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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Constipation With Mounjaro — What UK Patients Need to Know
Constipation is one of the most frequently reported side effects in patients taking Mounjaro (tirzepatide) for weight management in the UK. Unlike nausea or diarrhoea, which often improve after a few weeks, constipation can persist and require active management. This guide explains why Mounjaro causes constipation, when it's a concern, and what NHS-aligned strategies can help. If you're prescribed Mounjaro through an NHS specialist tier 3 weight management service or privately, understanding this side effect will help you manage your treatment effectively.
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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 — The Mechanism
Mounjaro works by slowing gastric emptying — the rate at which food moves from your stomach into your small intestine. This slowed digestion creates the "fullness" effect that helps reduce appetite. However, this same mechanism can slow movement through your entire digestive tract, including the colon, leading to constipation.
Additionally, Mounjaro may reduce overall food intake so significantly that there is less bulk moving through the bowel. Reduced water intake and decreased physical activity — common during weight loss phases — can compound the problem. The dual GIP and GLP-1 mechanism of tirzepatide may also affect gut muscle contractions.
Constipation typically appears within the first few weeks of starting Mounjaro or when the dose titration increases (e.g., moving from 5mg to 10mg weekly). For some patients, it resolves as the body adapts; for others, it requires intervention.
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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 Constipation — Evidence-Based NHS Strategies
First-line approaches that work alongside Mounjaro include increasing water intake to at least 2–3 litres daily, eating more soluble fibre (oats, beans, fruit), and increasing physical activity gradually. Most UK GPs recommend starting with these lifestyle changes before considering medication.
If constipation persists after 2–3 weeks of lifestyle changes, your GP may suggest a stool softener like docusate sodium or a mild osmotic laxative such as lactulose or polyethylene glycol (PEG, sold as MoviPrep or similar). Do not use stimulant laxatives long-term as they can reduce bowel function. If severe constipation develops alongside stomach pain, seek urgent medical advice — it may indicate a more serious bowel issue.
- 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 a serious side effect of Mounjaro?
Constipation is common but not usually serious. However, severe constipation lasting more than a week, accompanied by abdominal pain or bloating, requires GP assessment. Constipation itself is manageable; the concern is ensuring it doesn't lead to faecal impaction or mask other bowel problems.
Will constipation go away on its own with Mounjaro?
In some patients, constipation improves as the body adapts to Mounjaro — typically after 4–8 weeks. For others, it persists and requires active management through diet, hydration, and sometimes medication. Your individual response depends on baseline bowel habits and overall health.
Can I take laxatives while taking Mounjaro?
Yes. Over-the-counter stool softeners (docusate) and osmotic laxatives (lactulose, PEG) are safe with Mounjaro. Avoid stimulant laxatives long-term. Always discuss with your GP or prescriber if constipation is severe, as they may need to review your Mounjaro dose or underlying medications.
Is constipation worse at higher Mounjaro doses?
Yes, constipation often worsens during dose titration — moving from 2.5mg to 5mg, or 5mg to 10mg. This is because the gastric-slowing effect is dose-dependent. If constipation becomes intolerable during dose increase, discuss with your prescriber; dose escalation can sometimes be slowed or paused.
Should I stop Mounjaro if constipation is severe?
Do not stop Mounjaro without discussing with your prescriber. Severe constipation is usually manageable with dietary changes and medication. Your prescriber may suggest pausing dose increases or reducing dose temporarily. Stopping abruptly without guidance may affect your weight management and blood sugar control if you have diabetes.
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.