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
Can Diarrhoea Cause Weight Loss?
Yes, diarrhoea can cause weight loss, but it's usually temporary and not the same as sustainable weight loss for health. When your body loses fluids and electrolytes through diarrhoea, you may see the scales drop within hours or days. However, this is water and nutrient loss, not fat loss. Once you rehydrate and your digestive system returns to normal, the weight typically returns. Understanding the difference between fluid loss and intentional weight loss is important for your health and wellbeing.
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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.
How Diarrhoea Causes Weight Loss
Diarrhoea causes weight loss through fluid and nutrient loss, not fat loss. When you have diarrhoea, your body rapidly eliminates water, electrolytes, and undigested food from your digestive system. This creates an immediate drop on the scales — sometimes 1–3 kg within 24 hours depending on severity.
- Fluid loss: Your body loses significant amounts of water, which weighs considerably
- Reduced food absorption: Your intestines cannot absorb nutrients and calories properly
- Lower food intake: People often eat less when experiencing diarrhoea
- Temporary effect: Weight returns quickly once normal digestion resumes and you rehydrate
This is why diarrhoea-related weight loss is not considered meaningful or sustainable. Real weight loss for health and weight management in the UK context involves reducing body fat through a calorie deficit alongside increased physical activity — not losing fluids.
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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 Diarrhoea Weight Loss Is a Health Concern
Mild, short-term diarrhoea with temporary weight loss is usually not serious. However, contact your GP if diarrhoea persists for more than a few days or if you experience severe weight loss alongside other symptoms like blood in stools, severe abdominal pain, or signs of dehydration (dizziness, dark urine, extreme thirst).
Chronic diarrhoea causing ongoing weight loss may indicate underlying conditions like coeliac disease, irritable bowel syndrome (IBS), or inflammatory bowel disease (IBD). These require medical investigation and management. If you're considering NHS weight loss support or prescription weight loss medication, it's essential your GP assesses your full health picture — including any digestive issues.
- 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 weight loss from diarrhoea permanent?
No. Weight loss from diarrhoea is temporary and caused by fluid loss, not fat loss. Once you rehydrate and your digestive system normalises, the weight typically returns within days. Real, sustainable weight loss requires a calorie deficit and lifestyle changes.
What's the difference between diarrhoea weight loss and intentional weight loss?
Diarrhoea causes rapid fluid loss (water, electrolytes, undigested food) that returns quickly. Intentional weight loss involves reducing body fat through a sustained calorie deficit (around 500 kcal/day for ~0.5 kg/week loss) combined with physical activity. Our free BMI calculator and progress tracker help you monitor real, meaningful weight changes.
When should I see my GP about diarrhoea and weight loss?
Contact your GP if diarrhoea lasts more than a few days, causes severe weight loss, or is accompanied by blood in stools, severe pain, or dehydration signs. Persistent diarrhoea with unintended weight loss may signal conditions like IBS, coeliac disease, or IBD that need investigation.
Can I use diarrhoea as a weight loss method?
Absolutely not. Using diarrhoea intentionally for weight loss is dangerous and ineffective. It leads to dehydration, nutrient deficiency, electrolyte imbalances, and muscle loss — not fat loss. If you're interested in safe, evidence-based weight loss, speak to your GP about prescription weight loss medication or NHS weight loss programmes.
If I have diarrhoea from Wegovy or Mounjaro, will I lose weight?
These GLP-1 medications may cause diarrhoea as a side effect in some people, particularly during dose escalation. Any weight loss with these prescription medications comes from the medication's appetite-suppressing effect combined with a reduced-calorie diet — not from the diarrhoea itself. If diarrhoea is severe, contact your prescriber.
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.