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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Weight Loss Alli — What You Need to Know
Alli (orlistat) is an over-the-counter weight loss medication available in UK pharmacies without a prescription. It works differently from prescription weight management medications like Wegovy or Mounjaro — it reduces the amount of fat your body absorbs from food rather than affecting appetite. If you're exploring weight loss options in the UK, understanding how Alli compares to other approaches, including GLP-1 injections and lifestyle changes, can help you make an informed decision about what's right for you. This guide covers how Alli works, what to expect, and where it fits in UK weight management.
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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 What to Expect
Alli contains orlistat, a lipase inhibitor that blocks approximately 25–30% of dietary fat from being absorbed in your intestines. Rather than reducing hunger like GLP-1 weight loss injections, Alli works in your digestive system. The unabsorbed fat is then passed naturally through bowel movements.
- Standard dose: 60 mg capsule, three times daily with meals containing fat
- Average weight loss: approximately 2–3 kg over 6 months compared to diet alone
- Works best alongside a reduced-calorie, low-fat diet
- Most effective in the first 3–6 months of use
Alli is available over-the-counter from supermarkets and pharmacies, making it accessible without GP involvement. However, it's less effective than prescription weight loss medications — clinical trials show GLP-1 medications and prescription orlistat (120 mg dose) produce greater weight loss results. Use our free BMI calculator to establish your starting point before beginning any weight loss journey.
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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.
Alli Side Effects and Suitability
The main side effect of Alli is related to its mechanism — fatty stools, urgency, and leakage occur in many users because fat passes through unabsorbed. These effects are usually mild and reduce over time, especially when you eat a low-fat diet. Other common side effects include stomach pain, nausea, and increased bowel movements.
Alli is not suitable if you're pregnant, breastfeeding, have chronic malabsorption conditions, or take certain medications like warfarin. It's also less effective than prescription weight management options for significant weight loss. If you need more support, your GP can refer you to NHS weight management services or discuss prescription alternatives. Track your progress with our free progress tracker to monitor results over weeks and months.
- 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 available on the NHS?
No, Alli (orlistat 60 mg) is over-the-counter only and not available on the NHS. Prescription orlistat (120 mg) can be prescribed by GPs in some cases, but access is limited. NHS prescription weight loss medications include Mounjaro and Wegovy, available through specialist tier 3 weight management services.
How does Alli compare to prescription weight loss injections?
Alli works locally in the digestive system to reduce fat absorption, while GLP-1 medications like Wegovy affect appetite and fullness signals in the brain. Clinical trials show GLP-1 injections produce significantly greater average weight loss (14–21% body weight) compared to Alli (2–3 kg). Alli may suit those wanting a lower-intervention approach; GLP-1 medications suit those needing more substantial support.
Can I take Alli with other weight loss methods?
Yes, Alli works best alongside a reduced-calorie, low-fat diet and increased physical activity. You should not combine Alli with prescription orlistat (120 mg) or GLP-1 medications without GP guidance. Always inform your pharmacist or GP of all medications and supplements you're taking.
What diet should I follow while taking Alli?
Alli requires a low-fat diet (below 15 g fat per meal) to be effective and to minimise side effects. Focus on lean proteins, vegetables, whole grains, and low-fat dairy. High-fat meals will increase digestive side effects. A qualified dietitian can provide personalised meal plans if you need more structured support.
How long does it take to see weight loss results with Alli?
Most users see modest results within 4–8 weeks when combined with a reduced-calorie diet. Average weight loss is 2–3 kg over 6 months — results vary significantly between individuals. Use our weight converter and BMI calculator to track changes in different units and monitor progress toward your healthy weight range.
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.