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 Diarrhoea Cause Weight Loss?
Diarrhoea can lead to temporary weight loss because your body loses fluids and your digestive system moves food through faster than normal. However, this weight loss is usually short-term water loss, not fat loss, and it's not a healthy or sustainable way to lose weight. If you're experiencing persistent diarrhoea or considering weight management, speaking to your GP is important. For adults eligible under NHS weight loss criteria, there are prescription weight loss medications and structured support available.
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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.
Diarrhoea, Weight Loss, and Healthy Weight Management
Diarrhoea causes weight loss mainly through fluid and electrolyte loss, not fat loss. When your digestive system moves food through more quickly, your body has less time to absorb water and nutrients. Once you rehydrate and your digestion normalises, this weight typically returns.
This is very different from sustainable weight loss through nhs weight loss programmes, which focus on fat loss over time. If diarrhoea is a side effect of weight management medication like Mounjaro or Wegovy, it usually settles within weeks as your body adjusts. Our free BMI calculator can help you understand your current weight status and whether you might be eligible for prescription weight loss support. For genuine, lasting weight management, UK adults can access GLP-1 weight loss injections through NHS specialist services or private prescribing.
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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.
Prescription Weight Management Options in the UK
If you're interested in weight management uk support, several prescription options are available. Mounjaro and Wegovy are once-weekly injections licensed for weight loss in eligible UK adults. Both work alongside a reduced-calorie diet and physical activity — diarrhoea is a common early side effect but usually improves within 4–8 weeks as your body adjusts.
Eligibility includes a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes or high blood pressure. Access is available through NHS tier 3 specialist services or private online assessment. Your GP can discuss which route suits you best and assess whether prescription weight loss medication is appropriate for your individual health needs.
- 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 diarrhoea alone help you lose weight permanently?
No. Diarrhoea causes temporary fluid loss, not fat loss. Once you rehydrate, the weight typically returns. Sustainable weight loss requires a calorie deficit through diet and activity changes, not digestive issues. For lasting results, speak to your GP about evidence-based approaches.
Is diarrhoea a side effect of Mounjaro and Wegovy?
Yes. Diarrhoea is common, especially in the first 4–8 weeks of starting GLP-1 weight loss injections. It usually improves as your body adjusts. Staying hydrated and eating slowly can help. If severe or persistent, contact your prescriber.
How can I tell if diarrhoea is from medication or something else?
Medication-related diarrhoea typically starts after beginning a new injection and settles within weeks. Other causes include viral infection, food intolerance, or IBS. If you're unsure, speak to your GP. They can assess and advise whether to continue medication or adjust your dose.
What's the difference between water weight loss and fat loss?
Water weight loss happens quickly but is temporary — it returns when you rehydrate. Fat loss is slower, typically 0.5–1 kg per week, but is permanent because you've used more calories than you consumed. Sustainable weight management uk focuses on fat loss through calorie deficit.
How do I know if I'm eligible for prescription weight loss medication?
You may be eligible if you have a BMI of 30 or above, or 27 or above with a weight-related health condition. Use our free BMI calculator to check your BMI, then speak to your GP. They can assess your individual health and discuss whether medication like Mounjaro or Wegovy is suitable for you.
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.