How to Clean Grease Out of Sink Drains Mr. Rooter Plumbing Most people dump leftover grease down their sink drains because they find it to be convenient, and they also wrongly believe that it will drain all the way through their drain lines without any problems. But the truth is, grease should never go down sink drains because it can cause sink clogs and slow draining sinks. While grease in liquid form may not appear to be harmful, it will eventually cool and solidify in your drain lines and combine with food waste, soap scum, and other non-flushable items to form sink clogs.