Use Baking Soda for Drain Flies as a First-Step Fix To use baking soda for drain flies, pour 1/2 cup of baking soda into the affected drain, add 1 cup of white vinegar, let it fizz for 10 to 15 minutes, then flush with very hot water. For plastic or older pipes, use steaming hot tap water instead of rolling-boil water. This method can loosen light soap scum, food residue, and organic film where drain flies breed. But the fizz is not a complete cure on its own. Remove the stop