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 Orlistat Work for Weight Loss?
Orlistat is an over-the-counter weight loss medication available in the UK that has been used for weight management for over 20 years. The evidence shows it does help some people lose weight, but results are modest compared to newer prescription options. This guide explains how orlistat works, what the clinical evidence shows, and how it fits within the broader UK weight loss medication landscape for adults seeking evidence-based weight management options.
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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.
How Orlistat Works and What the Evidence Shows
Orlistat (sold as Xenical or Alli in the UK) works differently from newer prescription weight loss medication like Mounjaro and Wegovy. It works in the digestive system by blocking approximately 25–30% of dietary fat from being absorbed, meaning that fat passes through the body undigested. This reduces overall calorie intake.
Clinical trials show orlistat produces modest weight loss: on average, 2.7 kg more weight loss than diet alone over 6 months. The UK weight management guidelines acknowledge orlistat as a first-line medication option for eligible adults. However, this is significantly less than GLP-1 injections — Wegovy achieves approximately clinically studied weight loss, and Mounjaro achieves 15–21% on average. Orlistat works best alongside a reduced-calorie, low-fat diet. Without dietary changes, it is largely ineffective. Common side effects include oily spotting, urgency to use the toilet, and fatty stools.
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.
Prescription Weight Loss Options Available in the UK
If you have a BMI of 30 or above (or 27+ with weight-related health conditions), you may be eligible for prescription weight loss injections through the NHS or private prescribing. These include Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All are GLP-1 or dual GLP-1/GIP receptor agonists that reduce appetite and support feeling fuller after meals.
NHS weight loss medication is accessed through specialist tier 3 weight management services, typically via GP referral. Access varies by region. Private options are available through online prescribing following clinical assessment. Use our free BMI calculator above to check your BMI and understand your starting point. Your GP can discuss which approach — orlistat, prescription injections, or lifestyle changes — is most suitable for you.
- 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.
How much weight can you lose with orlistat?
Clinical trials show orlistat produces approximately 2.7 kg more weight loss than diet alone over 6 months. Results vary depending on diet adherence and individual factors. Orlistat works best alongside a reduced-calorie, low-fat diet — without dietary changes, weight loss is minimal.
Is orlistat available on the NHS in the UK?
Orlistat is available both over-the-counter (Alli) and as a prescription (Xenical) in the UK. The NHS may prescribe it within weight management services, but it is also widely available to buy without prescription from pharmacies and online retailers for around £10–15 per month.
How does orlistat compare to Mounjaro or Wegovy?
Mounjaro and Wegovy are prescription-only injections that achieve 15–clinically studied weight loss on average, compared to 2.7 kg with orlistat over 6 months. However, Mounjaro and Wegovy require clinical assessment, cost more, and may have different side effects. Your GP can advise which is appropriate for you.
What are the side effects of orlistat?
Common side effects include oily spotting, urgent bowel movements, and fatty stools (especially if you eat high-fat foods). These are usually mild and improve over time. Serious side effects are rare. Always read the patient information leaflet and speak to a pharmacist or GP if you have concerns.
Can I use orlistat without changing my diet?
Orlistat is much less effective without dietary changes. It works by blocking fat absorption, so you must eat a lower-fat diet to see results. A reduced-calorie diet combined with increased physical activity is essential for sustainable weight loss with any medication.
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.