Orlistat

What it means

Orlistat works in the gut, not the brain. It blocks lipase, the enzyme that digests fat, so part of the fat in a meal passes through undigested. The prescription strength, Xenical, was approved in 1999; a lower-dose version, Alli, became available without a prescription in 2007.

It is taken with meals that contain fat. Its weight effect is modest compared with newer medicines.

The side effects follow from how it works: oily spotting, gas with discharge, urgent or more frequent bowel movements and fatty stools, especially after high-fat meals. Because it also reduces absorption of vitamins A, D, E and K, labels advise a daily multivitamin taken at least two hours apart from orlistat. Rare cases of severe liver injury have been reported.

Orlistat can also affect how some other medicines are absorbed, including levothyroxine and certain seizure drugs.

Why it matters

Orlistat is the only FDA-approved weight-loss medicine sold over the counter, so it is often the first thing people find in a pharmacy aisle.