1 cup baking soda and let sit for 15 minutes. Add 1/2 cup white vinegar and sit for 15 minutes. Boil water and pour down the drain.
To keep them fresh, I empty the spent boxes of baking soda from the refrigerators and freezers and pour into all kitchen and bathroom drains and toilets. I let sit for a few hours or overnight and clean in the morning . I do this once a month. 2 refrigerators so 4 boxes a month.