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 short-term weight loss because your body loses water and passes food through your digestive system more quickly than normal. However, this is temporary fluid loss, not genuine fat loss — and it's a sign something isn't right with your health. If diarrhoea persists, especially when taking weight management medication, speak to your GP. For sustainable weight loss in the UK, prescription options like Mounjaro and Wegovy work through appetite regulation, not digestive upset.
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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.
Why Diarrhoea Causes Weight Loss — And Why It Matters
When you have diarrhoea, weight loss happens for specific reasons: dehydration (your body loses water rapidly), reduced food absorption (nutrients pass through before being fully absorbed), and faster gastric transit (food moves through your digestive system quicker). This weight loss is not fat loss — it's primarily water weight and undigested food.
Diarrhoea lasting more than a few days requires medical attention, especially if you're also experiencing abdominal pain, fever, or blood in stools. This is particularly important if you're taking prescription weight loss medication such as Mounjaro or Wegovy, as diarrhoea is a known side effect that can affect how your body absorbs other medicines and nutrients. Always report persistent diarrhoea to your GP, who can assess whether your medication needs adjusting or whether something else is causing the symptoms.
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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.
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Weight Management Medications and Digestive Side Effects in the UK
GLP-1 receptor agonist medications like Mounjaro and Wegovy can cause diarrhoea as a side effect — reported in clinical trials in up to 1 in 10 users. This is different from intentional weight loss: the medications work by reducing appetite and helping you feel fuller with smaller meals, not by causing malabsorption.
If you experience diarrhoea on prescription weight loss medication, don't stop taking it without speaking to your GP. Side effects often improve after a few weeks as your body adjusts. Your GP can also check whether your dose needs adjusting or if something else is contributing. For sustainable weight management in the UK, the focus should always be on appetite regulation and lifestyle change — never on digestive distress.
- 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 fluid and food loss. Once diarrhoea resolves, weight usually returns within days as your body rehydrates and normal eating resumes. This is not genuine fat loss and should never be considered a weight management strategy.
Which prescription weight loss medications cause diarrhoea?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) list diarrhoea as a common side effect. Saxenda (liraglutide) can also cause it. These are GLP-1 medications available on the NHS or privately in the UK. Side effects typically improve with time as your body adjusts.
Should I stop taking my weight loss medication if I have diarrhoea?
No — always speak to your GP first. Don't stop prescription medication without medical guidance. Your GP can assess whether the diarrhoea is medication-related, adjust your dose if needed, or identify other causes. Stopping suddenly may not be appropriate.
How do weight loss injections in the UK actually work?
Mounjaro and Wegovy work through appetite regulation, not digestive upset. They activate GLP-1 (and in Mounjaro's case, GIP) pathways that signal fullness to your brain, helping you eat less and feel satisfied with smaller portions — leading to sustainable weight loss alongside a reduced-calorie diet and increased physical activity.
When should I see my GP about persistent diarrhoea?
Contact your GP if diarrhoea lasts more than a few days, or sooner if you experience severe cramping, fever, bloody stools, or signs of dehydration. This is especially important if you're taking any prescription medication. Your GP can determine the cause and advise on next steps, including whether your weight management medication needs reviewing.
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.