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 — How It Compares to Wegovy
Yes, constipation is a commonly reported side effect of Mounjaro (tirzepatide), though it affects people differently. This guide compares constipation risk between Mounjaro and Wegovy, explains the underlying mechanisms, and helps you understand what to expect if you're considering either medication. Both are prescription-only weight loss injections available in the UK following clinical assessment. Individual experiences vary significantly, and your GP or prescriber can discuss which option might suit you best based on your health profile.
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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.
Constipation Rates — Clinical Trial Data and Mechanism Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — this dual mechanism may explain why constipation is particularly common. In the SURMOUNT clinical trials, constipation was reported by approximately 22% of participants on the maximum 15mg dose. Wegovy (semaglutide), a GLP-1 receptor agonist only, showed constipation in roughly 19% of trial participants at the 2.4mg dose.
- Mounjaro: Dual GIP/GLP-1 action slows gastric emptying more significantly — this slower digestion is thought to increase constipation risk
- Wegovy: Single GLP-1 mechanism also slows stomach function but may have slightly less impact on bowel transit
- Individual variation: Some users experience no constipation at all; others find it severe enough to adjust doses or switch medications
Both medications typically slow how food moves through your digestive system, which is part of how they reduce appetite — but this effect can lead to constipation, especially in the first few weeks of treatment.
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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
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Managing Constipation and Choosing Between Medications
If you experience constipation on either medication, practical steps include increasing water intake to 2–3 litres daily, eating high-fibre foods (vegetables, legumes, whole grains), taking regular physical activity, and considering a stool softener or fibre supplement. Never take stronger laxatives without consulting your prescriber first, as they can interact with weight loss medications.
The choice between Mounjaro and Wegovy depends on your individual health profile, not just constipation risk. Your prescriber will assess your medical history, other medications, and personal tolerances. Some people find one medication suits them better than the other. Use our GLP-1 weight loss calculator to explore projected outcomes at different doses, and discuss side effect management with your GP or prescriber before starting treatment.
- 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 worse with Mounjaro than Wegovy?
Clinical trial data suggests constipation rates are slightly higher with Mounjaro (22%) compared to Wegovy (19%), but individual experiences vary widely. Some people experience no constipation on either medication. Your prescriber can help you weigh this against other factors when deciding which is right for you.
How long does constipation last when starting Mounjaro or Wegovy?
Constipation is typically worst in the first 2–4 weeks as your body adjusts. For most people, it improves as your digestive system adapts. If it persists beyond 4 weeks or becomes severe, contact your GP — you may need dose adjustment or additional management strategies.
Can I take laxatives while on Mounjaro or Wegovy?
Always check with your prescriber before taking laxatives or stool softeners. They can advise on safe options that won't interfere with your medication. Gentle fibre supplements are usually safer than stimulant laxatives. Increasing water and physical activity often helps naturally.
Why do GLP-1 and tirzepatide injections cause constipation?
Both medications slow gastric emptying — how quickly food leaves your stomach. This slower digestion is part of how they reduce appetite, but it can lead to constipation. Tirzepatide's dual GIP/GLP-1 action may slow digestion more significantly than GLP-1 alone.
Will my constipation improve if I switch from Mounjaro to Wegovy?
Some people find constipation improves on Wegovy due to its single mechanism, but others experience similar side effects. Every person responds differently. If constipation is severe on Mounjaro, discuss switching options with your prescriber — they may recommend adjustments before changing medications. ```
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