Orlistat works in a completely different way to GLP-1 medications. Rather than acting on your brain or gut hormones, it works locally in your digestive system by blocking an enzyme called lipase, which your body needs to break down and absorb dietary fat.
When you take orlistat with a meal, approximately 30% of the fat you eat passes through your body unabsorbed. This means fewer calories are absorbed from fatty foods, creating a calorie deficit that leads to gradual weight loss.
The mechanism is entirely mechanical — orlistat doesn't suppress appetite, change your hormones, or act on your brain. It simply prevents some fat from being absorbed. This also means its side effects are directly related to how much fat you eat: the more fat in your diet, the more unabsorbed fat passes through your system, causing the characteristic gastrointestinal side effects.
All forms are taken up to 3 times daily, with each main meal that contains fat. If a meal has no fat, you skip the dose.
Alli (60mg OTC): Adults aged 18+ with a BMI of 28 or above. No prescription needed — available from pharmacies after a consultation with the pharmacist.1
Xenical/generic (120mg prescription): Adults with BMI ≥30, or BMI ≥28 with weight-related comorbidities. Available on NHS prescription.
Orlistat is one of the most accessible weight loss medications in the UK because Alli is available without a prescription and is significantly cheaper than GLP-1 injections.
The side effects of orlistat are distinctive and directly tied to fat intake:
These are very common but manageable by following a low-fat diet (which is the intended approach when taking orlistat). The side effects effectively act as a behavioural reinforcement — eating high-fat foods produces unpleasant consequences, encouraging healthier eating habits.
Rare but serious risks include liver injury and reduced absorption of fat-soluble vitamins (A, D, E, K). A daily multivitamin supplement taken at bedtime (separated from orlistat doses) is recommended during treatment.1
View full side effects breakdown →
Multiple studies have shown orlistat 120mg helps people lose approximately 5% of their body weight over 12+ weeks when combined with a reduced-calorie, low-fat diet. While this is modest compared to GLP-1 medications, it's clinically meaningful — a 5% weight loss can improve blood pressure, blood sugar, and cholesterol levels.2
Alli (60mg OTC): Approximately £20–£50 per month from pharmacies.
Xenical/generic (120mg): £30–£70 per month privately, or NHS prescription charge (£9.90 per item in England).
Orlistat is by far the cheapest weight loss medication available and the only option that doesn't require a prescription (at the Alli dose).
This page is for informational purposes only. Always consult a qualified healthcare professional or pharmacist before starting any weight loss medication.