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 on Mounjaro — What NHS Patients Should Know
If you're taking Mounjaro (tirzepatide) and experiencing constipation, you're not alone — difficulty with bowel movements is one of the most commonly reported side effects in UK clinical practice. This happens because Mounjaro slows gastric emptying, meaning food moves through your digestive system more slowly. The good news is that constipation is usually manageable with dietary adjustments and lifestyle changes, and it often improves as your body adapts to the medication. This guide explains what causes it, practical solutions, and when to speak to your GP.
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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 Constipation Happens on Mounjaro
Mounjaro works by activating GIP and GLP-1 receptor pathways, which regulate appetite and blood sugar. A side effect of this mechanism is that it slows gastric emptying — the rate at which food moves from your stomach into your small intestine. Combined with reduced food intake (because you feel fuller faster) and sometimes lower fluid consumption, this slower digestion can lead to constipation.
In UK clinical practice, constipation is reported in approximately 20–30% of people taking Mounjaro, making it one of the most frequent gastrointestinal side effects. It's typically mild to moderate and tends to improve within 4–8 weeks as your body adjusts to the medication. Constipation is also common with other GLP-1 medications like Wegovy (semaglutide), though the underlying mechanism is similar across this drug class.
- Slowed stomach emptying (expected pharmacological effect)
- Reduced overall food intake and calorie consumption
- Lower fluid and fibre intake during weight loss
- Reduced physical activity in some patients
- Dehydration or electrolyte changes
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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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NHS-Recommended Strategies to Manage Constipation
Practical dietary and lifestyle adjustments can significantly reduce constipation on Mounjaro: increase your daily water intake to at least 2–3 litres per day, as dehydration is a major contributor. Gradually increase dietary fibre through vegetables, fruits, pulses, and wholegrains — but do this slowly to avoid bloating. Gentle regular physical activity, even a 20–30 minute walk, helps stimulate bowel movements. If constipation persists after 2 weeks, your GP may recommend a stool softener like docusate sodium or a mild osmotic laxative such as lactulose, which are available over-the-counter at UK pharmacies without prescription. Avoid stimulant laxatives if possible, as they can become less effective with prolonged use.
- 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 when taking Mounjaro?
Constipation typically improves within 4–8 weeks as your body adjusts. However, it can persist longer in some people. If it continues beyond 8 weeks despite dietary and lifestyle changes, contact your GP. They may adjust your dose or recommend medication to help manage it.
Should I stop taking Mounjaro if I'm constipated?
No — constipation alone is not a reason to stop Mounjaro. Speak to your GP before making any changes to your prescription. In most cases, dietary adjustments and over-the-counter remedies resolve the issue. Your prescriber can advise whether dose adjustment is appropriate if constipation is severe.
Is constipation on Mounjaro different from constipation on Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) slow gastric emptying and cause constipation through similar mechanisms. The frequency and severity may vary slightly between individuals, but management strategies are the same. Your GP can advise which medication suits you best.
What over-the-counter remedies are safe to use with Mounjaro?
Docusate sodium (stool softener) and lactulose (osmotic laxative) are safe and commonly recommended in UK practice. Avoid stimulant laxatives as routine treatment. Always inform your prescriber or GP about any over-the-counter medications you're taking, as some may interact with your tirzepatide dose titration.
When should I contact my GP about constipation on Mounjaro?
Contact your GP if constipation is severe, lasts longer than 2 weeks despite self-help measures, or is accompanied by stomach pain, vomiting, or inability to pass stools. These could indicate a more serious issue like bowel obstruction, though this is rare. Your GP can assess whether the constipation is related to Mounjaro or another cause.
Ready to take control of your weight loss journey?
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