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
Free tracker included with every download this week
Mounjaro Constipation Relief vs Wegovy — What's the Difference?
Constipation is one of the most common side effects reported with both Mounjaro (tirzepatide) and Wegovy (semaglutide). While both GLP-1-based weight loss injections can cause digestive changes, the frequency and severity may differ between the two medications. Understanding how each works and what the clinical trial data shows can help you and your prescriber decide which option might be more suitable for your individual needs. This guide compares the two based on UK evidence and real-world experience.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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.
How Mounjaro and Wegovy Cause Constipation — Mechanism Differences
Both Mounjaro and Wegovy slow gastric emptying—the speed at which food leaves your stomach—which can lead to constipation. However, their mechanisms differ slightly.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. This dual action may cause a wider range of gastrointestinal effects, including constipation, as it affects appetite regulation and stomach emptying through two hormone pathways simultaneously. In the SURMOUNT clinical trials, constipation was reported in approximately 26% of participants at the highest dose (15 mg).
Wegovy (semaglutide) is a single GLP-1 receptor agonist. It works through one hormone pathway, which some users find causes fewer or less severe digestive side effects overall. In the STEP clinical trials, constipation was reported in approximately 20% of participants at the 2.4 mg dose.
The difference is modest, but individual experience varies considerably. Your prescriber will discuss which medication suits your medical history and baseline digestive health.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 Constipation on Either Medication — Practical Strategies
If you're taking either Mounjaro or Wegovy, constipation can usually be managed effectively without stopping the medication:
- Hydration: Drink 2–3 litres of water daily—dehydration worsens constipation significantly.
- Fibre intake: Gradually increase soluble fibre (oats, beans, vegetables) rather than sudden large increases, which can worsen bloating.
- Physical activity: Regular walking or gentle exercise stimulates bowel movement.
- Osmotic laxatives: Lactulose or polyethylene glycol (PEG) are safe and effective alongside GLP-1 medications.
- Stool softeners: Docusate sodium can help without causing urgency.
- Meal timing: Smaller, more frequent meals are often easier to digest than large meals.
Always discuss any constipation management with your GP or prescriber. Most people find that constipation improves naturally as their body adjusts to the medication—typically within 4–8 weeks.
- 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 more common with Mounjaro or Wegovy?
Clinical trial data suggests constipation is slightly more common with Mounjaro (26% at 15 mg) than Wegovy (20% at 2.4 mg), but individual experience varies widely. Your medical history, diet, and baseline digestion play a bigger role than the medication itself. Your prescriber will assess your personal risk factors.
Why do GLP-1 and GIP/GLP-1 injections cause constipation?
Both medication types slow how quickly food moves through your stomach (gastric emptying), which is part of how they help reduce appetite. This slower movement can lead to constipation. The effect usually lessens as your body adjusts over 4–8 weeks, though it may persist for some people.
Can I take laxatives while on Mounjaro or Wegovy?
Yes. Osmotic laxatives like lactulose and polyethylene glycol (PEG) are safe and effective alongside both medications. Stool softeners like docusate sodium are also suitable. Always inform your prescriber about any over-the-counter medications you're using.
Will constipation go away, or will it last as long as I'm on the injection?
Most people find constipation improves within the first 4–8 weeks as their body adapts. Some experience persistent mild constipation, which usually responds well to increased water, fibre, and gentle exercise. If it remains severe, discuss alternative options with your GP.
Should I choose Wegovy over Mounjaro because of lower constipation rates?
Constipation shouldn't be the only deciding factor. Both medications require a clinical assessment and prescription. Your prescriber will consider your eligibility, other side effects, medical history, and overall suitability. Discuss all side effects and what constipation management looks like for you specifically.
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.