A damaged drain pipe causing a leak can waste water, increase the water bill, allow hazardous sewer gases to leak in the bathroom and home, and cause water damage in the bathroom. When the drain pipe connected to a bathroom sink develops a leak, the drain pipe may often have to be replaced. You can do this yourself, however, if you are uncomfortable with the project, or the project may be more complicated that you thought, it is wise to call in a plumber. The drain pipe parts that are located under your sink includes two central parts – the straight tail piece that is connected to the drain, and the curved P-trap which prevents back up of water and it keeps sewer gases from leaking into the home.