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 Diarrhoea Cause Weight Loss?
Yes, diarrhoea can cause temporary weight loss, but this is not the same as losing fat or achieving sustainable weight management in the UK. When you have diarrhoea, your body loses water, electrolytes, and undigested food more quickly than normal, which reduces what you're carrying in your digestive system. However, this weight typically returns once your digestion normalises. For genuine, long-term weight management, a combination of dietary changes, physical activity, and (where appropriate) medical support works far better than relying on any gastrointestinal upset.
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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 Diarrhoea Affects Your Weight
Diarrhoea causes weight loss in several ways, but these changes are temporary and unhealthy. When your bowels move more frequently, less food stays in your digestive system long enough for your body to absorb nutrients and calories properly. You also lose significant amounts of water and electrolytes (minerals like sodium and potassium) that your body needs. This combination of reduced food in your system and fluid loss can show as a noticeable drop on the scales within hours.
- Water loss: Diarrhoea removes fluid from your body rapidly — most of the initial weight loss is water, not fat
- Nutrient malabsorption: Your intestines cannot absorb calories and nutrients effectively, so your body gets less energy from food
- Temporary nature: Once diarrhoea stops, your body reabsorbs water and you regain this weight quickly
- Health impact: Persistent diarrhoea can lead to dehydration, nutritional deficiencies, and fatigue
This is fundamentally different from sustainable weight loss, which involves fat loss combined with preserved muscle and proper nutrition.
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.
Diarrhoea vs Sustainable Weight Loss
For genuine weight management in the UK, the NHS recommends losing weight at a rate of 0.5 to 1 kg per week through a combination of reduced-calorie diet and increased physical activity. This approach preserves muscle mass, improves energy levels, and is sustainable long-term. Diarrhoea-related weight loss, by contrast, is water and waste — not fat — and is accompanied by weakness, dehydration, and nutritional loss.
If you're interested in evidence-based weight loss support, check your BMI using our free BMI calculator, and speak to your GP about prescription weight loss medication options (such as GLP-1 medications like Wegovy or Mounjaro) if eligible. These are prescription-only treatments that work alongside lifestyle changes for sustainable results.
- 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. Diarrhoea-related weight loss is temporary and caused by water loss and reduced food in your digestive system. Once your bowels return to normal and you rehydrate, you typically regain this weight within days. It's not fat loss and should not be considered a weight loss method.
Can diarrhoea be a sign of something serious?
Acute diarrhoea lasting a few days is usually mild viral gastroenteritis. However, persistent diarrhoea lasting more than 2 weeks, diarrhoea with blood, severe abdominal pain, or signs of dehydration require GP assessment. Contact your GP or NHS 111 if concerned.
How do weight loss injections like Wegovy differ from diarrhoea weight loss?
Wegovy (semaglutide) is a prescription GLP-1 medication that reduces appetite and supports gradual fat loss — typically 0.5–1 kg per week — alongside lifestyle changes. Unlike diarrhoea, it preserves muscle, maintains hydration, and produces sustainable results. Diarrhoea causes unhealthy water and nutrient loss with rapid weight regain.
Should I try to use diarrhoea for weight loss?
No. Intentionally causing or prolonging diarrhoea to lose weight is dangerous and can lead to severe dehydration, electrolyte imbalances, malnutrition, and organ damage. If you're struggling with weight management, speak to your GP about NHS weight loss services, prescription medication, or our free GLP-1 weight loss calculator for guidance.
What should I do if I have persistent diarrhoea?
Stay hydrated with water and oral rehydration salts (available from pharmacies). Eat bland, easy-to-digest foods. If diarrhoea lasts more than 2 weeks, is severe, or is accompanied by blood, fever, or severe pain, contact your GP. They can check for underlying causes and refer you to appropriate support.
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.