What causes a wet anus?

Sh*t + Fart = Shart. The anus is a rather remarkable muscle able to differentiate solid from liquid from gas. In fact, when healthy, one can pass gas while retaining stool. However, if you watch enough adolescent movies, you'll see scenes where someone loses control. Weight loss medication, orlistat, is well known to cause "oily spotting" & inability to control bowel movement. if this doesn't answer your Q, write back.