Really? I thought caustic soda was in those oven cleaning kits where you put the bags in a rack. They tell you to pour it down the sink.
Well, these were the crystals you can buy in tubs from hardware store and dilute in water. All I know is, either the pipes were made of paper, or caustic soda fucks up plastic pipes in certain circumstances. We were using fair quantities as it didn't seem to be clearing the blockage in the bath*. When I'd used the stuff first time out, it had worked a treat unclogging fat from a narrow metal pipe leading down the sink to the drains outside the flat.
*it was a clusterfuck of poor, faulty and fundamentally flawed design of the pipework in the bathroom as well as issues with the exterior piping outside that contributed. The blockages were insane, and haven't been repeated anywhere I've lived since. Naturally, the landlord blamed us rather than listening to the 6 seperate plumbers called out (who to be fair each only partly diagnosed the problem)