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 Side Effects — What You Should Know
Alli is an over-the-counter weight loss medication containing orlistat that's available in the UK without a prescription. Unlike prescription weight loss injections such as those used in prescription weight management uk, Alli works differently in your body. This guide explains the side effects you may experience, who should avoid it, and how it fits into your overall weight management strategy alongside diet and exercise.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
Common Side Effects of Alli (Orlistat)
Alli contains orlistat, which works by reducing the amount of fat your body absorbs from food — roughly 25–30% of dietary fat passes through unabsorbed. This mechanism causes most of Alli's side effects.
- Oily spotting or urgency to have bowel movements — the most common effect, occurring in up to 1 in 4 users
- Fatty or oily stools — particularly noticeable after high-fat meals
- Stomach pain and cramping — usually mild to moderate
- Urgent need to go to the toilet — especially within 2 hours of eating
- Loss of appetite — part of how the medication helps reduce calorie intake
- Headaches and fatigue — reported in some users during the first few weeks
These effects typically reduce or disappear within 2–4 weeks as your body adjusts, particularly if you follow a low-fat diet (under 15g fat per meal is recommended).
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.
How Alli Compares to Prescription Weight Loss Options
Alli is an over-the-counter option, but prescription weight loss medication nhs services now offer includes GLP-1 injections like semaglutide (Wegovy), which work through a completely different mechanism — reducing appetite rather than fat absorption. Try our free GLP-1 weight loss calculator to explore how prescription options might work differently for you.
Prescription weight loss injections uk typically show stronger results in clinical trials but require a clinical assessment and GP referral. Alli requires no prescription but may have more digestive side effects. Your choice depends on your health profile, BMI, and personal preferences. Use our free BMI calculator to determine whether you might be eligible for prescription weight loss medication through the NHS or private prescribing.
- 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.
Can I take Alli if I have a sensitive stomach?
If you have IBS, Crohn's disease, or other digestive conditions, Alli may worsen symptoms due to its fat-blocking mechanism. Speak to your GP or pharmacist before starting. Prescription alternatives like GLP-1 injections work differently and might be more suitable — ask your GP about weight loss medication nhs options.
Who should not take Alli?
Avoid Alli if you're pregnant, breastfeeding, allergic to orlistat, have chronic pancreatic disease, or take certain medications like vitamin K supplements. Always check with your pharmacist before starting. Prescription weight loss options have different eligibility criteria — your GP can advise.
How much weight will I lose with Alli?
Clinical studies show Alli helps people lose approximately 2–3 kg more than diet alone over 6 months. Individual results vary considerably. For more significant results, prescription glp-1 weight loss medications show greater average weight loss in clinical trials. Use our weight loss calculator to compare projections.
Is Alli safe to take long-term?
Alli can be used long-term, but most people use it for 6–12 months alongside lifestyle changes. Extended use without dietary modification is less effective. Your GP can advise on whether long-term use suits your situation, or whether prescription lose weight uk options might be more appropriate for sustainable management.
What should I do if side effects are severe?
If you experience severe stomach pain, persistent diarrhoea, or signs of an allergic reaction, stop taking Alli and contact your GP or call NHS 111. Severe side effects are uncommon but warrant professional assessment. Your GP can discuss whether an alternative approach to weight management might be better tolerated.
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.