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 and Weight Loss: What You Need to Know
Constipation is a common side effect during weight loss, particularly when using GLP-1 weight loss medications like Wegovy or making significant dietary changes. Understanding why constipation occurs and how to manage it safely is important for sustainable weight management. This guide explains the connection between weight loss and constipation, when to seek medical advice, and practical strategies to manage symptoms whilst pursuing your weight loss goals through prescription weight loss treatment or lifestyle changes.
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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.
Why Does Weight Loss Cause Constipation?
Constipation during weight loss happens for several reasons. When you reduce calorie intake significantly, your digestive system receives less bulk to process, slowing gastric transit time. GLP-1 medications like Wegovy and other weight loss injections can also slow how quickly food moves through your stomach and intestines — this is part of how they help with appetite control, but it can contribute to constipation.
Additional factors include reduced physical activity, lower fibre intake if diet changes aren't well-planned, and dehydration. The NHS notes that gradual weight management can also trigger temporary changes in digestive function. If you're using prescription weight loss medication through the NHS or private weight loss services, your prescriber should discuss potential digestive side effects and management strategies during your clinical assessment.
- Slower gastric emptying from weight loss medications
- Reduced calorie intake means less food bulk
- Potential dehydration during weight management
- Changes in physical activity levels
- Inadequate dietary fibre intake
Clinically BackedWeight Loss
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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 During Weight Loss
Effective constipation management starts with hydration and dietary fibre. Aim for 6–8 glasses of water daily, and gradually increase fibre through vegetables, whole grains, and legumes — doing this gradually prevents bloating. Regular physical activity supports digestive health; even a 20-minute walk after meals helps. Use our free GLP-1 weight loss calculator to track your weight management progress and discuss digestive symptoms with your GP if they persist beyond 2–3 weeks.
Over-the-counter laxatives can help, but speak to your pharmacist or GP before using them regularly, especially if you're on prescription weight loss medication. Some medications interact with constipation remedies. If constipation is severe or accompanied by stomach pain, contact your GP or call NHS 111 for guidance. Never adjust your weight loss medication dose without medical advice.
- 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 a side effect of Wegovy and other weight loss injections?
Yes. Wegovy (semaglutide) commonly causes constipation by slowing gastric emptying. Clinical trials showed constipation affected a significant proportion of participants. It typically improves as your body adjusts, but management strategies help. Always report persistent symptoms to your prescriber during follow-up appointments.
How long does constipation last when losing weight?
Constipation from weight loss medication often improves within 2–4 weeks as your body adjusts. Constipation from dietary changes typically resolves once fibre intake is optimised. If it persists beyond 3–4 weeks or worsens, contact your GP for personalised advice and possible further investigation.
Can I use laxatives whilst taking Wegovy or other weight loss medication?
Speak to your pharmacist or GP before using laxatives with weight loss medication, as some interactions are possible. Stool softeners and osmotic laxatives (like movicol) are often safer than stimulant laxatives. Your prescriber can recommend options suitable for you during your clinical assessment.
What's the best way to prevent constipation during weight loss?
Drink plenty of water (6–8 glasses daily), gradually increase dietary fibre through vegetables and whole grains, and maintain regular physical activity. Avoid sudden extreme dietary changes. If using weight loss medication, discuss prevention strategies with your prescriber. Monitor progress using our free progress tracker.
Should I stop taking my weight loss medication if I get constipation?
Never stop prescription weight loss medication without medical advice. Contact your GP or prescriber instead — they can adjust your dose, suggest management strategies, or explore alternatives if constipation is severe. Constipation is usually manageable and doesn't mean the medication isn't working.
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.