One and only one
There is only one Drano® product recommended for use in slow-running toilets: Drano® Max Build-Up Remover. It contains microorganisms that break down organic matter in pipes that can slow water flow. (However, it will not open a completely clogged toilet. For that problem, see How to Unclog a Toilet.)
Two cupfuls over three days
Simply flush 2 cupfuls of Drano® Max Build-Up Remover down the toilet for three consecutive days. Do this before leaving for work or going to bed, as it’s best to avoid flushing your toilet for 6-8 hours after each treatment.
Every four weeks
For preventative purposes, you can safely use Drano® Max Build-Up Remover monthly. It’s a good idea to be prepared, and keep some on hand!