How does Alli work?

The active substance in Alli, orlistat, is an anti-obesity medicine, which does not affect appetite.

Orlistat blocks gastrointestinal lipases (enzymes that digest fat). When these enzymes are blocked, they cannot digest some fats in the diet, and this allows about a quarter of the fat eaten in the meal to be passed out in the stools undigested.

The body does not absorb this fat and this helps the patient reduce their weight.