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 experienced by people taking Mounjaro (tirzepatide) in the UK. This gastrointestinal issue typically occurs because the medication slows how quickly food moves through your digestive system. While constipation can be uncomfortable, there are practical steps you can take to manage it effectively. Most people find their symptoms improve after the first few weeks as their body adjusts to the medication. If you're struggling with constipation whilst on Mounjaro, your GP can provide personalised guidance on managing this side effect safely.
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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.
Managing Constipation: Practical Steps You Can Take
Constipation on Mounjaro often improves with simple lifestyle adjustments that support healthy digestion:
- Increase water intake — aim for at least 2 litres daily, as dehydration worsens constipation
- Add fibre gradually — include more vegetables, fruits, and whole grains, but increase intake slowly to avoid bloating
- Move more — light physical activity like walking helps stimulate bowel movement
- Eat smaller, more frequent meals — large meals can slow digestion further when taking Mounjaro
- Consider natural remedies — prunes, kiwi fruit, and ground flaxseed may help gently
If constipation persists for more than a few days despite these changes, speak to your GP before taking any laxative medication. Some over-the-counter options may interact with your Mounjaro dose titration plan or other medications you're taking.
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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.
When To Contact Your GP — Red Flags and Concerns
Most constipation on Mounjaro resolves within 2–4 weeks as your body adjusts, but contact your GP immediately if you experience severe abdominal pain spreading to your back, persistent constipation lasting more than a week despite lifestyle changes, signs of bowel obstruction, or vomiting alongside constipation. These could indicate a more serious issue like pancreatitis or gallbladder problems, which require urgent medical attention. Your GP may recommend a stool softener or mild laxative suitable for use alongside your tirzepatide prescription. Never assume constipation will resolve on its own if it's causing significant discomfort — your prescriber can adjust your approach or dose if needed.
- 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 normal when starting Mounjaro?
Yes, constipation is very common on Mounjaro and affects up to 1 in 10 people. It typically occurs because the medication slows gastric emptying—how quickly food moves through your digestive system. Most people find constipation improves after 2–4 weeks as their body adjusts. If it persists, speak to your GP about management options.
What's the difference between constipation on Mounjaro versus Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) can cause constipation as they both slow digestion. However, Mounjaro's dual GIP and GLP-1 action may affect gut motility slightly differently than Wegovy's single GLP-1 mechanism. Individual experiences vary significantly—some people manage constipation better on one medication than the other. Your prescriber can discuss which may suit you.
Can I take laxatives whilst on Mounjaro?
You can take some laxatives, but speak to your GP or pharmacist first. They'll recommend options safe to use alongside your tirzepatide prescription and any other medications. Avoid strong or stimulant laxatives without medical advice, as they may interact with your weight management plan. Always start with lifestyle changes (hydration, fibre, movement) first.
How long does constipation last on Mounjaro?
For most people, constipation improves within 2–4 weeks as their body adjusts to the medication. However, some people experience ongoing mild constipation throughout their treatment. If it doesn't improve after a month despite lifestyle changes, contact your GP—they may suggest a safe stool softener or assess whether your Mounjaro dose needs adjusting.
Could severe constipation mean something serious?
Occasional constipation is expected on Mounjaro, but severe pain, vomiting, or constipation lasting more than a week may indicate a serious issue like pancreatitis or bowel obstruction. These require urgent medical attention. If you experience severe symptoms, contact your GP or NHS 111 immediately. Don't wait to see if it resolves on its own.
Ready to take control of your weight loss journey?
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