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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Alli Weight Loss Side Effects — What You Need to Know
Alli (orlistat 60mg) is an over-the-counter weight loss medication available in the UK without a prescription. It works by blocking some fat absorption in your digestive system. However, this mechanism means it can cause digestive side effects for many users. If you're considering Alli or exploring weight management options in the UK, it's important to understand what to expect and know that prescription weight loss alternatives are also available for eligible adults. This guide covers Alli's side effects, who can use it safely, and how it compares to NHS-approved prescription 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.
Common Alli Side Effects and What They Mean
Alli's main side effects are related to how it works — by reducing fat absorption in the gut. The most common effects include:
- Oily spotting or urgency — uncontrolled bowel movements with oily discharge, especially after high-fat meals
- Stomach pain and cramping — abdominal discomfort, particularly in the first few weeks
- Loose stools or diarrhoea — increased bowel frequency
- Urgency to have a bowel movement — may occur suddenly
- Fatty or oily stools — the unabsorbed fat being passed through the body
These effects are most noticeable when you eat high-fat foods. Many users manage side effects by eating a low-fat diet (less than 25g fat per meal) and avoiding fried or greasy foods. If side effects persist or are severe, speak to your GP. Prescription weight loss medications like Mounjaro and Wegovy work through different mechanisms and may suit you better — your GP can assess eligibility and discuss NHS weight loss services or private options.
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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.
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Prescription Weight Loss Options Available in the UK
If Alli side effects are bothersome, several prescription weight loss medications are available in the UK for eligible adults. Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly injections that work differently from Alli — they're GLP-1 receptor agonists that help reduce appetite rather than blocking fat absorption. Eligibility includes adults with a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes or high blood pressure. Access is through NHS specialist weight management services or private online prescribing following clinical assessment. Use our free GLP-1 weight loss calculator to understand potential outcomes with prescription options, and speak with your GP about what's available in your area.
- 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 Alli?
The most common side effects are digestive — oily spotting, loose stools, stomach cramping, and urgency to have a bowel movement. These occur because Alli blocks fat absorption, so unabsorbed fat passes through the digestive system. Side effects are usually worst after eating high-fat meals and often reduce if you eat a low-fat diet (under 25g fat per meal).
Can I take Alli with other medications?
Alli can interact with some medications, particularly oral contraceptives (it may reduce their effectiveness, so use a barrier method within 4 weeks of starting) and fat-soluble vitamin supplements. Always read the patient information leaflet or ask your pharmacist before taking Alli with any other medicines or supplements. Tell your GP if you're taking prescription medications.
Who shouldn't take Alli?
Alli is not suitable if you're pregnant, breastfeeding, have a BMI under 27, or have certain digestive conditions like chronic pancreatitis or Crohn's disease. It's also not recommended if you have a history of kidney stones or vitamin absorption problems. Ask your GP or pharmacist if Alli is safe for you.
How long does Alli take to work?
Alli starts working immediately when you take it, but weight loss results take time. Clinical evidence shows modest weight loss — typically 2–3 kg more than diet and exercise alone over 6 months. Weight loss requires Alli plus a reduced-calorie diet and increased physical activity. Results vary significantly between individuals.
Are prescription weight loss injections better than Alli?
Prescription medications like Mounjaro and Wegovy work through different mechanisms and typically produce greater average weight loss in clinical trials — around 15–21% body weight with Mounjaro. However, they require a prescription, clinical assessment, and are only available to eligible adults. Your GP can discuss which option suits you best based on your health, BMI, and preferences.
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.