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
Alli Weight Loss Supplement — What You Need to Know
Alli is an over-the-counter weight loss supplement containing orlistat, a lipase inhibitor that reduces fat absorption in the digestive system. Unlike prescription weight loss medications such as GLP-1 injections, Alli is available without a prescription from pharmacies across the UK. This guide explains how Alli works, its effectiveness for weight management, and how it compares to other weight loss options available to UK adults seeking to lose weight.
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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 Alli Works and UK Availability
Alli contains orlistat 60 mg, which blocks approximately 25–30% of dietary fat absorption in your intestines. The unabsorbed fat is then passed through your digestive system. Alli is available without prescription at UK pharmacies, supermarkets, and online retailers — making it one of the most accessible weight management options.
For more significant weight loss, prescription weight loss medications are available through the NHS or private prescribing services. These include GLP-1 injections like Wegovy (semaglutide) and Mounjaro (tirzepatide), which work through different mechanisms and typically produce greater weight loss than orlistat alone. Our GLP-1 weight loss calculator can help you estimate potential results with prescription medication if you're considering that route.
- Alli requires a reduced-calorie, low-fat diet to be effective
- Works best when combined with increased physical activity
- Results are typically modest — average 2–3 kg more weight loss than diet alone over 6 months
- Must be taken consistently (one capsule with each main meal)
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.
Side Effects, Limitations, and Realistic Expectations
Alli's main side effect is gastrointestinal — oily spotting, urgency to use the toilet, and loose stools are common, especially if you eat high-fat foods. These effects often improve as your body adjusts and as you reduce dietary fat intake. Alli has real limitations: it only works if you maintain a strict low-fat diet, and weight loss is modest compared to prescription alternatives.
For weight management in the UK, your GP can discuss all options with you. Use our BMI calculator to check your current BMI and see whether you might be eligible for NHS-supported weight loss services or prescription weight loss medication. Track your progress with our free progress tracker as part of any weight management plan.
- 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 Alli effective for weight loss?
Alli can help you lose weight, but results are modest — typically 2–3 kg more than diet alone over 6 months, according to clinical studies. It works best when combined with a reduced-calorie, low-fat diet and regular physical activity. Individual results vary significantly.
Can I buy Alli without a prescription in the UK?
Yes, Alli (orlistat 60 mg) is available over-the-counter at UK pharmacies and supermarkets without a prescription. However, some pharmacies may ask brief health screening questions before selling it to you.
How does Alli compare to prescription weight loss injections?
Alli is an over-the-counter fat blocker producing modest weight loss, while prescription medications like Wegovy and Mounjaro are GLP-1 agonists that reduce appetite and typically produce 15–clinically studied weight loss. Prescription options require clinical assessment and are more effective for significant weight loss.
What are the main side effects of Alli?
Common side effects include oily spotting, loose stools, and urgency to use the toilet — especially after eating fatty foods. These effects are usually mild and often improve over time as you adjust your diet to be lower in fat.
Should I try Alli or ask my GP about other weight loss options?
That depends on your health, BMI, and weight loss goals. Your GP can discuss all options — including Alli, NHS weight management services, and prescription weight loss medication if you're eligible. Use our BMI calculator to check your current BMI and discuss the results with your GP.
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.