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 common side effect of Mounjaro (tirzepatide) in the UK, affecting many patients during treatment. However, digestive side effects like constipation typically improve as your body adjusts to the medication over the first few weeks. This guide explains why Mounjaro may cause constipation, what NHS guidance says, and practical steps you can take to manage it. If you're considering this prescription-only weight management injection or currently taking it, understanding what to expect can help you stay comfortable and informed throughout your treatment.
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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 Does Mounjaro Cause Constipation?
Mounjaro (tirzepatide) works as a dual GIP and GLP-1 receptor agonist, activating two natural hormone pathways that regulate appetite and blood sugar. One of the key mechanisms is slowing gastric emptying — how quickly food moves from your stomach into your small intestine. This slower digestion helps you feel fuller for longer, which supports weight loss. However, this same effect can reduce overall gut movement and cause constipation, particularly during the first 4–8 weeks of treatment or when your dose increases.
Constipation is listed as a common side effect in UK prescribing information, meaning it may affect up to 1 in 10 people taking Mounjaro. Other digestive changes are also typical: nausea, diarrhoea, and stomach pain are very common (affecting more than 1 in 10 patients). These effects usually settle as your body acclimatises to the medication, though timing varies between individuals.
- Slower gastric emptying reduces overall intestinal movement
- Reduced appetite may mean less food volume moving through your digestive system
- Dose titration — higher doses may cause more pronounced effects initially
- Individual variation — some patients experience no constipation, others do
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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.
Managing Mounjaro Constipation — Practical Steps
If you experience constipation while taking Mounjaro, several practical steps can help. Increase your water intake significantly — aim for at least 2–3 litres daily, as dehydration worsens constipation. Add high-fibre foods gradually: leafy greens, oats, beans, and prunes. Gentle physical activity — even a 20-minute daily walk — can stimulate bowel movement. Your GP or prescriber can advise whether over-the-counter stool softeners (like docusate) are appropriate for you; avoid strong laxatives without medical guidance as they can mask underlying issues.
Most importantly, contact your GP or prescriber if constipation is severe, persistent beyond 2 weeks, or accompanied by abdominal pain. Rarely, severe constipation can indicate a more serious issue. Your dose may be adjusted or your prescriber may recommend a short-term stool softener. Document your symptoms using our free progress tracker to share with your healthcare team — this helps them understand your response to treatment and make informed decisions about your ongoing care.
- 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 side effect of Mounjaro in the UK?
Yes, constipation is a common side effect, listed in UK prescribing information as affecting up to 1 in 10 patients. It's caused by Mounjaro slowing gastric emptying — how quickly food moves through your stomach. Most patients find it improves within the first 4–8 weeks as their body adjusts.
How long does Mounjaro constipation last?
Constipation typically improves within 2–4 weeks as your body adjusts to the medication. However, timing varies — some people experience no constipation, whilst others may need longer. If it persists beyond 2 weeks or is severe, contact your GP or prescriber.
What should I do if Mounjaro makes me constipated?
Increase water intake to 2–3 litres daily, add high-fibre foods gradually, and take gentle exercise like walking. Avoid strong laxatives without medical advice. If constipation is severe or persistent, contact your GP — they may recommend a stool softener or adjust your dose.
Can I take laxatives whilst taking Mounjaro?
Over-the-counter stool softeners like docusate are generally safe, but avoid strong laxatives without speaking to your GP or prescriber first. They can mask underlying problems and may not address the root cause. Always seek professional guidance before taking any new medication alongside Mounjaro.
Should I stop taking Mounjaro if I get constipated?
Do not stop taking Mounjaro without speaking to your prescriber — stopping suddenly is not recommended. Instead, contact your GP or prescriber to discuss practical management options. Most constipation improves with hydration, fibre, and time, and your dose may be adjusted if needed.
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.