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 Pill — How It Works and What to Know
Alli is an over-the-counter weight loss pill containing orlistat, available to buy in UK pharmacies and supermarkets without a prescription. Unlike prescription weight loss medications, Alli works by preventing your body from absorbing some of the fat you eat. It's been used for weight management in the UK for over 15 years. While it may support weight loss alongside a reduced-calorie diet, results vary between individuals. Many people ask how Alli compares to other weight management approaches, including prescription options and NHS weight loss services.
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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.
How Alli Works and What to Expect
Alli contains orlistat 60mg, which works differently from prescription weight loss medications like Wegovy or Mounjaro. Instead of affecting your appetite, orlistat reduces how much fat your body absorbs from food — approximately 25-30% of dietary fat passes through unabsorbed.
Key points about Alli:
- No prescription needed — available over-the-counter in supermarkets and pharmacies
- Taken three times daily with meals containing fat
- Works best alongside a reduced-calorie diet (around 1500 kcal per day)
- Clinical evidence shows modest weight loss of approximately 2-3kg over 6 months on average, though individual results vary significantly
- Does not suppress appetite like GLP-1 weight loss injections
Alli may help with weight management uk goals, particularly if you find it easier to manage diet than appetite. However, weight loss with Alli tends to be slower than with prescription weight loss injections.
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.
Alli Side Effects and Considerations
The most common side effects of Alli relate to its mechanism — since unabsorbed fat passes through your digestive system, you may experience oily spotting, loose stools, or urgent bowel movements, especially if you eat fatty foods.
Other considerations:
- Not suitable if you take certain medications (discuss with your pharmacist)
- May reduce absorption of fat-soluble vitamins — a multivitamin is recommended
- Works best with a low-fat, calorie-controlled diet
- Takes 3-6 months to see meaningful results
- Requires ongoing use to maintain weight loss
For prescription weight loss medication nhs options or faster results, your GP can refer you to specialist weight management services, though waiting lists vary significantly by region.
- 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 60mg) is an over-the-counter product and not available on prescription through the NHS. However, a higher-dose version (Xenical, 120mg) may be prescribed on the NHS as part of specialist weight management services. Your GP can advise whether you're eligible for prescription weight loss medication.
How much weight can you lose with Alli?
Clinical studies show average weight loss of 2-3kg over six months with Alli, though individual results vary significantly. Weight loss is modest compared to prescription GLP-1 medications. Results depend heavily on following a reduced-calorie diet — Alli alone without dietary changes is ineffective.
Can I use Alli if I'm on other medications?
Some medications may interact with Alli, particularly fat-soluble drugs. Always check with your pharmacist before buying Alli. Discuss your full medical history, as Alli isn't suitable for everyone. Your GP can advise on safer alternatives if needed.
How does Alli compare to weight loss injections like Wegovy?
Alli is over-the-counter and requires no prescription, whereas Wegovy and similar GLP-1 weight loss injections are prescription-only and work by reducing appetite. Clinical data shows GLP-1 injections typically produce greater weight loss (10-20% of body weight) than Alli (2-3kg). Both require lifestyle changes for best results.
Do I need to take Alli forever to keep weight off?
Yes, Alli requires ongoing use to maintain weight loss. Once you stop taking it, weight tends to return without continued dietary management. This is why sustainable lifestyle changes — including reduced-calorie diet and physical activity — are essential for long-term success with any weight loss approach.
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.