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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Managing Mounjaro Constipation — UK Health Guidance
Constipation is one of the most common side effects of Mounjaro (tirzepatide), affecting many people during treatment. This happens because Mounjaro slows how quickly food moves through your digestive system as part of how it works. While it's usually manageable, understanding why it occurs and how to address it can help you continue your weight management journey comfortably. This guide covers practical strategies and when to seek GP support.
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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 Managing It
Mounjaro is a dual GIP and GLP-1 receptor agonist that slows gastric emptying — the rate at which food leaves your stomach. This mechanism helps reduce appetite, but it can also slow your entire digestive system, leading to constipation. The effect is often most noticeable when starting Mounjaro or increasing your dose during the titration schedule.
- Stay hydrated — aim for at least 2–3 litres of water daily, as dehydration worsens constipation
- Increase dietary fibre gradually — oats, beans, vegetables, and whole grains help but add slowly to avoid stomach discomfort
- Move regularly — even 10–15 minutes of walking daily can stimulate digestion
- Consider stool softeners — products containing docusate sodium are available over-the-counter at pharmacies
- Limit medications that worsen constipation — including some pain relief and iron supplements; discuss with your GP
Most people find constipation improves as their body adjusts to Mounjaro, typically within 2–4 weeks. If it persists beyond this, speak to your prescriber about adjustment options or GP support.
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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.
Accessing Mounjaro in the UK and Cost Considerations
In the UK, Mounjaro is available through two main routes: NHS specialist weight management services and private online prescribing. NHS access is through tier 3 specialist services following a GP referral, though availability and waiting times vary significantly by region. Private online prescribing is available without NHS referral following an online consultation with a UK-licensed prescriber, typically costing £129–£320 monthly depending on your current dose and KwikPen quantity. Both routes require clinical assessment to confirm eligibility. When considering your options, remember that managing side effects like constipation is part of ongoing support — your prescriber can advise whether dose adjustments or lifestyle changes help. Use our free BMI calculator above to check your eligibility status and understand your personal weight health context.
- 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 very common but usually not serious. It's a known side effect affecting many people and typically improves as your body adjusts. However, if you experience severe abdominal pain, persistent constipation lasting more than a few days, or signs of obstruction, contact your GP immediately as these are rare but require medical attention.
How long does Mounjaro constipation last?
Most people experience constipation for 2–4 weeks when starting Mounjaro or increasing their dose during titration. Some find it improves naturally as their body adapts. Others need ongoing management strategies. If constipation doesn't improve after 4 weeks despite lifestyle changes, discuss with your prescriber about possible adjustments or GP support.
Can I take laxatives while on Mounjaro?
Yes, over-the-counter stool softeners and mild laxatives are generally safe with Mounjaro. Start with gentle options like docusate sodium or lactulose. Avoid strong laxatives without GP advice. Always discuss any new medications or supplements with your prescriber, as some can interact with Mounjaro or affect its absorption.
Should I stop Mounjaro if constipation is severe?
No — do not stop Mounjaro without speaking to your prescriber first. Stopping suddenly isn't safe. Instead, contact your GP or prescriber to discuss the constipation. They may recommend dose adjustments, lifestyle changes, or additional support. Side effects often improve with time and management strategies.
What's the difference between Mounjaro and Wegovy for constipation risk?
Both Mounjaro and Wegovy (semaglutide) can cause constipation as they both slow digestion. Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy works on GLP-1 alone. Clinical data shows similar rates of gastrointestinal side effects between them. Individual experiences vary, and your prescriber can help determine which suits you best.
Ready to take control of your weight loss journey?
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