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 Weight Loss Cause Diarrhoea?
Yes, diarrhoea can occur during weight loss, especially when using prescription weight management medication in the UK. GLP-1 receptor agonist medications like Mounjaro and Wegovy commonly cause digestive side effects including diarrhoea, nausea, and constipation. These symptoms are often temporary and typically improve as your body adjusts to the medication. Understanding what to expect helps you manage these effects safely whilst working towards your weight loss goals.
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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.
Weight Loss Medication and Digestive Side Effects in the UK
GLP-1 weight loss injections approved for use in the UK—Mounjaro (tirzepatide) and Wegovy (semaglutide)—work by slowing how quickly food moves through your stomach and reducing appetite signals to your brain. This mechanism, whilst effective for weight management in the UK, often causes temporary digestive changes.
Diarrhoea and other common side effects include:
- Diarrhoea (very common)
- Nausea and vomiting
- Constipation
- Stomach pain or indigestion
- Reduced appetite
In clinical trials, diarrhoea affected up to 1 in 10 patients on these medications. Most people find symptoms improve within 4–8 weeks as their body adapts to the treatment. If diarrhoea persists or becomes severe, inform your prescriber—they may adjust your dose or recommend management strategies. Staying hydrated and eating bland foods can help during initial weeks.
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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 and Private Weight Loss Options in the UK
In the UK, prescription weight loss medication is available through two routes: NHS specialist weight management services or private online prescribing. NHS access requires referral to a tier 3 specialist service, though waiting lists vary significantly by region. Private prescribing offers faster access following an online clinical assessment.
To check if you're eligible, use our free GLP-1 weight loss calculator to estimate your suitability based on BMI and health factors. Your GP can also assess eligibility for NHS referral. Before starting any weight loss medication, discuss potential side effects like diarrhoea with your prescriber—they'll explain what to expect and how to manage digestive changes safely. This conversation ensures you're prepared and can adjust treatment if needed.
- 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.
Why does Mounjaro cause diarrhoea?
Mounjaro (tirzepatide) slows gastric emptying—how quickly food leaves your stomach—and affects gut hormone signalling. These changes can alter bowel movement frequency and consistency. Diarrhoea is a very common side effect affecting up to 1 in 10 patients, usually improving within 4–8 weeks as your body adjusts to the medication.
How long does diarrhoea last on weight loss medication?
Most people experience diarrhoea for 2–8 weeks when starting GLP-1 medication. Some find it improves as they progress to maintenance doses. If diarrhoea persists beyond 8 weeks or worsens, contact your prescriber. They may recommend dietary changes, hydration strategies, or dose adjustments to manage the side effect.
Can I prevent diarrhoea when starting Wegovy or Mounjaro?
You cannot prevent diarrhoea entirely, but you can minimise it. Eat small, bland meals; stay hydrated with water and electrolyte drinks; avoid high-fat and high-fibre foods initially; and follow the prescribed dose schedule carefully. Discuss digestive concerns with your prescriber before starting treatment—they may recommend over-the-counter remedies.
Is diarrhoea a sign that weight loss medication isn't right for me?
Not necessarily. Diarrhoea is an expected temporary side effect for many people, not a sign the medication is unsuitable. However, if diarrhoea is severe, persistent, or affecting your quality of life significantly, discuss this with your prescriber. They can assess whether dose adjustment, timing changes, or a different approach might suit you better.
What should I do if I have severe diarrhoea on weight loss injections?
Contact your prescriber or GP immediately. Severe or prolonged diarrhoea can lead to dehydration and electrolyte imbalances. Your healthcare provider may recommend temporary dose reduction, dietary modifications, or over-the-counter anti-diarrhoeal medication. Keep track of symptoms and report them during your follow-up appointments—this helps your prescriber optimise your treatment plan.
Ready to take control of your weight loss journey?
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