A cartridge is adapted to be inserted removably into a drain hole of a urinal, and includes a cartridge body, a guide plate member, a detecting module and a filter unit. The cartridge body has an internal space having an opening. The guide plate member is disposed in the opening, and is perforated to guide liquid which exits the urinal via the drain hole to flow therethrough into the internal space. The detecting module includes a drug-detecting chip for detecting presence of a specified drug, and a controlling chip for controlling operation of the drug-detecting chip according to measured displacement of the guide plate member. The filter unit utilizes a channel to direct the liquid from