Mounjaro. Avoid stimulant laxatives (like senna) unless advised by your GP.
Timing and meals: Eat smaller, regular meals rather than large portions. This may reduce nausea and constipation together.
Talk to your prescriber: If constipation is severe, persistent, or affecting your quality of life, contact your GP or the healthcare provider who prescribed your Mounjaro. They may adjust your dose, recommend specific treatments, or check for other causes.
- 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 constipation last on Mounjaro?
For most people, constipation improves within 2–4 weeks as the body adjusts to tirzepatide. However, this varies — some people experience constipation throughout their treatment, whilst others find it resolves quickly. If it persists beyond 4 weeks or worsens, contact your GP or prescriber.
Is constipation on Mounjaro dangerous?
Mild to moderate constipation is uncomfortable but not usually dangerous. However, severe constipation or pain in the upper abdomen could indicate a more serious issue like pancreatitis or gallbladder problems — seek urgent medical attention if you experience severe abdominal pain spreading to your back.
Can I take laxatives whilst on Mounjaro?
Yes, over-the-counter stool softeners and osmotic laxatives (like lactulose or polyethylene glycol) are generally safe to use with Mounjaro. Avoid stimulant laxatives unless your GP recommends them. Always read the label and ask your prescriber if you're unsure about a specific product.
Does constipation happen at every Mounjaro dose?
Constipation can occur at any dose, but it's often more noticeable when you first start Mounjaro or when your dose increases. As your body adjusts, symptoms typically improve — though not always. Dose titration (gradual increases) helps minimise side effects.
Should I stop taking Mounjaro if constipation is severe?
Do not stop Mounjaro without speaking to your prescriber. If constipation is severe or affecting your health, contact your GP or prescriber immediately. They can suggest management strategies, adjust your dose, or discuss whether the medication is right for you — stopping suddenly without guidance isn't recommended.
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.