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
Does Mounjaro Cause Constipation?
Yes, constipation is one of the most commonly reported side effects of Mounjaro (tirzepatide), the GLP-1 receptor agonist and GIP receptor weight loss injection available in the UK. According to clinical trial data, constipation affects many people taking this prescription weight loss medicine. The good news is that constipation often improves over time as your body adjusts to the medication, and there are proven strategies to manage it effectively.
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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.
Constipation as a Mounjaro Side Effect
Constipation is listed as a very common side effect of Mounjaro, meaning it may affect more than 1 in 10 people taking this weekly weight loss injection. In clinical trials, constipation occurred alongside other gastrointestinal symptoms like nausea, diarrhoea, and stomach pain.
The mechanism is straightforward: Mounjaro slows gastric emptying (how quickly food moves through your stomach and digestive system), which can lead to constipation. This effect is most pronounced during the dose escalation phase (typically the first 4–5 months) and often improves as your body adapts to the medication.
- Constipation severity varies between individuals
- It tends to be worse when starting or increasing doses
- Most people experience improvement by week 8–12 on a stable dose
- Staying hydrated and eating adequate fibre can help significantly
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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 Constipation While Taking Mounjaro
If you experience constipation on Mounjaro, practical steps can help. Increase water intake to at least 2–3 litres daily, as dehydration worsens constipation. Eat more high-fibre foods including vegetables, fruits, wholegrains, and pulses—but increase fibre gradually to avoid bloating.
Regular physical activity, even a 20-minute walk after meals, supports digestive movement. If constipation persists despite these changes, speak to your GP or prescriber about over-the-counter remedies like lactulose or docusate. Never take strong laxatives without medical advice, as they can interact with Mounjaro's effects on digestion. Track your digestive changes using our free progress tracker to share patterns with your prescriber.
- 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 dangerous when taking Mounjaro?
Mild to moderate constipation is common but not dangerous. However, severe abdominal pain or constipation lasting more than a week despite management strategies warrants a GP consultation. Mounjaro can occasionally cause pancreatitis (inflammation of the pancreas), which presents as severe, persistent stomach pain—seek urgent medical attention if this occurs.
How long does Mounjaro-related constipation last?
Constipation is typically worst during the first 4–8 weeks and during dose escalations. Most people report improvement by week 12 on a stable dose as their digestive system adjusts. If constipation persists beyond 4 weeks on a stable dose, discuss management options with your prescriber.
Can I take laxatives while on Mounjaro?
Mild over-the-counter options like lactulose or docusate are generally safe, but always check with your GP or pharmacist first. Avoid strong stimulant laxatives without medical advice, as Mounjaro already affects how your stomach empties. Your prescriber can recommend the safest options for your situation.
Does constipation mean Mounjaro isn't working for weight loss?
No. Constipation is a side effect of how Mounjaro works (slowing gastric emptying), not a sign of effectiveness or failure. Many people experience constipation and still achieve significant weight loss. Use our free GLP-1 weight loss calculator to track expected progress independent of side effects.
What if constipation gets worse instead of better?
Contact your GP or your prescriber's clinic if constipation worsens after 2–3 weeks, or if you develop severe abdominal pain. Your prescriber may recommend stool softeners, adjust your fluid intake plan, or in rare cases, consider dose adjustments or switching medications.
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.