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
Will IBS Cause Weight Loss?
Irritable bowel syndrome (IBS) can affect weight in different ways depending on your symptoms and how your body responds. Some people with IBS experience weight loss, whilst others may gain weight or remain stable. Weight changes aren't inevitable with IBS, but certain symptoms—like chronic diarrhoea, reduced appetite, or malabsorption—can contribute to losing weight. If you're experiencing unexplained weight loss alongside digestive symptoms, it's important to speak with your GP. Understanding your individual situation helps you maintain a healthy weight and manage your IBS 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.
How IBS Affects Weight Loss
IBS affects weight through several mechanisms. Diarrhoea-predominant IBS (IBS-D) can reduce how much nutrition your body absorbs from food, potentially leading to weight loss over time. Reduced appetite during flare-ups means you eat less, which naturally lowers calorie intake. Some people with IBS avoid certain foods (triggers like dairy, wheat, or high-fat items), further reducing food intake and energy consumption.
- Chronic diarrhoea may prevent proper nutrient absorption
- Abdominal pain and bloating often suppress appetite
- Food avoidance can create unintentional calorie deficit
- Anxiety about symptoms may reduce eating
However, constipation-predominant IBS (IBS-C) or stress-related eating may cause weight gain instead. Weight changes with IBS vary significantly between individuals—your GP can assess whether your weight loss is related to IBS or another factor, and whether you need further investigation.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 Seek Medical Advice
Contact your GP if you experience unintentional weight loss, especially if it's rapid (more than a few kilograms over weeks) or accompanied by other symptoms like blood in stools, severe pain, or persistent fatigue. Your GP may investigate whether your weight loss is genuinely IBS-related or caused by another condition like coeliac disease, thyroid problems, or malabsorption disorders.
If you're concerned about your weight management or want to achieve a healthy weight range for your height, use our free BMI calculator to assess your current status against NHS guidelines. If you need structured support for weight loss alongside managing IBS, your GP can refer you to NHS weight management services. Sustainable weight management requires addressing both your digestive health and your nutritional intake—not restrictive dieting.
- 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.
Can IBS cause sudden weight loss?
Yes, IBS-D with chronic diarrhoea can cause unintentional weight loss by reducing nutrient absorption and calorie intake. However, sudden or gradual weight management is not typical and warrants GP evaluation to rule out other causes like coeliac disease, thyroid dysfunction, or inflammatory bowel disease (IBD, which is different from IBS).
Does everyone with IBS lose weight?
No. Weight changes depend on IBS type and individual factors. IBS-D may cause weight loss, whilst IBS-C or stress-eating patterns may cause weight gain. Many people with IBS maintain stable weight by managing symptoms and eating a balanced diet.
Is weight loss from IBS dangerous?
Unintentional weight loss can indicate nutritional deficiency or malabsorption. If you're losing weight with IBS, discuss it with your GP. They can check whether you're meeting nutritional needs and whether you need dietician support or further investigation.
How can I manage weight safely if I have IBS?
Work with a dietician experienced in IBS to identify trigger foods without unnecessarily restricting your diet. Focus on nutrient-dense, well-tolerated foods. Avoid extreme calorie restriction, which can worsen symptoms. Your GP can refer you to NHS dietetic services.
Does IBS medication cause weight loss?
Most IBS medications (antispasmodics, laxatives, antimotility agents) don't directly cause weight loss. However, if medication improves your symptoms, you may eat more normally and gain weight. Always discuss side effects with your GP.
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.