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
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Orlistat For Weight Loss — How It Works In The UK
Orlistat is a prescription-only weight management medication available in the UK through the NHS and private prescribing. Unlike GLP-1 weight loss injections like Wegovy or newer medications like Mounjaro, orlistat works differently in your digestive system. It has been used for weight management in the UK for over two decades and remains one of the most commonly prescribed weight loss medications. This guide explains what orlistat is, how it works, who can take it, and how effective it is for sustainable weight management.
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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 For Weight Loss
Orlistat works by blocking the absorption of dietary fat in your intestines. Approximately 30% of the fat you eat is prevented from being absorbed, which reduces overall calorie intake and supports weight loss alongside a reduced-calorie diet. Unlike GLP-1 medications such as Wegovy which signal to the brain that you are full, orlistat acts directly in the digestive system.
- Mechanism: Inhibits pancreatic and gastric lipase enzymes that break down dietary fat
- Fat blocked: Around 30% of dietary fat intake is not absorbed
- Calorie reduction: Results in approximately 100–150 kcal per day reduction from fat alone
- Onset: Effects begin within 24–48 hours of starting the medication
Orlistat is most effective when combined with a reduced-calorie diet and increased physical activity. Clinical evidence shows that weight loss occurs gradually over 12 weeks and beyond. The medication does not suppress appetite or affect fullness signals like GLP-1 medications do.
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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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Orlistat Eligibility And Effectiveness In The UK
Adults with a BMI of 30 or above (or BMI 27–29.9 with weight-related health conditions such as type 2 diabetes or high blood pressure) may be eligible for orlistat in the UK. Your GP or prescriber will assess your medical history and determine suitability. In clinical trials, adults taking orlistat lost an average of 5–10% of their body weight over 12 weeks, with continued weight loss possible with ongoing use and lifestyle changes.
Orlistat is available as 60 mg and 120 mg capsules taken three times daily with meals. Regular follow-up appointments are important to monitor progress and manage side effects. Use our free BMI calculator above to check your personal BMI and see if you may be eligible for weight management support.
- 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.
What are the main side effects of orlistat?
The most common side effects relate to the digestive system: oily spotting, urgent bowel movements, and fatty stools. These typically occur when eating high-fat meals and often reduce over time. Most side effects are mild and managed by following a low-fat diet (less than 15g fat per meal). Serious side effects are rare.
Can orlistat be used on the NHS?
Yes, orlistat (120 mg) is available on the NHS through your GP or a tier 3 weight management service. Prescribing varies by region. The 60 mg version is available over-the-counter at pharmacies without prescription. NHS access depends on your BMI and whether you have weight-related health conditions.
How does orlistat compare to Wegovy or other weight loss injections?
Orlistat blocks fat absorption in the intestines, whilst GLP-1 medications like Wegovy work on appetite signals in the brain. GLP-1 injections typically produce greater average weight loss (15–20% body weight) than orlistat (5–10%), but both require reduced-calorie diets and exercise for best results. Orlistat is a capsule taken three times daily; Wegovy is a weekly injection.
How long does orlistat take to work?
Effects begin within 24–48 hours as dietary fat is not absorbed. Noticeable weight loss typically occurs over 2–4 weeks when combined with a reduced-calorie diet. Most weight loss happens in the first 12 weeks. Continued use alongside lifestyle changes supports sustained weight management over months.
What do I need to do whilst taking orlistat?
Follow a low-fat, reduced-calorie diet (aim for less than 15g fat per meal to minimise side effects). Eat a balanced diet with plenty of fruits, vegetables, and lean protein. Take regular physical activity. Attend follow-up appointments with your GP. Track your weight using our free progress tracker to monitor your progress over time.
Ready to take control of your weight loss journey?
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