If caustic soda doesn’t clear the drain after two attempts, the blockage is likely non-organic (e.g., a foreign object, collapsed pipe, or tree root). In that case, stop using chemicals and call a plumber – continued use could damage your pipes or leave dangerous standing solution that injures the plumber.
Here’s a short, practical piece on using caustic soda (sodium hydroxide) to unblock a drain, written in a clear, instructional style. Unblock a Drain with Caustic Soda: Fast, Powerful, but Handle with Care unblocking a drain with caustic soda
Caustic soda is a powerful DIY solution for organic clogs, but respect it like a strong acid. With proper protection and careful use, it can save you a plumber’s visit. When in doubt, choose a safer mechanical method or call for help. If caustic soda doesn’t clear the drain after