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 Diarrhea — When It's a Health Concern
Unintended weight loss caused by diarrhoea can be a sign your body isn't absorbing nutrients properly, or a side effect of medication. While some prescription weight management medications do cause gastrointestinal effects as a side effect, unexpected weight loss from diarrhoea — especially if it's persistent or accompanied by other symptoms — should always be discussed with your GP. This guide explains what to watch for and what support is available through the NHS.
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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.
Prescription Weight Management Options in the UK
If you're exploring prescription weight management in the UK, several options are available through NHS specialist services or private prescribing. Mounjaro (tirzepatide) and Wegovy (semaglutide) are the main GLP-1 and dual GIP/GLP-1 receptor agonists licensed for weight loss, and Saxenda (liraglutide) is also available. These are prescription-only medicines that may help with weight management alongside a reduced-calorie diet and increased physical activity.
However, if you're experiencing weight loss from diarrhoea — whether related to medication, diet, or another cause — this needs assessment first. Common gastrointestinal side effects of weight loss injections include nausea, diarrhoea, and constipation, typically occurring during dose escalation. These usually settle as your body adjusts. If weight loss from diarrhoea is severe, persistent, or accompanied by abdominal pain, fever, or bloody stools, contact your GP urgently — this may indicate a separate health issue requiring investigation.
- Mounjaro: dual mechanism, doses 2.5–15 mg weekly
- Wegovy: GLP-1 agonist, doses 0.25–2.4 mg weekly
- Saxenda: GLP-1 agonist, daily injection option
- All require clinical assessment before prescription
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.
NHS Access and Private Options — What's Right for You
In the UK, prescription weight loss medications are accessed through two main routes. NHS weight management is available through tier 3 specialist services — your GP can refer you if you meet eligibility criteria (BMI 30 or above, or 27 or above with weight-related health conditions). However, NHS waiting lists can be long in many areas, and availability varies by region.
Private prescribing is available through UK-licensed prescribers via online consultation and GPhC-registered pharmacies, without needing an NHS referral. Whichever route you choose, your prescriber will assess whether medication is appropriate for you — especially important if you have ongoing gastrointestinal symptoms. Use our free BMI calculator to check if you might be eligible, and discuss any concerns about weight loss and diarrhoea with your GP before starting any weight management treatment.
- 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 diarrhoea from weight loss injections normal?
Yes — nausea, diarrhoea, and constipation are common side effects of GLP-1 and GIP/GLP-1 medications, particularly during dose increases. They typically improve as your body adjusts over weeks. If symptoms are severe or persistent beyond 4 weeks, contact your prescriber or GP for guidance.
When should I see my GP about weight loss and diarrhoea?
See your GP if: weight loss is unintended and persistent, diarrhoea lasts more than 2 weeks, you have severe abdominal pain, fever, blood in stools, or signs of dehydration (dark urine, dizziness, extreme thirst). These may indicate infection, malabsorption, or another condition needing investigation.
Can I take weight loss injections if I have irritable bowel syndrome (IBS)?
This depends on your specific situation. GLP-1 medications may worsen IBS symptoms in some people. Always inform your GP or prescriber if you have IBS, inflammatory bowel disease, or other gastrointestinal conditions before starting any weight management treatment. They can assess whether it's safe for you.
What's the difference between weight loss medication side effects and a medical problem?
Medication side effects are usually mild-to-moderate, temporary, and linked to dose changes. Red flags requiring urgent GP attention include: severe or worsening abdominal pain (possible pancreatitis), persistent diarrhoea with weight loss over weeks, blood in stools, or signs of dehydration or infection.
How can I check if I'm eligible for NHS weight loss medication?
Use our free BMI calculator to find your current BMI. If it's 30 or above (or 27 with weight-related conditions like diabetes or high blood pressure), you may be eligible. Speak to your GP about referral to a specialist weight management service in your area — they can formally assess eligibility and discuss options.
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.