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 Constipation Cause Weight Loss?
No, constipation does not cause weight loss. If your weight drops when constipated, you're likely seeing temporary water weight or digestive changes—not actual fat loss. However, constipation is a common side effect of weight management medication, and understanding the difference between scale fluctuations and real weight loss is important when using prescription weight loss treatments. This guide explains what's happening and how to support healthy weight management in the UK.
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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.
Understanding Constipation and Weight Changes
When you're constipated, your body retains stool in the digestive tract, which can temporarily increase your weight by 0.5–2 kg (1–4 lbs). Once you're regular again, that weight drops—but this isn't real fat loss, it's just your digestive system returning to normal. Real weight loss occurs when you burn more calories than you consume, which results in your body using stored fat for energy.
If you're using prescription weight loss medication like Wegovy (semaglutide) or Mounjaro (tirzepatide), constipation is actually a very common side effect affecting up to 1 in 10 people. These GLP-1 and GIP/GLP-1 medications slow how quickly food moves through your stomach (gastric emptying), which can reduce bowel movements. The weight loss you may see while on these medications is from reduced calorie intake and increased fat burning—not from constipation itself.
To track real weight loss progress, weigh yourself once weekly at the same time of day (ideally after using the bathroom). Use our free progress tracker to log your weight changes over time and spot true trends rather than daily fluctuations caused by constipation, water retention, or meal timing.
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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 Losing Weight
If you're experiencing constipation during weight management, it's important to address it for comfort and bowel health—not because it's preventing weight loss. Effective strategies include: drinking more water (aim for 6–8 glasses daily), eating high-fibre foods (vegetables, wholemeal bread, oats), staying physically active, and addressing any underlying causes.
If you're taking prescription weight loss medication, inform your GP or prescriber about constipation, as they may recommend stool softeners or osmotic laxatives like movicol. Never assume weight loss stalls because you're constipated—use the BMI calculator and weight converter on this page to track your actual progress in kg or stone. For NHS weight loss support and personalised guidance on managing medication side effects, speak with your GP or specialist weight management service.
- 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.
Does constipation actually cause weight loss?
No. Constipation may temporarily increase your weight (0.5–2 kg) because stool remains in your digestive tract. Once constipation resolves, that weight drops—but it's not real fat loss. Actual weight loss comes from burning more calories than you consume, typically resulting in 0.5–1 kg loss per week with healthy practices.
Why am I constipated while taking Wegovy or Mounjaro?
Both Wegovy (semaglutide) and Mounjaro (tirzepatide) slow gastric emptying—how quickly food leaves your stomach. This is part of how they reduce appetite, but it can reduce bowel movement frequency. Drinking water, eating fibre, and staying active usually help. Inform your prescriber if constipation is severe or persistent.
How do I know if I'm losing real fat and not just having digestive changes?
Track your weight weekly at the same time, ideally after using the bathroom. Real weight loss shows a downward trend over 2–4 weeks. Daily fluctuations of 0.5–1.5 kg are normal due to water, food timing, and digestion. Our free progress tracker helps you spot genuine weight loss patterns over time.
Is constipation a sign that weight loss medication is working?
Constipation is a common side effect but not a sign of effectiveness. The medication works by reducing appetite and helping you eat less—whether or not you experience constipation. Weight loss depends on consistent calorie deficit and lifestyle changes, not on side effects.
Should I stop taking my weight loss medication if I'm constipated?
No—speak to your prescriber instead. Constipation is usually manageable with dietary and lifestyle adjustments (water, fibre, movement, and sometimes stool softeners). Your GP can recommend safe options and may adjust your dose if needed. Never stop prescription medication without medical guidance.
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.