How Does Orlistat Work? How Long It Takes, and Does It Work?

Short answer: Orlistat is a lipase inhibitor: it blocks the enzymes in your stomach and small intestine that digest fat, so part of the fat in each meal is not absorbed and passes out in your stools, about a third with orlistat 120mg and about a quarter with orlistat 60mg (the Xenical and alli patient leaflets, section 1). It acts on the fat in the meal you take it with, from the first dose: the fat in your stools rises within 24 to 48 hours (Xenical SmPC, section 4.2). It does not burn fat already stored in your body, and its UK product information describes no effect on appetite. In pooled data from five 2-year studies in the UK product information, with a reduced-calorie diet in both groups, people taking orlistat 120mg lost 3.2kg more on average after one year than people taking a dummy capsule (placebo) (Xenical SmPC, section 5.1). Individual results vary. Orlistat is reviewed at 12 weeks and only continued if you have lost at least 5% of your starting weight. After that, our pharmacist reviews it at every order.

This guide sets out what the UK product information for orlistat 120mg (Xenical) and orlistat 60mg (alli) and the NHS say, as of October 2026: what orlistat is, how it works in the gut, whether it affects appetite or stored fat, how long it takes, what the studies found, how you know it is working, why it might not be, and how long it stays in your system. A GPhC-registered pharmacist reviews every orlistat consultation before anything is supplied, and orlistat 60mg, a pharmacy medicine, is supplied after a pharmacist consultation.

What is orlistat?

Orlistat is a medicine for weight loss in adults, taken as a capsule with meals alongside a reduced-calorie diet (Xenical and alli SmPCs, section 4.1). The NHS says orlistat treats obesity by helping you lose weight alongside exercise and changes to your diet, and lists its common brands as Alli, Orlos and Xenical: orlistat 120mg (Xenical) is a prescription medicine, and the lower 60mg strength (alli) is a pharmacy medicine, supplied after a pharmacist consultation.

The UK product information classes orlistat as a "peripherally acting antiobesity agent" (Xenical SmPC, section 5.1). In plain words, it works inside your stomach and small intestine rather than on your brain, and very little of it reaches your blood: its absorption is "minimal" (section 5.2). Orlistat and the other weight-loss medicines taken by mouth are set out side by side on our weight-loss tablets page.

How does orlistat work?

Orlistat works by attaching to the lipases in your stomach and small intestine, the enzymes that break down fat, so they cannot split the fat in your food (triglycerides) into the smaller fatty acids your gut can absorb. The UK product information describes it as an "inhibitor of gastrointestinal lipases" that works "in the lumen of the stomach and small intestine", the space inside them where food is digested (Xenical and alli SmPCs, section 5.1). The fat that is not broken down cannot be absorbed, and it leaves your body in your stools (Xenical leaflet, section 1).

How much fat it blocks depends on the strength. Orlistat 120mg blocks "about one-third of the fat in the food you eat" (Xenical leaflet, section 1), and orlistat 60mg "about a quarter of the fat in your meals" (alli leaflet, section 1), which its product information gives as "approximately 25% of dietary fat" (alli SmPC, section 5.1). The NHS puts it as a third of the fat from your food being passed out in your poo.

Because it acts on the fat in your meal, it needs fat to do anything: "Xenical only works in the presence of dietary fat" (Xenical leaflet, section 3), so if you miss a meal, or a meal contains no fat, you skip that capsule. The full routine is in how to take orlistat.

Does orlistat reduce appetite or make you feel full?

No: orlistat is not a GLP-1 medicine, and the UK product information for Xenical and alli describes no effect on appetite or on feeling full, because it works on the fat in your gut rather than on hunger. How it differs from the medicines that do act on appetite is in orlistat vs Mounjaro and Wegovy and what is a GLP-1 medication.

Does orlistat burn existing fat?

No. Orlistat acts on the fat in the food in your gut, not on fat already stored in your body. Weight comes off when, with the fat it stops you absorbing and a reduced-calorie diet, you take in less energy than your body uses. That is why orlistat 120mg is licensed "in conjunction with a mildly hypocaloric diet", a diet slightly lower in calories than you need (Xenical SmPC, section 4.1), and orlistat 60mg with "a mildly hypocaloric, lower-fat diet" (alli SmPC, section 4.1).

With orlistat, the diet is part of the treatment. The Xenical leaflet says that "over-consumption of fat and calories may reduce any weight loss effect" (section 2).

How long does orlistat take to work?

Orlistat starts acting on the fat in the first meal you take it with: the fat in your stools rises "as early as 24 to 48 hours after dosing" (Xenical SmPC, section 4.2). Whether it is helping your weight takes longer to judge, and the UK product information sets that check at 12 weeks (Xenical SmPC, section 4.1; alli SmPC, section 4.2). In the two orlistat 60mg studies in its product information, "most weight loss occurred within the first 6 months" (alli SmPC, section 5.1).

We cannot tell you how much weight you will have lost by a given week. The product information gives no week-by-week figure, and how much you lose depends on your diet and on you.

Does orlistat work?

Orlistat does help with weight loss alongside a reduced-calorie diet, but the effect in its UK product information is modest and varies from person to person. In pooled data from five 2-year studies, in which both the orlistat and the placebo groups followed a reduced-calorie diet, people taking orlistat 120mg lost 3.2kg more on average after one year than people taking a dummy capsule (placebo) (Xenical SmPC, section 5.1). That is an average across groups of people, not a prediction for any one person. Individual results vary.

The 3.2kg figure cannot be set beside the results of GLP-1 medicines, which come from different trials in different groups of people; how the medicines differ is in orlistat vs Mounjaro and Wegovy.

Weight loss was smaller in people with type 2 diabetes: "In clinical trials, the decrease in bodyweight with orlistat treatment was less in type II diabetic patients than in non-diabetic patients" (Xenical SmPC, section 4.4).

How do you know if orlistat is working?

The check is your weight at 12 weeks, measured against your weight when you started. This is the rule our pharmacist works to: Orlistat is reviewed at 12 weeks and only continued if you have lost at least 5% of your starting weight. After that, our pharmacist reviews it at every order.

For orlistat 60mg, the alli leaflet says: "If you do not lose weight after taking alli for 12 weeks, see your doctor or pharmacist for advice. You may need to stop taking alli." (section 3; alli SmPC, section 4.2). The NHS puts it this way for orlistat in general: taking it for more than 3 months is not recommended if you have not lost 5% of your body weight.

Changes in your stools, such as oily or fatty stools, are side effects of the fat that is not absorbed. They usually ease as you keep to the recommended diet (Xenical leaflet, section 4), so they are not the way to judge whether orlistat is helping your weight: the scales are. What to expect, and what to do about them, is in orlistat side effects.

Why might orlistat not be working?

The usual reasons the product information points to are eating more fat and calories than the diet plan allows, fatty snacks between meals, and taking a capsule with meals that contain no fat, when it has nothing to act on (Xenical leaflet, sections 2 and 3). Each one, in the product information's words:

• More fat and calories than planned. "Over-consumption of fat and calories may reduce any weight loss effect" (Xenical leaflet, section 2).
• Fatty snacks between meals. The leaflet says to avoid "food containing fat between meals, such as biscuits, chocolate and savoury snacks" (Xenical leaflet, section 3).
• Meals with no fat. "alli does not work unless there is some fat in the meal" (alli leaflet, section 2), so a capsule with a fat-free meal does nothing, and it is skipped.
• Taking more than the dose. "Doses of orlistat above 120 mg three times daily have not been shown to provide additional benefit" (Xenical SmPC, section 4.2), so more capsules are not the answer, even after a high-fat meal (alli leaflet, section 2).
• Missed capsules. Tell your pharmacist if you have not taken it as advised: the leaflet warns that otherwise it may be thought not to have worked (Xenical leaflet, section 3).
• Type 2 diabetes. Weight loss with orlistat was smaller in people with type 2 diabetes (Xenical SmPC, section 4.4).

The diet that goes with orlistat, meal by meal, is in our orlistat diet plan.

Can orlistat stop working?

Weight loss slowing down over time is what the studies found too: in the two orlistat 60mg studies, most of the weight loss came within the first 6 months (alli SmPC, section 5.1). If your weight has stopped changing, talk to your pharmacist, who reviews orlistat at every order.

How long does orlistat stay in your system?

Very little orlistat gets into your body, and it does not build up: its absorption is "minimal", intact orlistat could not be measured in the blood 8 hours after a dose, and there was "no evidence of accumulation" (Xenical SmPC, section 5.2). About 97% of a dose leaves in the faeces, most of it unchanged, less than 2% in the urine, and complete excretion took 3 to 5 days in studies (section 5.2). After you stop taking it, the fat in your stools usually returns to its usual level within 48 to 72 hours (Xenical SmPC, section 4.2).

• Stools back to usual after stopping48 to 72 hours

  After you stop orlistat, the fat in your stools usually returns to its usual level within 48 to 72 hours (Xenical SmPC, section 4.2).

• Fully cleared from the body in studies3 to 5 days

  In studies, complete excretion of a dose of orlistat, in the faeces and urine, took 3 to 5 days (Xenical SmPC, section 5.2).

What is the difference between orlistat 60mg and 120mg?

They are the same medicine at two strengths: the 120mg capsule blocks about a third of the fat in a meal and the 60mg capsule about a quarter (each leaflet, section 1), and Xenical is a brand of orlistat 120mg and alli a brand of orlistat 60mg. Who each may suit differs, and these are the rules our pharmacist works to, as of October 2026:

The two strengths side by side, with how each is taken, are in how to take orlistat. Orlistat has its own orlistat consultation, and a GPhC-registered pharmacist reviews every answer before anything is supplied.