Most homes built after 1970 use PVC (plastic) pipes for drains. Bleach is generally safe for PVC in small, diluted amounts. But if you pour undiluted bleach down a slow drain, it sits in the trap (the U-bend under your sink) for hours. Over time, concentrated bleach can make PVC brittle and prone to cracking.
But a clean-smelling drain is not an unclogged drain. You have simply sterilized a blockage. Now, instead of a living, decomposing clog, you have a sterile, solid plug of hair and soap. And you may have made the problem worse. This is where the "household hack" becomes a plumbing nightmare. can bleach unclog drain
What you are seeing when the water level drops isn't the clog dissolving. It is the bleach, which is less viscous than standing water, seeping through the gaps in the clog. The clog is still there—you’ve just found a temporary leak. Why do people think it works? Because of the smell. A clogged drain often stinks because of anaerobic bacteria (the kind that thrives without oxygen) feeding on the gunk. Bleach annihilates those bacteria instantly. The sulfurous, rotten-egg odor vanishes. Most homes built after 1970 use PVC (plastic)