Anal leakage

Anal leakage is a type of fecal incontinence that comes from eating non-digestible, artificial fats such as Olestra and from medicines that prevent the absorption of dietary fats such as Orlistat, where the fat leaks out from the rectum and stains the underpants.

Artificial fats
The fat substitute Olestra, used in some reduced-calorie foods, has been proven to cause leakage in some consumers. The United States Food and Drug Administration warning indicated that excessive consumption of Olestra could result in "loose stools." As of 2003, the FDA no longer requires this warning.

Medications
Orlistat (Xenical) is a diet pill that works by blocking the enzymes that digest fat. As a result, fat cannot be absorbed from the gut, and instead some fat is excreted in the feces instead of being metabolically digested, and as a result, can cause oily anal leakage.