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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How To Help Constipation On Mounjaro
Constipation is one of the most common side effects experienced by people taking Mounjaro (tirzepatide) in the UK. Because Mounjaro slows how quickly food moves through your digestive system, many patients notice changes in bowel habits, especially during the first few weeks or when increasing doses. The good news is that constipation can be managed effectively through practical lifestyle changes and, if needed, support from your GP. This guide covers evidence-based strategies recommended by NHS professionals to help you stay comfortable whilst taking this prescription-only 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 Mounjaro Causes Constipation And What You Can Do
How Mounjaro affects digestion: Mounjaro works by activating GLP-1 and GIP receptors, which slow gastric emptying — the rate at which food leaves your stomach. This slower movement through your digestive system can reduce appetite and support weight loss, but it also reduces how often your bowels move, leading to constipation. This effect is often more noticeable in the first 4 weeks or when you increase to a higher dose, though it typically improves as your body adjusts.
- Increase water intake significantly — aim for 2–3 litres daily. Dehydration worsens constipation dramatically, so prioritise hydration throughout the day
- Add soluble fibre gradually — increase oats, beans, and vegetables slowly to avoid bloating. Sudden fibre increases can cause stomach discomfort when taking Mounjaro
- Move your body regularly — even 15–20 minutes of walking after meals can stimulate bowel movement and improve overall digestion
- Avoid alcohol and caffeine — both dehydrate you further and can worsen constipation
If constipation persists after 1–2 weeks despite these changes, contact your GP. They may suggest a stool softener like docusate or a gentle osmotic laxative such as lactulose — both are safe to use alongside Mounjaro whilst your body adapts to the dose titration.
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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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Everything you need to understand your weight, explore medication options, and track your progress — completely free.
When To Seek GP Support And Medication Options
Whilst mild constipation is expected and usually resolves within 2–3 weeks as your body adjusts, you should contact your GP if constipation is severe, lasts more than 3 weeks, or is accompanied by severe stomach pain. Severe abdominal pain could indicate a more serious complication like pancreatitis or bowel obstruction, both of which require urgent medical attention.
Your GP can recommend laxatives safe to use with tirzepatide, including osmotic laxatives (lactulose, polyethylene glycol) or stool softeners (docusate). Avoid stimulant laxatives like senna long-term. If you use the Mounjaro KwikPen and experience ongoing digestive issues after dose titration completes, your prescriber may discuss whether your current dose is appropriate for you. Track your symptoms using our free progress tracker so you have clear information to share with your GP.
- 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 on Mounjaro normal?
Yes, constipation is very common with Mounjaro — it affects more than 1 in 10 people during treatment. It typically occurs because tirzepatide slows gastric emptying. Most cases improve within 2–3 weeks as your body adjusts, but it's worth contacting your GP if it persists or worsens.
What's the fastest way to relieve Mounjaro constipation?
Increase water intake to 2–3 litres daily — this is the most effective immediate strategy. Add soluble fibre gradually (oats, beans), walk regularly after meals, and avoid dehydrating substances like caffeine and alcohol. These changes work within days for most people.
Can I take laxatives whilst on Mounjaro?
Yes, many laxatives are safe to use with Mounjaro. Osmotic laxatives like lactulose or polyethylene glycol (Movicol) and stool softeners like docusate are suitable. Avoid stimulant laxatives long-term. Always check with your GP or pharmacist before starting any new medication, as some may interact with other medicines you take.
Should I stop taking Mounjaro if constipation is severe?
Do not stop Mounjaro without speaking to your GP or prescriber — stopping suddenly without medical guidance isn't recommended. If constipation is severe, contact your GP immediately. They may adjust your dose, recommend laxatives, or discuss whether a lower dose would be better for you at this stage.
How long does constipation last on Mounjaro?
For most people, constipation improves within 2–3 weeks as your body adjusts to the medication and you implement lifestyle changes. However, it can persist at higher doses. Staying hydrated, moving regularly, and eating adequate fibre help significantly. If it doesn't improve after 3 weeks, your GP can offer additional support or review whether your current dose is right for you.
Ready to take control of your weight loss journey?
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