Let it sit for 20–30 minutes. Do not flush. Do not lean over the bowl. You may hear fizzing or feel heat radiating from the porcelain.
There’s a mess in the bathroom. The water level is high, the plunger has failed, and the chemical drain cleaners under the sink look suspiciously mild. You’ve heard rumors about a "nuclear option" for plumbing: Caustic Soda. caustic soda for blocked toilet
After half an hour, flush the toilet. If the water level drops, the clog is clearing. If it remains high, repeat the process once more. Let it sit for 20–30 minutes
For 90% of toilet clogs, a (with a good seal) or a toilet auger (a $30 snake) is safer, cheaper, and more effective. Use caustic soda only for slow-draining toilets caused by organic sludge, and only if you are comfortable handling industrial chemicals. You may hear fizzing or feel heat radiating
If the water is already near the rim of the bowl, do not add chemicals. You will cause an overflow of caustic liquid. In that scenario, turn off the water supply, bail the bowl, and call a plumber immediately.