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 You Need to Know
Constipation is one of the most common side effects reported by people taking Mounjaro (tirzepatide) in the UK. If you've been prescribed Mounjaro through an NHS specialist weight management service or privately, understanding why constipation occurs and how to manage it can help you stay on track with your treatment. This guide explains the causes, duration, and practical strategies to manage this side effect whilst using this prescription medication.
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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 Does Mounjaro Cause Constipation?
Mounjaro works by activating GIP and GLP-1 receptor pathways, which slows gastric emptying — the rate at which food leaves your stomach. This effect is what helps you feel fuller for longer and reduces appetite. However, this same mechanism also slows movement throughout your entire digestive system, which can result in constipation.
Constipation is listed as a very common side effect affecting more than 1 in 10 people taking Mounjaro in clinical trials. It's particularly common during the initial dose titration phase (when you're escalating from 2.5 mg through to 15 mg over several weeks). The slower your dose increases, the more time your body has to adapt. Additionally, if you reduce your food intake significantly whilst on Mounjaro, you may consume less dietary fibre — a key factor that exacerbates constipation.
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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 — Practical NHS-Aligned Strategies
Increase water intake: Drink at least 6–8 glasses of water daily. Dehydration worsens constipation significantly.
Gradually increase fibre: Eat more whole grains, vegetables, and fruits — but increase gradually to avoid stomach discomfort (a common Mounjaro side effect). Soluble fibre (oats, beans) is often easier to tolerate than insoluble fibre initially.
Stay physically active: Regular movement, even gentle walking, stimulates bowel motility. The NHS recommends 150 minutes of moderate activity weekly.
Consider over-the-counter options: Bulk-forming laxatives (like psyllium husk) or osmotic laxatives (like lactulose) are often recommended before stimulant laxatives. Always check with your GP or pharmacist before starting any new medication.
When to contact your GP: If constipation persists beyond a few weeks, causes severe abdominal pain, or you're concerned, speak to your prescriber.
Most people find constipation improves as their body adjusts to Mounjaro — usually within 2–4 weeks of reaching a stable dose. Your GP can assess whether constipation is significantly impacting your quality of life and whether your Mounjaro dose or other medications need adjusting. If you're using our GLP-1 weight loss calculator to project your progress on Mounjaro, remember that managing side effects like constipation helps you stay consistent with your treatment plan.
- 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 2–4 weeks of reaching your stable maintenance dose as your digestive system adapts. During the dose titration phase (weeks 1–20), constipation may fluctuate with each dose increase. If it persists beyond 4 weeks at a stable dose, speak to your GP.
Is constipation a serious side effect of Mounjaro in the UK?
Constipation is very common (affecting >1 in 10 people) but not usually serious. However, severe abdominal pain, especially pain radiating to the back, could indicate pancreatitis — a rare but serious condition. Seek immediate medical attention if you experience severe pain.
Can I take laxatives whilst on Mounjaro?
Yes, over-the-counter laxatives are often recommended. Bulk-forming laxatives (psyllium husk) and osmotic laxatives (lactulose) are typically first-line options. Always inform your GP or pharmacist that you're taking Mounjaro, as it may affect medication absorption.
Should I stop Mounjaro if constipation is severe?
Do not stop Mounjaro without speaking to your prescriber. If constipation significantly impacts your quality of life, your GP may suggest a slower dose titration, a lower maintenance dose, or lifestyle adjustments. Your prescriber can help find the right solution.
Does increasing fibre help Mounjaro constipation?
Yes, gradually increasing soluble fibre (oats, beans, vegetables) helps significantly. However, increase fibre slowly over 1–2 weeks to avoid worsening stomach pain. Combined with increased water intake and physical activity, fibre is often very effective.
Ready to take control of your weight loss journey?
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