Have you used caustic soda before? Share your experience (or warning) in the comments.
Pour slowly and low to avoid splashing. Immediately close the toilet lid.
If a plunger, hot water, and dish soap have all failed, you’re facing a serious clog. For stubborn organic blockages (toilet paper, feces, vomit, or grease), caustic soda (sodium hydroxide) can be an effective last-resort chemical cleaner.