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 IBS Cause Weight Loss?
IBS (Irritable Bowel Syndrome) can sometimes lead to unintended weight loss, particularly if digestive symptoms reduce your appetite or nutrient absorption. However, weight loss isn't inevitable with IBS — many people maintain stable weight despite symptoms. If you're experiencing unexplained weight loss alongside IBS symptoms like diarrhoea, constipation, or stomach pain, it's important to discuss this with your GP. They can assess whether your weight changes are IBS-related or linked to another condition. If you're looking to manage your weight intentionally, UK-licensed prescription options are available for eligible adults.
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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 have a BMI of 30 or above (or 27 or above with weight-related health conditions), you may be eligible for prescription weight loss medication through the NHS or private prescribing. The main options available in the UK include:
- Mounjaro (tirzepatide) — a once-weekly GLP-1 injection licensed for weight management, which may help reduce appetite and support weight loss alongside dietary changes
- Wegovy (semaglutide) — another once-weekly injection that works on similar principles, with clinical evidence showing average weight loss of around 15% of body weight
- Saxenda (liraglutide) — a daily injection option, also licensed for weight management in UK adults
All three are prescription-only medicines requiring a clinical assessment before being prescribed. Access varies — you can be referred to NHS specialist weight management services through your GP, or consult a UK-licensed prescriber privately. Our GLP-1 weight loss calculator can help you understand potential outcomes at different doses, while our BMI calculator shows whether you may be eligible.
Clinically BackedWeight Loss
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.
NHS vs Private Access — How to Get Started
On the NHS route, your GP can refer you to a tier 3 specialist weight management service if you meet eligibility criteria. However, NHS availability varies significantly by region, and waiting lists can be lengthy in many areas. On the private route, you can have an online consultation with a UK-licensed prescriber without a NHS referral — if approved, you'll receive a prescription dispensed by a GPhC-registered pharmacy.
Whichever route you choose, prescription weight loss medications work best alongside a reduced-calorie diet and increased physical activity. If you're unsure whether you're eligible, your GP is the best starting point. They can assess your BMI, health conditions, and whether medication is appropriate for you. Use our free progress tracker to monitor your weight management journey once you've started 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.
Can IBS lead to weight loss without trying?
Yes, IBS can cause unintended weight loss in some people, especially if diarrhoea, reduced appetite, or poor nutrient absorption are present. However, others with IBS maintain stable weight. Unexplained weight loss should always be discussed with your GP to rule out other causes and ensure your digestive health is being properly managed.
Are prescription weight loss injections safe if I have IBS?
Prescription weight loss medications like Mounjaro and Wegovy can affect digestion — they slow gastric emptying, which may cause nausea or changes in bowel habits. If you have IBS, discuss this with your prescriber before starting treatment. They can assess whether it's appropriate for you and monitor you during treatment.
How do I know if I'm eligible for weight loss medication in the UK?
You're potentially eligible if you have a BMI of 30 or above, or 27 or above with weight-related health conditions like high blood pressure or type 2 diabetes. Your GP can assess your individual situation and refer you to NHS specialist services or you can consult a UK-licensed prescriber privately. Use our free BMI calculator to check your starting point.
What's the difference between Mounjaro and Wegovy for weight management?
Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist with average weight loss of around 15–21% in clinical trials, while Wegovy (semaglutide) is a single GLP-1 agonist with average weight loss of around 15%. Both are once-weekly injections. Individual results vary significantly, and your prescriber will determine which is more suitable for you.
How quickly will I lose weight on prescription medication?
Weight loss with GLP-1 medications typically becomes noticeable after 4–8 weeks, with gradual continued loss over months. Average weight loss varies — clinical trials showed 15–21% of body weight over 12–18 months alongside diet and exercise changes. Results are individual; some people lose more, others less. Track your progress using our free progress tracker. ```
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.