Orlistat

Key facts

• Orlistat (sold as Xenical and Alli, and in generic form) blocks the enzyme lipase in your gut, so it prevents about a third of the fat you eat from being absorbed. It is used alongside a reduced-calorie, lower-fat diet for weight management.
• Take it with meals that contain fat, or up to an hour afterward; skip the dose if a meal has no fat.
• Because it blocks fat absorption, eating a high-fat meal causes oily, loose stools and sudden, hard-to-control urges to go to the toilet; keeping meals lower in fat reduces this.
• It also reduces absorption of the fat-soluble vitamins A, D, E, and K, so take a multivitamin at bedtime. Rare but serious liver injury has been reported: seek care for yellowing skin or eyes, dark urine, or unusual tiredness.

What orlistat treats

Orlistat is used for weight management in adults who are overweight or living with obesity, alongside a reduced-calorie, lower-fat diet, and sometimes to help maintain weight already lost. It is not effective, and is not intended to be used, without dietary changes, and it does not treat any underlying cause of weight gain such as thyroid or hormonal conditions.

How orlistat works

Orlistat stays within the gut rather than being absorbed into the bloodstream in any meaningful amount. There it binds to the enzyme lipase, which normally breaks dietary fat down into a form the body can absorb. With lipase blocked, roughly a third of the fat you eat passes through the gut undigested and leaves the body in the stool instead of being absorbed as calories.

Before you take it

• Do not take orlistat if you have chronic malabsorption syndrome, cholestasis (reduced bile flow), or are pregnant or breastfeeding.
• Tell your prescriber if you have gallstones, kidney stones, or a history of pancreatitis.
• Take orlistat and other oral medicines, including levothyroxine, anticonvulsants, ciclosporin, and warfarin, at least a few hours apart, since orlistat can reduce their absorption.
• People with a history of eating disorders should discuss whether orlistat is appropriate.

Side effects

Common effects are tied directly to how much fat you eat: oily spotting, increased flatulence, oily or fatty stools, and urgent, frequent bowel movements.

Stop and seek urgent medical care for any of these:

• Yellowing of the skin or eyes, dark urine, severe itching, or loss of appetite, which can signal liver injury.
• Severe or worsening abdominal pain.
• Signs of an allergic reaction: rash, swelling of the face or throat, difficulty breathing.

Safety essentials

• Take a multivitamin containing vitamins A, D, E, and K at bedtime, well apart from your orlistat dose, since the drug reduces how much of these you absorb from food.
• Keep dietary fat to roughly a third of your calories per meal; higher-fat meals make the oily stools and urgency markedly worse.
• Rare cases of serious liver injury have been reported. Stop orlistat and seek care promptly for jaundice, dark urine, or unexplained fatigue.
• Separate orlistat from other oral medicines by a few hours, since it can blunt their absorption, particularly thyroid and anti-seizure medicines.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.