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
Weight Loss From Diarrhoea — What You Should Know
Diarrhoea can cause temporary weight loss because your body loses water and undigested food passes through the digestive system more quickly than normal. However, this is not the same as fat loss, and diarrhoea as a weight loss method is neither safe nor sustainable. If you're experiencing diarrhoea alongside weight loss medications like Wegovy or Mounjaro, it's important to understand what's happening and when to contact your GP. This guide explains the difference between weight loss caused by illness and weight loss achieved through evidence-based prescription medication and 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.
How Diarrhoea Affects Weight — The Difference Between Water Loss and Fat Loss
When you have diarrhoea, weight loss occurs mainly due to water and electrolyte loss, not fat loss. Food moves through your digestive system faster than usual, meaning fewer calories are absorbed from what you eat. This can result in gradual weight management — sometimes 1–2 kg within days — but this weight returns once your bowel function normalises and you rehydrate.
- Dehydration accounts for most gradual weight management during diarrhoea
- Fat loss typically requires a sustained calorie deficit over weeks or months
- Diarrhoea-related weight loss is temporary and unhealthy
- Electrolyte imbalance can cause serious health complications
In contrast, sustainable weight management in the UK through prescription medications like Wegovy (semaglutide) works by reducing appetite and supporting a calorie deficit over time, alongside diet and physical activity. The STEP clinical trials showed Wegovy may help participants lose an average of approximately 15% body weight — but this is achieved safely, with medical monitoring, not through illness.
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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 as a Side Effect of Weight Loss Medication — What to Expect
Diarrhoea is a common side effect of GLP-1 weight loss injections like Wegovy and Mounjaro. It typically occurs during the dose escalation phase and may improve over time as your body adjusts. This is different from using diarrhoea to lose weight — it's an unintended effect of medication that works through appetite regulation, not fluid loss.
- Diarrhoea affects up to 1 in 10 people taking these medications
- Usually improves after 4–8 weeks of continued use
- Staying hydrated and eating fibre-rich foods can help manage symptoms
- Contact your GP if diarrhoea is severe or persists beyond 2 weeks
If you're considering prescription weight loss medication in the UK, your GP or prescriber will assess whether it's suitable for you and monitor side effects. Our GLP-1 weight loss calculator can help you understand how these medications may support your weight management goals when used correctly.
- 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 lost during diarrhoea is mainly water loss and returns once your bowel function normalises and you rehydrate. It is not fat loss and is not sustainable. True, lasting weight loss requires a calorie deficit maintained over weeks and months, ideally supported by diet, physical activity, and where appropriate, prescription medication prescribed by your GP.
Can diarrhoea be used as a weight loss method?
Absolutely not. Using diarrhoea intentionally to lose weight is dangerous and can cause severe dehydration, electrolyte imbalance, muscle loss, and malnutrition. It is not a recognised weight loss strategy. If you're struggling with your weight, speak to your GP about evidence-based options including lifestyle changes or prescription weight loss treatment.
What should I do if I have diarrhoea while taking Wegovy or Mounjaro?
Diarrhoea is a known side effect that often improves within a few weeks. Stay hydrated, eat mild foods, and avoid high-fat or high-fibre foods temporarily. Contact your GP if diarrhoea is severe, bloody, or persists beyond 2 weeks. Never stop your medication without medical advice — your prescriber can help manage side effects.
How is weight loss from prescription medication different from diarrhoea-related loss?
Prescription weight loss medications like Wegovy work by reducing appetite and supporting a calorie deficit over time. They cause fat loss, not just water loss. Clinical trials show sustained weight loss of 14–21% body weight over months. This is monitored by healthcare professionals and combined with lifestyle changes for safe, lasting results.
Should I use the BMI calculator to track weight loss from diarrhoea?
Yes — use our free BMI calculator to track your overall health metrics. However, remember that temporary weight fluctuations from diarrhoea won't reflect real progress. For meaningful weight loss tracking, use our free progress tracker to monitor changes over weeks and months, which gives a clearer picture of actual fat loss alongside diet and exercise.
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.