My plumber recommends not putting toxic chemicals in the drains too. This is the best natural drain cleaner.
1/2 cup of baking soda
1/2cup of white vinegar
1 kettle of boiling water
Pour baking soda down drain. Add the vinegar and it will start bubbling up. Let sit for 30 minutes OR overnight . Boil a kettle of water to pour down the drain. Repeat if still sluggish. Use at least once a month, maybe more often for older houses with original pipes.