How did prostitutes in ancient China practice contraception?

Before the advent of modern contraceptives, how did the courtesans in ancient brothels prevent pregnancy? There are few records in historical books, even less in unofficial histories, and they are only briefly mentioned in films and TV dramas. In a movie starring Gong Li, there is such a plot: a prostitute drinks a kind of soup medicine before receiving guests, saying that she won't get pregnant after drinking it. Of course, there is no way to verify what ingredients this medicine contains, so there are various opinions about the contraceptive methods of ancient prostitutes.